DESCRIPTION WHITE POLYESTER FILM AND SURFACE LIGHT SOURCE THEREWITH TECHNICAL FIELD The invention relates to an improvernent in white polyester films and more specifically to a white polyester film suitable as a reflecting plate for surface light sources and as a reflector and useful to form brighter surface light sources having high lighting efficiency. BACKGROUND ART In recent years, various liquid crystal-based displays have been used in personal computers, televisions, cellular phones, and so on. Since such liquid crystal displays themselves are not light-emitting devices, surface light sources, called backlights, are placed on the back side for illumination to enable the display. To meet the requirement that not only illumination itself but also uniform illumination over the screen should be provided, backlights have a surface light source structure called side light type or direct type. In particular, side light type backlights, which illuminate the screen from the side, are used in slim liquid crystal display applications for slimness or small size requiring notebook computers and so on. In general, such side light type backlights use a light guide plate system that uniformly, illuminates the whole of a liquid crystal display through a light guide plate for uniformly transmitting and diffusing light from a cold cathode fluorescent lamp as an illuminating light source placed on the edge of the light guide plate. In this illumination system, a reflector is provided around the cold cathode fluorescent lamp to use light more efficiently, and a reflecting plate is provided under the light guide plate to efficiently reflect, to the liquid crystal screen side, the light diffused by the light guide plate. This structure reduces the loss of light from the cold cathode fluorescent lamp and provides the function of brightening the liquid crystal screen. On the other hand, large-screen applications such as liquid crystal televisions use a direct light system, because the edge light system does not promise the desired high screen brightness. In the direct light system, cold cathode fluorescent lamps are arranged in parallel under the liquid crystal screen and placed parallel to one another above a reflecting plate. A flat reflecting plate or a reflecting plate shaped like a semicircular arch along a part of the cold cathode fluorescent lamp is used. Such a reflector or reflecting plate for use in surface light sources for liquid crystal screen (generically called "surface light source reflecting member") is required to be a thin film and to have high reflection performance. Conventionally, a white pigment-containing film, a film containing fine voids inside, or a laminate of such a film and a metal plate, a plastic plate or the like has been used. In particular, the film containing fine voids inside is widely used, because it is highly effective in increasing brightness and has high uniformity.(Patent Literatures 1 and 2) Concerning the film containing fine voids inside, a nucleating agent is added to form the fin,e voids. An acyclic olefin resin such as polypropylene or polymethylpentene has been used as the nucleating agent. Liquid crystal screen applications have been extended from conventional notebook computers to other various devices such as desktop personal computers, televisions, and cellular phone displays in recent years. As high definition images on the liquid crystal screen have been required, improvements have been made to enhance the liquid crystal screen brightness and to make the image clearer and highly visible, and therefore, high-brightness and high-power illuminating light sources (such as cold cathode fluorescent lamps) have been used. Patent Literature 1: Japanese Patent Application Laid-open (JP-A) No. 06-322153 I Patent Literature 2: JP-A No. 07-118433 DISCLOSURE OF INVENTION Problems to be Solved by the Invention However, when the above conventional film is used as a surface light source reflecting member such as a reflecting plate or a reflector, light from the illuminating light source is partially transmitted to the opposite side, because of its low light reflectivity, so that the brightness (luminosity) of the liquid crystal screen may be insufficient and that the lighting efficiency may be reduced by the loss of the light transmitted from the illuminating light source. Such a problem has been pointed out, and there has been a strong demand for an improvement in the reflectivity and opacity of the white film. For this purpose, it is necessary to make a fine dispersion of the nucleating agent. However, there have been the problems that (1) in recent years, making a fine dispersion of the nucleating agent is reaching a limit; and that (2) if fine dispersion is achieved, voids cannot be stably formed or maintained in a film production process, because the nucleating agent has low stiffness or low deformation temperature. In addition, if the film production conditions are changed (for example, the heat treatment temperature is lowered) in order to solve the problem (2), a new problem such as low dimensional stability may occur. Means for Solving the Problems To solve the problems-, the invention has the features described below. Specifically, the invention is directed to a white polyester film, including a white polyester layer (W layer) containing a polyester resin component (A) and a component (B) incompatible with the resin component (A) and having voids inside, wherein the incompatible component (B) is a cyclic-olefin resin (b) having a glass transition temperature of 110°C or more, and the content Z of the cyclic-olefin resin (b) in the white polyester layer (W layer) is from 5% by weight to 50% by weight, based on the amount of the white polyester layer (W layer). Effects of the Invention The white polyester film of the invention has a high level of reflection properties, thermal dimensional stability, lightness, or the like, and is useful as a reflecting plate or a reflector in a surface light source to make it possible to brightly illuminate a liquid crystal screen and to make a liquid crystal display image clearer and highly visible. BEST MODE FOR CARRYING OUT THE INVENTION The white polyester film of the invention includes a white polyester layer (W layer) containing a polyester resin component (A) and a component (B) incompatible with the resin component (A) and having voids inside, wherein the incompatible component (B) is a cyclic-olefin resin (b) having a glass transition temperature of 110°C or more, and the content Z of the cyclic-olefin resin (b) in the white polyester layer (W layer) is from 5% by weight to 50% by weight, based on the amount of the white polyester layer (W layer). This feature dramatically improves the whiteness, light reflectivity, opacity, or the like of the white polyester film. In the invention, the white polyester layer (W layer) has voids inside, while the voids may be of any shape. The voids may be closed voids or two-dimensionally or three-dimensionally connected voids. While the voids may be of any shape, a large number of gas-solid interfaces are preferably formed in the film thickness direction. Therefore, the voids preferably have circular cross-sectional shapes or elliptical cross-sectional shapes elongated in the film in-plane direction. This is because the whiteness or light reflectivity of the film is produced by the reflection of rays incident on the film from the internal gas-solid interfaces (interfaces between the voids and the polyester resin as a matrix resin). In order to form such voids, the white polyester film of the invention is preferably produced by a process including melt extruding a mixture containing the polyester resin component (A) for serving as a matrix resin to form the white polyester layer (W layer) and the component (B) incompatible with the resin component (A) and then stretching the mixture in at least one direction to form voids inside so that interfaces can be formed. This technique produces flat voids based on the fact that flaking occurs at the interface between the resin component (A) and the incompatible component (B) as main components of the light-reflecting film during the stretching. When this technique is used, therefore, biaxial stretching is more preferred than uniaxial stretching in increasing the volume of the voids in the film and increasing the number of the interfaces per unit thickness. The polyester resin component (A) may be composed of a single polyester resin or a plurality of polyester resins -(namely, a mixture of a plurality of polyester resins). The same may apply to the incompatible component (B). In the white polyester film of the invention, the content Z of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more and serving as the incompatible component (B) is from 5% by weight to 50% by weight, based on the amount of the white polyester layer (W layer). The cyclic-olefin resin (b) used in the invention is a resin containing, as a monomer unit, at least one cyclic olefin selected from the group consisting of cycloalkene, bicycloalkene, tricycloalkene, and tetracycloalkene. The cyclic-olefin resin (b) may be a resin composed of the cyclic olefin or a copolymer of the cyclic olefin and a straight-chain olefin such as ethylene or propylene. Typical examples of the cyclic olefin include bicyclo[2,2,1]hept-2-en, 6-methylbicyclo[2,2,1]hept-2-en, 5,6-dimethylbicyclo[2,2,l]hept-2-en, 1- methylbicyclo[2,2,l]hept-2-en, 6-ethylbicyclo[2,2,l]hept-2-en, 6-n-butylbicyclo[2,2,1]hept-2-en, 6-isobutylbicyclo[2,2,1]hept-2-en, 7- methylbicyclo [2,2,1] hept-2-en, tricyclo [4,3,0,1^*^] -3-decene, 2-methyl-tricyclo [4,3,0,1^"^] -3-decene, 5-methyl-tricyclo [4,3,0,1^'^] -3-decene, tricyclo [4, 4,0,1^-^] -3-decene, and 10-methyl-tricyclo [4,4,0,1^'^] -3-decene. In particular, bicyclo[2,2,1]hept-2-en (norbornene) or derivatives thereof are preferred in view of productivity, transparency, or making Tg higher as described later. The cyclic-olefin resin (b) of the invention has a glass transition temperature of 110°C or more. When the cyclic-olefin resin (b) is used as the incompatible component (B), a large number of fine flat voids more than those produced with conventional incompatible components such as polymethylpentene, polypropylene, and polystyrene can be formed in the film. This makes it possible to dramatically improve the whiteness, light reflectivity, opacity, or the like of the film. This also makes it possible to further increase the brightness, when the film is used in a backlight unit. The cyclic-olefin resin (b) used as the incompatible component (B) may be finely dispersed by kneading it into the matrix resin (A), while both are melted. How finely the resin should be dispersed significantly depends on the interfacial tension difference between the matrix resin (A) and the incompatible component (B) and shearing during the kneading, while it may depend on various factors. The cyclic-olefin resin (b) having a glass transition temperature of 110°C or more, which is used in the invention, can more finely dispersed than conventional polyolefins (such as polypropylene and polymethylpentene). It may be because during the melt kneading process, the cyclic-olefin resin (b) has an interfacial_ tension closer to that of the polyester resin component (A) than that of conventional polyolefins. It is also considered that when the cyclic-olefin resin (b) has a glass transition temperature of 110°C or more, shearing can be easily applied during the kneading, so that the finely dispersing effect can also be produced. Flat voids are formed by flaking during a stretching process, in which the lower the stiffness of the incompatible component (B), the less likely the flaking at the interface between the matrix resin component (A) and the incompatible component (B) (the incompatible component (B) will be deformed together with the matrix resin component (A)). Therefore, it is considered that as the glass transition temperature of the cyclic-olefin resin (b) increases, the interfacial flaking may proceed so that a large number of fine flat voids can be stably formed. In particular, the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more dramatically improves the void forming ability. It is also considered that the use of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more will be effective in suppressing the loss of voids in the process of heat-treating the film. In general. the film is heat-treated at around 200°C so as to have thermal dimensional stability, after it is stretched. However, if the thermal deformation temperature of the incompatible component (B) is low, the incompatible resin may be deformed in the heat-treating process, so that some voids may be lost (or allowed to contract). The thermal deformation temperature generally depends on the glass transition temperature. Therefore, the use of the cyclic-olefin resin (b) having a relatively high glass transition temperature makes it possible to significantly suppress the loss of voids, even when heat treatment is performed. For the reason described above, the use of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more makes it possible to form a large number of fine flat voids in the process of forming the film, so that the whiteness, light reflectivity, lightness, thermal dimensional stability, or the like of the film can be dramatically improved and that higher brightness can be obtained when the film is used in a backlight unit. Specifically, the glass transition temperature is preferably 130°C or more, more preferably 160°C or more, even more preferably 180°C or more. This range makes possible finer dispersion into the matrix resin in the 11 kneading process, more stable formation of voids in the stretching process, and higher suppression of the loss of voids in the heat-treating process. The upper limit of the glass transition temperature is preferably equal to or lower than the melting point of the polyester resin component (A), while it is not particularly specified. The upper limit of the glass transition temperature is more preferably the melting point-20°C (20°C lower than the melting point), even more preferably the melting point-40°C (40°C lower than the melting point). More specifically, when the polyester resin component (A) is polyethylene terephthalate or the like, the glass transition temperature is preferably ■250°C or less, more preferably 230°C or less, particularly preferably 210°C or less. It is because if the glass transition temperature is higher than 250°C, the cyclic-olefin resin (b) may be insufficiently melted in the melt kneading process with the matrix resin (A), so that fine dispersion may not be promoted. On the other hand, if the glass transition temperature is less than 110°C, the incompatible component (B) may not be finely dispersed, so that the desired whiteness, reflectivity, or lightness cannot be obtained, or uneven stretching may occur. The glass transition temperature (Tg) may be the 12 midpoint glass transition temperature (Tmg) according to JIS K 7121 (1987), which may be determined by a process that includes heating the resin from 25°C to 300°C at a heating rate of 20°C/minute under a nitrogen atmosphere using a differential scanning calorimeter (e.g., RDC220 Robot DSC (Seiko Instruments Inc.)), holding the heated state for 10 minutes, then quenching the resin to 25°C or lower, and raising the temperature again from room temperature to 300°C at a heating rate of 20°C/minute to obtain endothermic and exothermic curves (DSC curve). » The cyclic-olefin resin (b) for use in the invention may be produced by known liquid-phase polymerization methods (see for example JP-A No. 61-271308), or a commercially available product (such as TOPAS (Polyplastics Co., Ltd.)) may be used as the cyclic-olefin resin (b). Examples of methods for setting the glass transition temperature to 110°C or more include increasing the content of the cyclic-olefin component in the cyclic-olefin resin (b) and reducing the content of the straight-chain olefin component such as ethylene in the cyclic-olefin resin (b). Specifically, the content of the cyclic-olefin component is preferably 60% by mole or more, more preferably 70% by mole or more, even more preferably 80% by mole or more, while the content of the straight-chain olefin component such as ethylene is preferably less than 13 40% by mole, more preferably less than 30% by mole, even more preferably less than 20% by mole. In these ranges, the cyclic-olefin resin (b) can have a high glass transition temperature. • While the content of each component is not limited within the above range, it is preferable that the content of the cyclic-olefin component is not 95% by mole or more, or that the content of the straight-chain olefin component such as ethylene is not less than 5% by mole. It is because within such a range, the polymerization may require a very long time, which is disadvantageous in productivity or cost. The straight-chain olefin component is preferably, but not limited to, an ethylene component in view of reactivity. The cyclic-olefin component is preferably, but not limited to, bicyclo[2,2,1]hept-2-en (norbornene) or a derivative thereof in view of productivity, transparency or making Tg high. Besides the above two components, if necessary, any other copolymerizable unsaturated monomer component may be copolymerized, as long as it does not interfere with the objects of the invention. Examples of such a copolymerizable unsaturated monomer include a-olefins of 3 to 20 carbon atoms, such as propylene, 1-butene, 4-methyl- 14 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, • and 1-eicosen, cyclopentene, cyclohexene, 3-methylcyclohexene, cyclooctene, 1, 4-hexadiene, 4-methyl-l, 4-hexadiene, 5-inethyl-l, 4-hexadiene, 1,7-octadiene, dicyclopentadiene, 5-ethylidene-2-norbornene, 5-vinyl-2-norbornene, tetracyclododecene, 2-methyltetracyclododecene, and 2-ethyltetracyclododecene, In the invention, the content Z of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more is from 5% by weight to 50% by weight, based on the amount of the white polyester layer (W layer). When the content Z is in this range, whiteness, reflectivity and lightness can be sufficiently produced. If the content Z of the cyclic-olefin resin (b) is less than 5% by weight, a low level of whiteness or light reflectivity may be provided. If it is more than 50% by weight, the film may have reduced strength and tend to be broken during stretching. In view of whiteness or light reflectivity, the content Z of the cyclic-olefin resin (b) is preferably 10% by weight or more, more preferably 20% by weight or more, even more preferably 30% by weight or more. The cyclic-olefin resin (b) for use in the invention is preferably amorphous. As used herein, the term "amorphous" means that the 15 heat of fusion of crystal is less than 1 cal/g as measured with a differential scanning calorimeter at a heating rate of 20°C/minute. The use of the amorphous cyclic-olefin resin (b2) makes it possible to further facilitate the fine dispersion in the matrix resin (A) and to significantly improve the whiteness or light reflectivity of the film. Although the detailed reason for the production of such an effect is not clear, it is considered that when it is amorphous, the temperature dependence of the melt viscosity is reduced so that shearing can be efficiently applied during kneading. For example, the cyclic olefin resin (b) may be made amorphous by increasing the content of the cyclic-olefin component in the cyclic-olefin resin (b). It is because the introduction of the cyclic-olefin component can increase the steric hindrance and reduce the crystallinity. The preferred content of the cyclic-olefin component depends on the type of the introduced cyclic-olefin component. For example, when ethylene and norbornene are selected as the structural components, the content of the norbornene component is preferably 60% by weight or more (the ethylene component is preferably 40% by weight or less), more preferably 70% by weight or more (the ethylene component is more preferably 30% by weight or less), even 16 more preferably 80% by weight or more (the ethylene component is even more preferably 20% by weight or less), based on the weight of the final resin product after the completion of the polymerization. The upper limit of the content of the cyclic-olefin component is preferably, but not limited to, 99% by weight or less. If it is more than 99% by weight, the resin will be close to a homopolymer so that its viscosity may be significantly high, which may make mass production impossible. The polyester resin for use in the polyester resin component (A) in the invention includes a dicarboxylic acid and a diol as structural components and may be obtained by polycondensation thereof. Typical examples of the dicarboxylic acid component as a structural component include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 5-sodium sulfoisophthalic acid, phthalic acid, and diphenic acid, and ester derivatives thereof; aliphatic dicarboxylic acids such as adipic acid, sebacic acid, dodecadionic acid, eicosanoic acid, and dimer acid, and ester derivatives thereof; aliphatic cyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and ester derivatives thereof; and polyfunctional acids such as trimellitic acid and pyromellitic acid, and ester derivatives thereof. Typical examples of the diol 17 component include ethylene glycol, propanediol, butanediol, neopentylglycol, pentanediol, hexanediol, octanediol, decanediol, cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyethylene glycol, tetramethylene glycol, polyethylene glycol, and polyethers such as polytetramethylene glycol. Examples of polyesters that are preferably used include polyethylene terephthalate (hereinafter also abbreviated as PET), polyethylene-2,6-naphthalene dicarboxylate, polypropylene terephthalate, polybutylene terephthalate, and poly-1,4-cyclohexylenedimethylene terephthalate. Using a polyester resin as the matrix resin, high mechanical strength can be imparted to the film, while high colorlessness is maintained. In addition, it is also inexpensive. In the invention, a copolyester resin having a basic structure of polyethylene terephthalate or the like and a copolymerized component introduced therein may also be used as the polyester resin component (A). The introduction of the copolymerizable component into the . polyester resin may be performed by a method including adding the copolymerizable component in the process of polymerization of polyester pellets as a raw material and using the pellets in which the component has already been copolymerized, or by a method including supplying a mixture of, for example, pellets of a homopolymer such as polybutylene terephthalate and pellets of polyethylene terephthalate to an extruder and copolymerizing them by transesterification when they are melted. In the invention, a polyester resin free of copolymerized components may be used in combination with a copolyester resin having a copolymerized component to form the polyester resin component (A). In the case of copolymerization, the content of the copolymerized component is preferably, but not limited to, 1 to 70% by mole, more preferably 10 to 40% by mole, based on the amount of each of the dicarboxylic acid component and the diol component, in view of transparency, formability or the like and in view of making the resin amorphous as described below. In a preferred aspect of the invention, amorphous polyester (a2) produced by copolymerization is used. As used herein, the term "amorphous" refers to a resin having a heat of fusion of crystal of less than 1 cal/g as described in detail later. In contrast, a resin having a heat of fusion of crystal of 1 cal/g or more is referred to as a crystalline resin. Preferred examples of the amorphous polyester resin (a2) include PET copolymer resins in which an aliphatic cyclic diol is copolymerized as a diol component, PET 19 copolymer resins in which isophthalic acid is copolymeri^eri as a dicarboxylic acid component, and PET copolymer resi_j^ in which an aliphatic cyclic dicarboxylic acid is copolymerized as a dicarboxylic acid component. In particular, an amorphous PET copolymer resin in which cyclohexanedimethanol (an aliphatic cyclic glycol) is copolymerized as a diol component, or a PET copolymer resi in which an aliphatic cyclic dicarboxylic acid is copolymerized as a dicarboxylic acid component is preferably used in view of transparency or formability or in view of the effect of finely dispersing the iricompatih] resin as described below. In view of easy availabili-ty ^ monomers, the amorphous PET resin in which cyclohexanedimethanol (an aliphatic cyclic glycol) ig copolymerized as a diol component is more preferred. When cyclohexanedimethanol is used to form an amorphous resin, the content of the copolymerized cyclohexanedimethanol component is preferably 25% by mole or more, more preferably 30% by mole or more, based on the amount of the diol components of the polyester resin, in view of forming an amorphous resin. orm an by on When cyclohexanedicarboxylic acid is used to f amorphous resin, the content of the copolymerized cyclohexanedicarboxylic acid component is preferably 25% mole or more, more preferably 30% by mole or more, based 20 the amount of the dicarboxylic acid components of the polyester resin, in view of forming an amorphous resin. The amorphous polyester resin (a2) used as the structural component of the polyester resin component (A) is effective in making a more stable dispersion of the incompatible component in the matrix resin and in finely dispersing the incompatible component. Although the detailed reason for the production of such an effect is not clear, this makes it possible to form a large number of voids in the film, so that high reflectivity, high whiteness, or lightness can be achieved. In addition, the stretchability of the film or the film forming ability can also be improved. The white polyester film of the invention preferably includes both the crystalline polyester (al) and the amorphous polyester resin (a2). In this case, the resulting white polyester film can have a high level of mechanical properties and reflection properties. Examples of the type of the crystalline polyester (al) include, but not limited to, PET, polyethylene-2,6-naphthalene dicarboxylate, polypropylene terephthalate, and polybutylene terephthalate. Particularly, in view of mechanical properties, PET is preferably used. In the white polyester film of the invention, the content a2w of the amorphous polyester resin (a2) in the 21 white polyester layer (W layer) is preferably 10% by weight or more, based on the amount of the white polyester layer (W layer). The content alw of the crystalline polyester (al) in the white polyester layer (W layer) is preferably less than 50% by weight, based on the amount of the white polyester layer (W layer). when the content alw or a2w is in the above range, the film forming ability and the mechanical properties can be maintained, while the above effect of dispersing the incompatible component (B) is sufficiently produced. While the upper limit of the content a2w of the amorphous polyester resin (a2) or the lower limit of the content alw of the crystalline polyester (al) is not limited, the content a2w of the amorphous polyester resin (a2) is preferably 50% by weight or less, based on the amount of the white polyester layer (W layer), and the content alw of the crystalline polyester (al) is preferably 10% by weight or more, based on the amount of the white polyester layer (W layer). If the content is outside the above range, the film forming ability or the mechanical properties may be reduced. In the white polyester film of the invention, the polyester resin component (A) preferably contains the crystalline polyester resin (al) and the amorphous polyester resin (a2), and the ratio (a2w/Z) of the content 22 a2w (% by weight) of the amorphous polyester resin (a2) in the white polyester layer (W layer) to the content Z (% by weight) of the cyclic-olefin resin (b) in the white polyester layer (W layer) is preferably from 0.01 to 0.85 or from 0.95 to 1.75. If the ratio is less than 0.01, the finely dispersing effect of the addition of the amorphous polyester resin may be insufficient, so that the reflection properties may tend to be significantly reduced, or the film forming ability may tend to be reduced. If the ratio is more than 0.85 and less than 0.95, the reflectivity may not be improved, and the heat resistance may tend to be reduced. If the ratio is more than 1.75, the heat resistance of the film may be significantly reduced, which is not preferred. When the ratio is in the range of 0.01 to 0.85, a reflective film with high heat resistance is provided. When the ratio is in the range of 0.95 to 1.75, a reflective film with a high level of reflection properties is provided. The ratio is more preferably from 0.4 to 0.85 or from 0.95 to 1.5. In the white polyester film of the invention, the white polyester layer (W layer) preferably includes an aliphatic cyclic diol component or an aliphatic cyclic dicarboxylic acid component and an isophthalic acid component, and preferably satisfies a formula 1 or 2: 0.01<(X+Y)/Z<0.33 (formula 1) 23 0.37<(X+Y)/Z<0.75 (formula 2) wherein X is the content (% by mole) of the aliphatic cyclic diol component based on the total amount of all the diol components in the W layer, Y is the sum (% by mole) of the contents (% by mole) of the aliphatic cyclic dicarboxylic acid component and the isophthalic acid component based on the total amount of all the dicarboxylic acid components in the W layer, and Z is the content (% by weight) of the cyclic-olefin resin (b) in the W layer based on the amount of the whole of the W layer. If (X+Y)/Z is less than 0.01, the finely dispersing effect of the addition of the amorphous polyester resin (a2) may be insufficient, so that the reflection properties may tend to be significantly reduced, or the film forming ability may tend to be reduced. If it is more than 0.33' and less than 0.37, the reflectivity may not be improved, and the heat resistance may tend to be reduced. If it is more than 0.75, the heat resistance of the film may be significantly reduced, which is not preferred. When it is in the range of more than 0.01 to 0.33, a reflective film having high reflection properties and particularly high heat resistance is provided. When it is in the range of more than 0.37 to 0.75, a reflective film having high heat resistance and particularly high reflection properties is provided. It is more preferably more than 0.05 to 0.33 or 24 more than 0.37 to 0.62. Besides the amorphous/crystalline polyester resin (al, a2) as the polyester resin component (A) and the cyclic-olefin resin (b) as the incompatible component (B), a dispersing agent (C) is preferably added to the white polyester film of the invention so that the cyclic-olefin resin (b) as the incompatible component (B) can be more finely dispersed. When the dispersing agent (C) is added, the cyclic-olefin resin (b) can be dispersed in smaller diameters, and therefore finer flat voids formed by stretching can be finely dispersed, so that the whiteness, reflectivity, or lightness of the film can be improved. Examples of the dispersing agent (C) that may be used include, but are not limited to, an olefin polymer or copolymer having a polar group such as a carboxyl or epoxy group or a functional group reactive with polyester; diethylene glycol, polyalkylene glycol, a surfactant, a heat-sensitive adhesive resin, and so on. Of course, these may be used singly or in combination of two or more thereof. In particular, a polyester-polyalkylene glycol copolymer including a polyester component and a polyalkyleneglycol component is preferred, and a crystalline polyester-polyalkyleneglycol copolymer (cl) is more preferred. 25 In this case, the polyester component preferably includes an aliphatic diol component of 2 to 6 carbon atoms and a terephthalic acid component and/or an isophthalic acid component. The polyalkylene glycol component is preferably a polyethylene glycol component, a polypropylene glycol component, a polytetramethylene glycol component, or the like. In a particularly preferred mode, polyethylene terephthalate or polybutylene terephthalate is used as the polyester component in combination with polyethylene glycol or polytetramethylene glycol as the polyalkylene glycol component. In particular, polybutylene terephthalate is preferably used as the polyester component in combination with polytetramethylene glycol as the polyalkylene glycol component, or polyethylene terephthalate is preferably used as the polyester component in combination with polyethylene glycol as the polyalkylene glycol component. The added amount Cw of the dispersing agent (C) used in the invention is preferably, but not limited to, 0.1 to 30% by weight, more preferably 2 to 25% by weight, even more preferably 5 to 20% by weight, based on the amount of the white polyester layer (W layer) containing voids. If the added amount Cw is less than 0.1% by weight, the effect of making fine voids may be reduced. If the added amount Cw is more than 30% by weight, the problem of 26 a reduction in production stability, an increase in cost, or the like may occur. In the white polyester film of the invention, the sum (alw+clw) of the content alw of the crystalline polyester resin (al) and the content clw of the crystalline polyester-polyalkyleneglycol copolymer (cl) is preferably less than 50% by weight, based on the amount of the white polyester layer (W layer). In this range, the film forming ability and the mechanical properties can be maintained, while the above effect of dispersing the incompatible component (B) is sufficiently produced. In the white polyester film of the invention, 99% by number or more of the cyclic-olefin resin (b) dispersions in the white polyester layer (W layer) preferably have a dispersion size (diameter) of 7 lam or less. Namely, the content of dispersions having dispersion sizes of more than 7 )im is preferably less than 1% by number. If the content of dispersions having dispersion sizes (diameters) of more than 7 ym is more than 1%, coarse voids may increase so that a low level of whiteness, light reflectivity or lightness may be provided. The content of dispersions having dispersion sizes of more than 4 \im is more preferably less than 1% by number, and the content of dispersions having dispersion sizes of 27 more than 2 ym is even more preferably less than 1% by number. Therefore, 99% by number or more of the dispersions more preferably have dispersion sizes (diameters) of 4 pm or less, even more preferably 2 ym or less. When the dispersion sizes are in the above range, a large number of voids can be efficiently formed in the film, so that a high level of whiteness, reflectivity or lightness can be provided. In a non-limited manner, 99% by number or more of the cyclic-olefin resin (b) dispersions in the white polyester layer (W layer) preferably have dispersion sizes (diameters) of 0.1 pm or more. If they are less than 0.1 ym, the desired whiteness or light reflectivity may not be obtained in some cases, because they are significantly smaller than the visible light wavelength, and therefore even when voids are formed in the film, the thickness of voids in the thickness direction is significantly smaller than the visible light wavelength, so that the efficiency of visible light reflection at the interfaces is reduced. The dispersion size may be measured by the method described below. First, the film is cut using a microtome in such a manner that the cross-section of the film is not deformed in the thickness direction, and a magnified image of the observed cross-section is obtained using an electron 28 microscope. At this time, the film is cut parallel to the TD direction (transverse direction). In the image, the maximum length of the cyclic-olefin resin (b) in the white polyester layer (W layer) is measured in the film surface direction and defined as the dispersion size. Examples of methods for adjusting the dispersion size of the cyclic-olefin resin (b) in the above range include, but are not limited to, (1) raising the glass transition temperature of the cyclic-olefin resin (b), (2) adding the amorphous polyester resin (a2) to the polyester resin component (A), and (3) adding the dispersing agent (C). In an extrusion process, the screw speed of the extruder is preferably high, and the time for which shearing is applied in the screw unit is preferably long. When the screw speed and the extrusion time are set high and long, respectively, the cyclic-olefin resin (b) is well dispersed in the polyester resin component (A), so that the dispersion sizes tend to be small and uniform. In addition, allowing the resin to pass once through a filter with an average mesh size of 40 ym or less in the extrusion process is also effective in making the dispersion sizes fine and uniform. In a preferred aspect, a thermoplastic resin layer such as a polyester layer is formed, by coextrusion or any other method, on at least one side of the white polyester 29 layer (W layer) of the film in which voids are formed by the above method. The formation of the thermoplastic resin layer on the film can impart surface smoothness and high mechanical strength to the- film. In this process, organic or inorganic fine particles or the incompatible resin may be added to the thermoplastic resin layer formed on the film. In this case, stretching may be performed in at least one direction in the process of producing the film, so that voids can also be formed in the thermoplastic resin layer formed thereon. To impart easy adhesion properties, antistatic properties or the like to the white polyester film of the invention, well-known techniques may be used to apply various coating liquids or form a hard coat layer to increase impact resistance. Examples of coating methods include gravure coating, roll coating, spin coating, reverse coating, bar coating, screen coating, blade coating, air knife coating, and dipping. After the coating process, the coating layer may be cured using known methods such as heat curing, methods using active rays such as ultraviolet rays, electron beams or radioactive rays, and combinations thereof. In this case, a curing agent such as a cross-linking agent is preferably used in combination with the methods. The coating layer may be formed by a coating method at the time 30 of the base film production (in-line coating) or a method of forming a coating on the white polyester film after the completion of crystal orientation (off-line coating). Various types of particles for increasing the surface smoothness or the running durability during the film production may also be added to the white polyester film of the invention. Particle species that may be used is preferably, but not limited to, silica, barium sulfate, titanium dioxide, calcium carbonate, or the like. In general, the white polyester film of the invention preferably has a high degree of whiteness and preferably has a bluish color tone rather than a yellowish color tone. In view of this point, a fluorescent brightening agent is preferably added to the white polyester film. An appropriate commercially-available fluorescent brightening agent may be used, such as UVITEX (manufactured by Ciba-Geigy Corporation), OB-1 (manufactured by Eastman Chemical Company), TBO (manufactured by SUMITOMO SEIKA CHEMICALS CO., LTD.), Keikol (manufactured by NIPPON SODA CO., LTD.), Kayalight (manufactured by NIPPON KAYAKU CO., LTD.), and Leucophor EGM (manufactured by Clariant (Japan) K.K.). The content of the fluorescent brightening agent in the film is preferably from 0.005 to 1% by weight, more preferably from 0.007 to 0.7% by weight, even more 31 preferably from 0.01 to 0.5% by weight. If it is less than 0.005% by weight, its effect may be small. If it is more than 1% by weight, the film may have a yellowish color. When the white polyester film is a laminated film, the fluorescent brightening agent may be more effectively added to the surface part. The white polyester film of the invention may also contain a photostabilizer. When a photostabilizer is added, ultraviolet-induced changes in the color of the film can be prevented. While any photostabilizer that does not degrade other properties may be preferably used, it is recommended that a photostabilizer having high heat resistance, being compatible with the polyester resin, uniformly dispersible, and less colored, and not affecting the reflection properties of the resin and the film be selected. Examples of such a photostabilizer include ultraviolet absorbers such as a salicylate type, a benzophenone type, a benzotriazole type, a cyanoacrylate type, and a triazine type, and ultraviolet stabilizers such as a hindered amine type. Specific examples include salicylate type ultraviolet absorbers such as p-tert-butylphenyl salicylate and p-octylphenyl salicylate; benzophenone type ultraviolet absorbers such as 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy'-4-methoxy-5-sulfobenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 32 bis(2-methoxy-4-hydroxy-5-benzoylphenyl)methane; benzotriazole type ultraviolet absorbers such as 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, and 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol]; cyanoacrylate type ultraviolet absorbers such as ethyl-2-cyano-3,3'-diphenyl acrylate; and triazine type ultraviolet absorbers such as 2-(4,6-diphenyl-l,3,5-triazine-2-yl)-5-[(hexyl)oxy]-phenol. Examples of ultraviolet stabilizers include hindered amine type ultraviolet stabilizers such as bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensate, and other ultraviolet stabilizers such as nickelbis(octylphenyl)sulfide and 2,4-di-tert-butylphenyl-3', 5'-di-tert-butyl-4'-hydroxybenzoate. Among these photostabilizers, 2,2',4,4'-tetrahydroxy-benzophenone, bis(2-methoxy-4-hydroxy-5-benzoylphenyl)methane, 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol], and 2-(4,6-diphenyl-l,3,5-triazine-2-yl)-5-[(hexyl)oxy]-phenol are highly compatible with polyester and therefore preferably used. The photostabilizers may be used singly or in combination of two or more thereof. The content of the photostabilizer in the white 33 polyester film of the invention is preferably from 0.05 to 10% by weight, more preferably from 0.1 to 5% by weight, even more preferably from 0.15 to 3% by weight, based on the amount of the photostabilizer-containing layer. If the content of the photostabilizer is less than 0.05% by weight, the light resistance may be insufficient so that the color tone may significantly change during long-term storage. If it is more than 10% by weight, the color tone of the film may be changed by coloration with the photostabilizer. In the invention, a coating layer having ultraviolet absorbing ability is preferably.formed on at least one side so that yellowing of the film during long-term use can be prevented. The ultraviolet absorbing layer may be a single layer or a plurality of layers. In the case of a plurality of layers, any one of the layers is an ultraviolet absorber-containing layer, and in view of maintaining light resistance, two or more of the layers are preferably ultraviolet absorber-containing layers. The ultraviolet absorbing layer may be obtained by adding or copolymerizing an ultraviolet absorber (such as a benzophenone type, a benzotriazole type, a triazine type, a cyanoacrylate type, a salicylate type, or a benzoate type) or an inorganic ultraviolet blocker or the like into a resin component such as a thermoplastic resin, a thermosetting resin, or an active ray-curable resin and forming a layer of the resin. 34 In particular, a benzotriazole type ultraviolet absorber is more preferred. Any benzotriazole type ultraviolet absorbing monomer having a benzotriazole moiety in the basic skeleton and having an unsaturated double bond may be used. Preferred examples of such a monomer include 2-(2'-hydroxy-5'-acryloyloxyethylphenyl)-2H-benzotriazole, 2-(2'-hydroxy-5'-methacryloxyethylphenyl)-2H-benzotriazole, and 2-(2'-hydroxy-3'-tert-butyl-5'-acryloyloxyethylphenyl)-5-chloro-2H-benzotriazole. Examples of an acrylic monomer and/or oligomer copolymerizable with these monomers include alkyl acrylate, alkyl methacrylate, and cross-linkable functional group-containing monomers such as monomers having a carboxyl group, a methylol group, an acid anhydride group, a sulfonic acid group, an amide group, an amino group, a hydroxyl group, an epoxy group, or the like. In the coating layer having ultraviolet absorbing ability, which is preferably used in the invention, one or more of the acrylic monomers and/or oligomers may be copolymerized in any ratio. In view of the hardness of the laminated film, methyl methacrylate or styrene is preferably polymerized in an amount of 20% by weight or more, more preferably 30% by weight or more, based on the amount of an acrylic monomer. Concerning the copolymerization ratio between the benzotriazole type 35 monomer and the acrylic type monomer, the ratio of the benzotriazole type monomer is preferably from 10 to 70% by weight, more preferably from 20 to 65% by weight, even more preferably from 25 to 60% by weight, in view of durability or adhesion to the base film. The molecular weight of the copolymer is preferably, but not limited to, 5,000 or more, more preferably 10,000 or more, in view of the durability of the coating layer. The copolymer may be produced by any appropriate method such as radical polymerization. The copolymer may be applied in the form of a dispersion in an organic solvent or water to the base film, and the thickness of the layer formed thereon is generally from 0.5 to 15 \im, preferably from 1 to 10 jam, more preferably from 1 to 5 ym, particularly in view of light resistance. In the invention, organic particles and/or inorganic particles for controlling the surface glossiness or the like may be added to the coating layer having ultraviolet absorbing ability. Inorganic particles may be made of silica, alumina, titanium dioxide, zinc oxide, barium sulfate, calcium carbonate, zeolite, kaolin, talc, or the like. Organic particles may be made of a silicone compound, cross-linked styrene, cross-linked acrylic, cross-linked melamine, or the like. The organic particles and/or inorganic particles preferably have a particle size (number average particle size) of 0.05 to 15 pm, more preferably 36 0.1 to 10 ]im. The content of the particles is preferably from 5 to 50% by weight, more preferably from 6 to 30% by weight, even more preferably from 7 to 20% by weight, based on the dry weight of the coating layer having ultraviolet absorbing ability. The size of the particles is preferably in the above range, so that the particles can be prevented from dropping off and the surface glossiness can be controlled. In the invention, various types of additives may be added to the coating layer having ultraviolet absorbing ability, as long as the effects of the invention is not inhibited. Examples of such additives include a fluorescent brightening agent, a cross-linking agent, a heat stabilizer, an electrification inhibitor, a coupling agent, and the like. The coating layer having ultraviolet absorbing ability may be formed by any coating method. For example, gravure coating, roll coating, spin coating, reverse coating, bar coating, screen coating, blade coating, air knife coating, dipping, extrusion lamination, or the like may be used. In particular, a kiss coating method with a micro-gravure roll is preferred, because of good coating appearance and highly uniform glossiness. After the coating process, the coating layer may be cured using known methods such as heat curing, methods using activ.e rays such 37 as ultraviolet rays, electron beams or radioactive rays, and combinations thereof. In the invention, a heat curing method with a hot air oven or an ultraviolet curing method by ultraviolet irradiation is preferred. The coating layer may be formed by a coating method at the time of the base film production (in-line coating) or a method of forming a coating on the base film after the completion of crystal orientation (off-line coating). The white polyester film of the invention preferably has a specific gravity of 0.4 to 1.5, more preferably 1.3 or less. If the specific gravity is less than 0.4, the film may have a disadvantage such as insufficient mechanical strength, buckling tendency, or low handleability. If it is more than 1.5, the void occupancy may be too low so that low r;eflectivity may be provided and that the film may tend to produce insufficient brightness, when used as a reflecting plat in a surface light source. The white polyester film of the invention preferably has a thickness of 10 to 500 jam, more preferably 20 to 300 Vim. If the thickness is less than 10 |im, it may be difficult to ensure the flatness of the film, so that uneven brightness may be easily produced, when the film is used in a surface light source. If the thickness is more than 500 ]im, a liquid crystal display in which the film is used as a light reflecting film may be too thick. When the film is a laminated film, the thickness ratio of its surface part/its internal part is preferably from 1/200 to 1/3, more preferably from 1/50 to 1/4. In the case of a three-layer laminated film having the structure surface part/internal part/surface part, the ratio is expressed by the total thickness of both surface parts/the thickness of the internal part. The white polyester film layer (W layer) is preferably an internal layer in view of whiteness or light reflectivity. Based on the above description, preferred examples of the content of each component according to the invention are illustrated below. Preferred are 5% by weight to 50% by weight of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more, 10% by weight to less than 50% by weight of the crystalline polyester resin (al), 10% by weight to 50% by weight of the amorphous polyester resin (a2), and 0.1% by weight to 30% by weight of the dispersing agent (C) (preferably a crystalline polyester-polyalkyleneglycol copolymer (cl)), based on the amount of the white polyester layer (W layer). More preferred are 20% by weight to 40% by weight of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more, 15% by weight to 45% by weight of the crystalline polyester resin (al), 15% by weight to 40% by weight of the amorphous polyester resin 39 (a2), and 2% by weight to 25% by weight of the dispersing agent (C) (preferably a crystalline polyester-polyalkyleneglycol copolymer (cl)), based on the amount of the white polyester layer (W layer). Particularly preferred are 25% by weight to 35% by weight of the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more, 20% by weight to 45% by weight of the crystalline polyester resin (al), 20% by weight to 30% by weight of the amorphous polyester resin (a2), and 5% by weight to 20% by weight of the dispersing agent (G) (preferably a crystalline polyester-polyalkyleneglycol copolymer (cl)), based on the amount of the white polyester layer (W layer). An exemplary method for producing the white polyester film of the invention is described below, which is not intended to limit the scope of the invention. A film producing machine equipped with an extruder (main extruder) is used to form the white polyester layer (W layer). A mixture of chips of the polyester resin component (A) and the cyclic-olefin resin (b) having a glass transition temperature of 110°C or more (if necessary, each vacuum-dried sufficiently) is supplied to the extruder being heated. The cyclic-olefin resin (b) may be added in the form of master chips, which are prepared in advance by uniformly melting and )