1. A focus measuring method measuring a focus of a picture displayed on a screen of a video display apparatus having a cathode-ray tube by employing a measuring apparatus having a rod lens array, an optical low-pass filter, and a line sensor, comprising the steps of: detecting a picture displayed on a screen of the video display apparatus having said cathode-ray tube by said line sensor through said rod lens array and said optical low-pass filter; and obtaining a focus measurement data from a signal detected by said line sensor.

2. A focus measuring method according to claim 1, wherein a focus adjustment picture having a bright line with wide widths in vertical and horizontal directions is employed as the picture displayed on said screen of the video display apparatus having the cathode-ray tube.

3. A focus measuring method according to claim 1, wherein said rod lens array is formed by arranging cylindrical minute lenses in lines.

4. A focus measuring method according to claim 1, wherein the picture displayed on said screen of the video display apparatus having the cathode-ray tube is detected by slightly displacing a focus of each of lens forming said rod lens array from said screen of the video display apparatus.

5. A focus measuring method according to claim 1, wherein said optical low-pass filter is a tape-like member bonded to a light receiving surface of said line sensor and having a light diffusion function.

6. A focus measuring method according to claim 1, wherein said optical low-pass filter is disposed in an optical path from said screen of the video display apparatus having the cathode-ray tube to said line sensor.

7. A focus measuring method of measuring a focus of a picture displayed on a screen of a video display apparatus having a cathode-ray tube, comprising the steps of: detecting a focus adjustment picture displayed on said screen of the video display apparatus having the cathode-ray tube and having a bright line with wide widths in the vertical and horizontal directions by a line sensor through a rod lens array and an optical low-pass filter; calculating a diameter of a beam spot based on vertical and horizontal line widths detected by said line sensor to determine whether a condition of said beam spot is an under-focussed condition or an over-focussed condition relative to a jut-focus condition; calculating, when it is determined that the condition is the under-focussed condition, a calculated value of the diameter of said beam spot with a function prepared for the under-focussed condition to obtain a focus quantitative value; and calculating, when it is determined that the condition is the over-focussed condition, a calculated value of the diameter of said beam spot with a function prepared for the over-focussed condition to obtain a focus quantitative value.

8. A focus measuring method according to claim 1, wherein the video display apparatus having the cathode-ray tube and to be adjusted comprises a memory for storing a focus control value and when a focus control is carried out based on a control data stored in said memory, a focus adjustment is carried out with the control data stored in said memory being replaced with a measured data.

9. A focus measuring method according to claim 7, wherein said rod lens array is formed by arranging cylindrical minute lenses in lines.

10. A focus measuring method according to claim 7, wherein a focus adjustment picture having a bright line with wide widths in vertical and horizontal directions and displayed on said screen of the video display apparatus having the cathode-ray tube is detected by slightly displacing a focus of each of lens forming said rod lens array from said screen of the video display apparatus.

11. A focus measuring method according to claim 7, wherein said optical low-pass filter is a tape-like member bonded to a light receiving surface of said line sensor and having a light diffusion function.

12. A focus measuring method according to claim 7, wherein said optical low-pass filter is disposed in an optical path from said screen of the video display apparatus having the cathode-ray tube to said line sensor.

13. A focus measuring method according to claim 7, wherein a predetermined focus evaluation membership function is employed to thereby obtain a focus quantitative value from a calculated value of the diameter of said beam spot based on a result of determination whether the condition is the under-focussed condition or the over-focussed condition.

14. An apparatus for measuring the focus substantially as herein described with reference to and as illustrated in the accompanying drawings.

APPARSTUSHFOR AND METHOD OF MEASURING FOCUS BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to an apparatus for and a method of measuring a focus of a television receiver or a monitoring image receiver using a cathode ray tube. Background of the Invention:
A television receiver using a cathode ray tube is required to be adjusted to an appropriate condition by measuring the focus of an electron beam so that a satisfactory picture may be displayed on a screen of the cathode ray tube before shipping it after manufacturing process in a factory. In a conventional method of measuring the focus, an operator used to control a volume for focus adjustment of the image receiver while checking by his eye a picture displayed on the cathode ray tube of the image receiver. However, such a method of adjustment has an inconvenient drawback in which an accuracy of the focus adjustment depends on a judgement of the operator because he directly views the picture to estimate, so that the accuracy will tend to be ununiform.
In order to overcome this inconvenience, the present applicant previously proposed a method of measuring a focus in which a displayed picture is detected by a sensor so as to enable a correct measurement of a shape of a beam spot (Patent Application No. 4-304032, etc.)
According to this method of measurement, the picture
displayed on the screen of the cathode ray tube is made incident on to a CCD line sensor through a rod lens array for measuring the shape of the beam spot from an output of the CCD line sensor. Specifically, for example, as is shown in FIG. 1A, the beam spot emitting a green light is shifted from a position shown by a broken line to a position shown by a solid line for displaying a bright line having a broad width. Here, a position denoted by G shows a position at which a green fluorescent substance is arranged and it is only this portion that actually emits light. Accordingly, the fluorescent substance emits light in a manner that each position of the green fluorescent substance emits light with a brightness corresponding to the beam spot that passed therethrough, as is shown in FIG. IB.
If the emitted light of the green fluorescent substance is detected by the line sensor, there is provided an output which is almost proportional to the light emission of the fluorescent substance, as shown in FIG. 1C. Since the output of the line sensor includes a noise component, the output from the line sensor is caused to pass through an electric low pass filter (e.g. a filter formed of a capacitor and a resistor) for obtaining a detected output without the noise component, as shown in FIG. ID. The bright line width is measured from this detected output and then a processing for measuring a diameter of the beam spot is performed based on that result. By measuring according to the process as shown in FIGS. 1A to ID, a uniform measurement can be made without any influence from a change of the light emission due to a portional relationship
between the fluorescent substance and the beam spot.
However, a ripple is introduced in the output of the line sensor that passed through the low pass filter- Thus, a ripple component L is generated in the output of the low pass filter, as is shown in FIG. ID. If this ripple component is large, the ripple component itself can be decided to be the detected output of the beam, thereby disturbing the measurement of the bright line width.
In order to eliminate this ripple component, it may be enough to set a passing frequency fc of the low pass filter to an appropriate value. However, since the bright line width of the cathode ray tube is different due to difference of a size of the screen or a mode of scanning of the image receiver to be measured, it is necessary to change the passing frequency fc of the low pass filter whenever they differ, which in turn brings an inconvenience in which a configuration of the detector circuit becomes extremely complicated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus for and a method of measuring the focus of the image receiver of this type in which the focus measurement can be performed correctly by a simple work.
The present invention provides an apparatus for and a method of measuring the focus of the picture displayed on the screen of a video display apparatus having a cathode ray tube, wherein the picture displayed on the screen of an image receiver is detected by a line sensor through a rod lens array and an
optical low pass filter and then data of focus measurement is obtained from a signal detected by the line sensor. At this time, in order to make it hard to be affected by a color sorting mechanism (aperture grille, etc.), a beam width of the picture is broadened and a focus of the rod lens array is slightly offset form the screen- At the same time, the optical low pass filter is arranged in an optical system such as an incident part of the line sensor or the like.
On the basis of the output of the line sensor thus obtained, widths of vertical and horizontal lines are displayed in a two-dimensional graph or the like. From the displayed position, etc. a decision from a just-focussed condition to an under-focused condition or an over-focussed condition is performed. At the same time, a diameter of the beam spot is calculated from the widths of the vertical and horizontal lines, and on the basis of the decision of the under-focussed condition or the over-focussed condition and the calculated value of the diameter of the beam spot, a quantitative value of the focus is acquired.
According to the present invention, since the beam width (bright line width) of a crosshatched image is broadened and an image on the screen is detected by the line sensor through the rod lens array whose focus is slightly offset and the optical low pass filter, a light energy stored in the line sensor is hard to be affected by the aperture grille, etc. and what is close to an actual shape of the beam can be detected for one field period, which in turn leads to a satisfactory focus
measurement. Moreover, by obtaining the quantitative focus value on the basis of the decision of the under-focussed condition or the over-focussed condition and the calculated value of the beam spot diameter, it will be possible to judge the correct focussed condition quantitatively.
According to the present invention, since the crosshatched image having the broadened widths of bright lines displayed on the screen of a video display apparatus having a cathode ray tube is detected by the line sensor through the rod lens array slightly out of focus and the optical low pass filter, an optical energy stored in the line sensor approximates an actual shape of the beam, thereby enabling a satisfactory focus measurement to be performed from the displayed image in which the bright line width is broad. At the same time, since the optical low pass filter is provided, the line sensor is able to store the optical energy under a good condition, which in turn leads to a good measurement with high accuracy without any influence from the color sorting mechanism (e.g. aperture grille etc.)* In addition, when the optical low pass filter is utilized as in the present invention, as compared with a case where the output of the line sensor is passed through an electric low pass filter, it is not necessary to change the setting due to a difference in an image size and a scanning made of the image receiver to be measured, thus allowing a constantly good measurement to be performed using the same measuring apparatus.
In this case, by applying the tape-like member having
the light-diffusing function to the light-receiving surface of the line sensor to form the optical low pass filter, it is possible for the optical low pass filter to be simply fixed.
Also, according to the present invention, since the focus quantitative value is obtained on the bases of the decision of the under-focussed condition or the over-focussed condition and the calculated value of the beam spot diameter, it is possible to decide quantitatively respective adjusted conditions, thereby enabling the focussed condition to be evaluated correctly.
In this case, by finding the focus quantitative value using the predetermined membership function for focus estimation, it is possible to evaluate correctly the focussed condition in consideration of the adjustment data obtained empirically from those skilled in the art and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to ID are wave form diagrams showing an example of a focus measurement;
FIG. 2 is a configuration diagram showing an embodiment according to the present invention;
FIG. 3 is an explanatory diagram showing the condition of measuring a screen according to the embodiment.;
FIG. 4 is a perspective view showing the structure of a sensor according to the embodiment;
FIG. 5 is an explanatory diagram showing a displayed image during measurement according to the embodiment;
FIG. 6 is an explanatory diagram showing the
arranging state of the sensor according to the embodiment;
FIG- 7 is a block diagram showing the structure of a detector circuit according to the embodiment;
FIGS. 8A to 8H are timing charts showing the focus measuring condition according to the embodiment;
FIG. 9 is an explanatory diagram showing an example of display of focussed condition according to the embodiment;
FIG. 10 is an explanatory graph showing a focus membership function to be applied to the embodiment;
FIG. 11 is an explanatory graph showing the values of the membership function in an measuring example (an example of an over-focussed condition and offset by 45µm) according to one
embodiment; and
FIG. 12 is an explanatory graph showing the values of the membership function in an measuring example (an example of
an under-focussed condition and offset by 45µm) according to the
embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, an embodiment according to the present invention will described below with reference to FIG. 2 to FIG. 12.
FIG. 2 is diagram showing a system configuration to which the measuring apparatus and the method according to the present embodiment are applied. Describing a processing to perform the measurement by the configuration shown in FIG. 2, in a case of this embodiment, the focus adjustment is carried out by measuring a crosshatched image 14 displayed on the screen of
the cathode ray tube of a television receiver 13. In order words, a measurement control device 11 comprised of a computer device controls a video signal generator 12 for measurement to produce a crosshatching signal for focus adjustment equivalent to a monoscope signal, this crosshatching signal for focus adjustment is supplied to the television receiver 13 as an image signal, and the crosshatched image 14 is displayed on the screen of the television receiver 13. This crosshatched image 14 is, for example, as is shown in FIG. 5, such a that only an area in the vicinity which the sensor detects is made black and two crossing vertical and horizontal bright lines with a predetermined width and of a predetermined color are displayed in the black background, other parts being made gray. A brightness level of the gray part is made variable to be equivalent to the monoscope signal. In addition, a light emitting color of the vertical and horizontal lines is made, for example, white (or green and the like of the light emitting color from a single fluorescent substance).
In this condition, a CCD line sensor 15 to which a rod lens array 16 is mounted is adhered closely to the screen of the television receiver 13 for detecting the vertical and horizontal bright lines in the crosshatched image 14 displayed. At this time, the focus of each lens forming the rod lens array 16 is slightly offset from the screen of the cathode ray tube. Specifically, as is shown in FIG. 3, the rod lens array 16 is comprised of a plurality of cylindrical microlenses 16a arranged linearly and functions as an optical relay member for making the
image displayed on a screen 13a of the television receiver 13 incident on the CCD line sensor 15. The focus of each of the microlenses 16a relative to the screen 13a is adjusted so that a position which is slightly offset from the screen 13a may be in focus.
The CCD line sensor 15 is comprised of light receivers arranged linearly and reads a signal charge stored in each light receiver and supplies the same to a beam size measuring unit 3 0 as a detected signal. Here, in this example, as shown in FIG. 4, a light-diffusing tape 21 is applied to the CCD line sensor 15 so as to cover entire light receivers 15a thereof. For this light-diffusing tape 21, what is called a semitransparent adhesive tape (e.g. a tape sold in a trade name of mending tape, etc. made by Sumitomo three M corporation) is utilized. By arranging the light-diffusing tape 21 in front of the light receivers 15a, an optical low pass filter is formed which cuts off a higher range component of the light emitted from the rod lens array 16 and the remaining light is made incident on the light receivers 15a.
Furthermore, in stead of the light-diffusing tape 21, other materials which function as the optical low pass filter may be arranged. Also, it may be arranged anywhere in the light path from the screen 13a to the light receivers 15a of the CCD line sensor 15.
The beam size measuring unit 30 measures from the detected signal of the line sensor 15 a horizontal width Hw of
the vertical line and a vertical width Vw of the horizontal line
in FIG. 6. In this case, the measurement process is performed in a manner that the sensor output supplied from the CCD line sensor 15 to the beam size measuring unit 30 is compared with a reference level and then the widths Hw and Vw are measured from
a width of a signal that exceeds the reference level.
In addition to these widths Hw and Vw, a position HL
from a reference position to an edge of the vertical line and a position VL from the reference position to an edge of the
horizontal line are also measured. These positions HL and VL
are determined as follows. The timing when the widths Hw and Vw
begin to be detected from widths of the vertical and horizontal lines is estimated as. Then, distances from a predetermined position (in this case, an end portion) of the CCD line sensor 15 to the respective edges of the vertical and horizontal lines are determined as the position HL and VL, respectively.
Particularly, as is shown in FIG. 6, the above measurement is performed in such a manner that the CCD line sensor 15 and the rod lens array 16 are put in an oblique position relative to the vertical and horizontal bright lines of the crosshatched image 14, that is, under a condition that they cross over both of the vertical and horizontal lines. In this condition, the width Hw and Vw are detected from a width of
respective signals when the output signals of the CCD line sensor 15 exceed the reference level, and then the positions HL
and VL are detected by detecting the distances from one end
portion 15a of the CCD line sensor 15 to the respective edges of
the widths Hw and Vw.
Here, a structure for detecting the widths Hw and Vw
as well as the positions HL and VL in the beam size measuring
unit 3 0 according to the present embodiment will be described with reference to a block diagram of FIG. 7 and a timing chart of FIG. 7 illustrating its operation. Further, signals denoted in FIGS. 8a, 8B, 8C, 8D, 8E, 8F, 8H correspond to signals transferred through signal lines denoted by reference symbols a, b, c, d, e, f, g and h in FIG. 7, respectively.
To begin with, a read start signal (FIG. 8A) is supplied from a terminal 31 at the side of the measuring unit 30 to the CCD line sensor 15 and at the same time a transfer clock signal (FIG. 8B) is supplied thereto from a terminal 32 at the side of the measuring unit 30. A sensor output (FIG. 8C) is read out of the CCD line sensor 15 at a speed synchronized with the transfer clock signal from a time when the read start signal is supplied thereto and then the sensor output is supplied to a positive side input terminal of a comparator 33 within the measuring unit 30. To a negative side input terminal of the comparator 33 is connected a reference voltage source 34. Therefore, the sensor output over a reference level which is a voltage of this reference voltage source 34 is taken out of the comparator 33 as a detected output of the bright line. That is, as shown in FIG. 8D, a pulse which rises to a high level when the bright line is detected and which falls to a low level when the bright line is not detected is output form the comparator 33.
The output of the comparator 3 3 is then supplied to a vertical/horizontal separator circuit 35. This vertical/ horizontal separator circuit 35 receives the read start signal (FIG. 8A) at a reset terminal R as a reset pulse and then supplies to a Q1 output terminal a first pulse received at a
pulse input terminal C after the reset pulse has been supplied, and supplies to a Q2 output terminal a second pulse received
thereat. The Q1. output of the vertical/horizontal separator
circuit 35 is supplied to one input end of an AND gate 3 6 as a vertical width detecting pulse (FIG. 8E). This AND gate 36 receives the transfer clock signal from the terminal 32 at the other input end and so the clock signal is output from a terminal 51 connected to the AND gate 36 only when the vertical width detecting pulse rises. A counter (not shown) connected to this terminal 51 counts that clock signal output and thus a value of this count corresponds to the vertical width measured. The Q2 output of the vertical/horizontal separator
circuit 35 is supplied to one input end of an AND gate 37 as a horizontal width detecting pulse (FIG. 8F). This AND gate 37 also receives the transfer clock signal form the terminal 32 at the other input end and so the clock signal is output from a terminal 52 connected to the AND gate 3 7 only when the horizontal width detecting pulse rises. A counter (not shown) connected to this terminal 52 counts that output clock signal and thus a value of this count corresponds to the measured horizontal width.
The Q1 output of the vertical/horizontal separator
circuit 35 is also supplied to a vertical position detector circuit 38. This circuit 38 receives the read start signal {FIG. 8A) at a reset terminal R as a reset pulse and then makes its Q output to rise at a time when the pulse supplied to a pulse input terminal C rises after the reception of the reset pulse. By inverting the Q output of the vertical position detector circuit 3 8 through an invertor gate 3 9, as shown in FIG. 8G, a vertical position detecting pulse is obtained a duration of which lasts from the rising time of the read start signal to the rising time of the vertical width detecting pulse. This vertical position detecting pulse is supplied to one input end of an AND gate 40. This AND gate 4 0 receives at its other input end the transfer clock signal form the terminal 32 and then outputs the clock signal from a terminal 5 3 connected to the AND gate 40 only when the vertical position detecting pulse rises. A counter (not shown) connected to the terminal 53 counts the output clock signal and its counted value corresponds to the measured vertical position.
Also, the Q2 output of the vertical/horizontal
separator circuit 35 is supplied to a horizontal position detector circuit 41. This circuit 41 receives the read start signal (FIG. 8A) at a reset terminal R as a reset pulse and then makes its Q output to rise at a time when the pulse supplied to its pulse input terminal C rises after the reception of the reset pulse. By inverting the Q output of the horizontal position detector circuit 3 9 through an invertor gate 42, as shown in FIG. 8H, a horizontal position detecting pulse is
obtained a duration of which lasts from the rising time of the read start signal to the rising time of the horizontal width detecting pulse. This horizontal position detecting pulse is supplied to one input end of an AND gate 43. This AND gate 43 receives the transfer clock signal from the terminal 32 at the other input end and then outputs the clock signal from a terminal 54 connected to the AND gate 4 3 only when the horizontal position detecting pulse rises. A counter (not shown) connected to the terminal 54 counts the output clock signal and its counted value corresponds to the measured horizontal position.
The measured data of the vertical width Hw and the
horizontal width Vw as well as the vertical position HL and the
horizontal position VL detected in this way by the counters
within the beam size measuring unit 30 are transferred to the measurement control device 11. In addition, the transfer clock signal supplied from the beam size measuring unit 30 to the CCD line sensor 15 is a clock signal generated based on a synchronizing signal received from the generator 12 of video signal for measurement, thereby enabling the image to be read out by the CCD line sensor 15 in synchronization with the image display on the screen of the television receiver 13.
The measurement control device 11 rotates a stepping motor 19 to vary the focussed condition while estimating the vertical width Hw and the horizontal width Vw as well as the
vertical position HL and the horizontal position VL indicated by
the transferred data. Specifically, the measurement control
device 11 controls a bit inserting robot 18 to insert automatically a bit fixed at a tip end of the stepping motor 19 into a focus adjusting volume 2 0 of the television receiver 13, so as to drive and control the focus adjusting volume 2 0 by the rotation of the stepping motor 19. Concerning the drive/control at this time, the focus adjusting volume 2 0 is driven, for example, over such an entire angular range that the focus adjusting volume can be rotated and then the measurement control device 11 stores respective change of each of data Hw, Vw, HL and
VL at that time.
The measurement control device 11 displays the changing state of each of data Hw, Vw, HL and VL on a screen of a
display device 11a connected to the measurement control device 11. FIG. 9 shows an example of the display in this case, in which the changes of the vertical position HL and the horizontal
position VL are displayed as a graph of horizontal/vertical
position 100 and the change of the vertical width Hw and the
horizontal width Vw are displayed as a beam area graph 110.
In this case, the graph of horizontal/vertical position 100, in which a vertical axis represents the detected state of the vertical position and a horizontal axis represents the detected state of the horizontal position, indicates changing states of each position on the coordinate axis composed of orthogonal vertical and horizontal axes. A curve 101 of change shown on the graph of horizontal/vertical position 100 is a changing curve obtained by varying the rotation of the focus
adjusting volume 20. Generally, the vertical position HL has
the maximum value at a certain adjusted position, while the horizontal position VL has the minimum value at the certain
adjusted position. This position is represented as a minimum point 102 and it is denoted by a mark (here, a solid circle) displayed. This minimum point 102 forms a boundary between a characteristic curve 103 representing the under-focussed condition and a characteristic curve 104 representing the over-focussed condition. A detected position along the curve of change 101 for the current rotated position of the volume 2 0 is displayed as a current point 105 (here, an open circle).
The beam area meter 110 displays the detected data of the vertical and horizontal widths Hw and Vw as a beam area
obtained by multiplying these data within the measurement control device 11. A left end of the meter 110 is denoted as a minimum point 111 and a right end is denoted as a maximum point 112. A beam area for the current adjusted position of the volume 20 is displayed as a current point 113 in a predetermined mode between the minimum point 111 and the maximum point 112.
By displaying the focussed condition measured in this manner, it will be possible to estimate correctly the changing state of focus of the beam of the cathode ray tube of the television receiver 13 which is an equipment to be adjusted. In other words, by observing the current point 105 along the curve of change 101 displayed as the graph of horizontal/vertical position 100, it will be possible to decide readily whether the current focussed condition is the under-focussed one or the
over-focussed one.
Moreover, in this example, on the basis of the estimation of the under-focussed condition or the over-focussed condition based on the display of the focussed condition and the diameter of the beam spot found from the data of the vertical width Hw and the horizontal width Vw which are the detected
bright line widths, a processing to find the focus quantitative value is performed within the measurement control device 11 for displaying that focus quantitative value found on the screen of the display 11a connected to the measurement control device 11. Here, the diameter of the beam spot is found from the following expression using the data of the vertical width Hw and the
horizontal width Vw detected.
Diameter of beam spot - J~ (Hw2 + vw2) (1)
For the processing to find the focus quantitative value from the calculated diameter of the beam spot and the estimation of the under-focussed or over-focussed condition, a membership function for focus estimation is utilized. Data of this membership function for focus estimation is previously stored in the measurement control device 11. FIG. 10 shows an example of the membership function for focus estimation to be used in the present embodiment. The membership function for focus estimation has been established as follows. A plurality of those skilled in the focus adjusting operation were made to perform the focus adjustment of a standard cathode ray tube. During the adjusting operation, three steps of adjusted
condition of CD good, (2) not well and (3) bad were set for
allowing respective ranges to be empirically determined, in which ranges the three steps of adjusted condition of good, not well and bad regarding the diameter of the beam spot could be estimated from respective focussing voltages, etc. during the adjustment.
In FIG. 10, a horizontal axis represents a difference from the minimum value of the beam spot diameter. A position
indicated by 0 µm represents a beam spot diameter at the just-
focussed position (i.e. corresponding to the minimum point 102 in FIG. 9). The left side from this position represents the over-focussed condition while the right side from this position represents the under-focussed condition. A vertical axis represents an adaptive value to be used for finding the focus quantitative value.
Next, a processing for finding the focus quantitative value using the membership function for the focus estimation will be described with reference to FIG. 11 and FIG. 12. The focus quantitative value in this example is a value which changes within a range form + 100 to -100. As a premise for finding the focus quantitative value, a calculated value of zero is set for the good focus-adjusted condition, a calculated value +50 is set for the not well focussed condition in the over-focussed condition and a calculated value of +100 is set for the bad focus-adjusted condition. Also, a calculated value -50 is set for the not well focus-adjusted condition in the under-focussed condition and a calculated value of -100 is set for the
bad focus-adjusted condition.
Firstly, an example of finding the focus quantitative value when a measured diameter of the beam spot is in the over-
focussed condition by 45u.m from the just-focussed position will
be described- in the membership function of FIG. 10, for a position offset by 45µm in a direction of the over-focussed
condition (in FIG. 10, a position denoted by a broken line of x), the adaptive value of the good focus-adjusted condition is 0.5 and at the same time the adaptive value of the not well focus-adjusted condition is 0.5. Accordingly, as is illustrated in FIG. 11, an adaptive value 0.5 is stood at the position of a value 0, and an adaptive value 0.5 is stood at the position of a value +50. Subsequently, a processing for finding a center of gravity of these adaptive values stood is carried out. In other words, the center of gravity is found by calculating the following expression and the value of the center of gravity is
determined to be the focus quantitative value for the 45µm over-focussed condition from the just-focussed position.
(Formula Removed)
From this expression, the focus quantitative value for the 45µm over-focussed condition from the just-focussed
condition will become +25 and this focus quantitative value +25 is displayed on the screen 11a as a meter indication. Secondly, an example of finding the focus
quantitative value when a measured diameter of the beam spot is in the under-focussed condition by 45µm from the just-focussed
position will be described. In the membership function of FIG. 10, for a position offset by 45µm in a direction of the under-focussed condition (in FIG. 10, a position denoted by a broken line of y), the adaptive value of the not well focus-adjusted condition is 0.2 and at the same time the adaptive value of the bad focus-adjusted condition is 0.8. Accordingly, as is illustrated in FIG. 12, the adaptive value 0.2 is stood at the position of a value -50 and the adaptive value 0.8 is stood at the position of a value -10 0. Subsequently, a processing for finding a center of gravity of these adaptive values stood is carried out. In other words, the center of gravity is found by calculating the following expression and the value of the center of gravity is determined to be the focus quantitative value for
45µm under-focussed condition from the just-focussed position.
(Formula Removed)
From this expression, the focus quantitative value for the 45µm under-focussed condition from the just-focussed
condition will become -90 and this focus quantitative value -90 is displayed on the screen 11a as a meter indication.
By displaying the focus quantitative value found in this way as a meter indication, it is possible to estimate a current focus-adjusted condition quantitatively. In other
words, when the focus quantitative value is zero, the just-focussed condition is obtained and it is found that as the value approaches +100, the focussed condition becomes gradually worse in the direction of the over-focussed condition and as the value approaches -100, the focussed condition becomes gradually worse in the direction of the under-focussed condition. Therefore, when estimating only by the beam spot diameter, there is a large difference in merits between the direction of the over-focussed condition and the direction of the under-focussed condition even in a ease where its difference has the same value, but when the focus quantitative value is found by applying the membership function as the present example, it will be possible to estimate the focussed condition quantitatively.
While, in the above embodiment, the processing which performs one measurement by means of the image displayed at a predetermined place in the cathode ray tube is explained, the image for measurement (in the above embodiment, the crosshatched image) may be displayed at a plurality of places on the screen for measuring the focussed condition at a plurality of places, thereby performing an adjustment in consideration of a balance of the entire screen to an appropriate focussed condition from an overall viewpoint. In this case, for example, it is conceivable to adjust so that the total focus quantitative value may fall to the minimum value on the whole screen.
Moreover, while, in the above embodiment, the adjustment is performed by rotating the focus adjustment volume 20 of the television receiver 13, if a television receiver to be
adjusted comprises a memory for storing the focus control value and the focus control is performed on the basis of the control data stored in this memory, the control data of the memory needs only to be rewritten based on the measured data, so that the configuration for adjustment can be made simpler.
Furthermore, while, in the above embodiment, the measurement of the focus of the cathode ray tube of the television receiver is performed, the present invention is of course applicable to the measurement of a focus of other image display devices having the cathode ray tube (e.g. a display for computer etc.).
Having described a preferred embodiment of the present invention with reference to the accompanying drawings, it is to be understood that the present invention is not limited to the above-mentioned embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the spirit or scope of the present invention as defined in the appended claims.

We Claim:-
1. A focus measuring method measuring a focus of a
picture displayed on a screen of a video display apparatus
having a cathode-ray tube by employing a measuring apparatus
having a rod lens array, an optical low-pass filter, and a line
sensor, comprising the steps of:
detecting a picture displayed on a screen of the video display apparatus having said cathode-ray tube by said line sensor through said rod lens array and said optical low-pass filter; and
obtaining a focus measurement data from a signal detected by said line sensor.
2. A focus measuring method according to claim 1, wherein a focus adjustment picture having a bright line with wide widths in vertical and horizontal directions is employed as the picture displayed on said screen of the video display apparatus having the cathode-ray tube.
3. A focus measuring method according to claim 1, wherein said rod lens array is formed by arranging cylindrical minute lenses in lines.
4. A focus measuring method according to claim 1, wherein the picture displayed on said screen of the video display apparatus having the cathode-ray tube is detected by
slightly displacing a focus of each of lens forming said rod lens array from said screen of the video display apparatus.
5. A focus measuring method according to claim 1, wherein said optical low-pass filter is a tape-like member bonded to a light receiving surface of said line sensor and having a light diffusion function.
6. A focus measuring method according to claim 1, wherein said optical low-pass filter is disposed in an optical path from said screen of the video display apparatus having the cathode-ray tube to said line sensor.
7. A focus measuring method of measuring a focus of a picture displayed on a screen of a video display apparatus having a cathode-ray tube, comprising the steps of:
detecting a focus adjustment picture displayed on said screen of the video display apparatus having the cathode-ray tube and having a bright line with wide widths in the vertical and horizontal directions by a line sensor through a rod lens array and an optical low-pass filter;
calculating a diameter of a beam spot based on vertical and horizontal line widths detected by said line sensor to determine whether a condition of said beam spot is an under-focussed condition or an over-focussed condition relative to a jut-focus condition;
calculating, when it is determined that the condition
is the under-focussed condition, a calculated value of the diameter of said beam spot with a function prepared for the under-focussed condition to obtain a focus quantitative value; and
calculating, when it is determined that the condition is the over-focussed condition, a calculated value of the diameter of said beam spot with a function prepared for the over-focussed condition to obtain a focus quantitative value.
8. A focus measuring method according to claim 1, wherein the video display apparatus having the cathode-ray tube and to be adjusted comprises a memory for storing a focus control value and when a focus control is carried out based on a control data stored in said memory, a focus adjustment is carried out with the control data stored in said memory being replaced with a measured data.
9. A focus measuring method according to claim 7, wherein said rod lens array is formed by arranging cylindrical minute lenses in lines.
10. A focus measuring method according to claim 7,
wherein a focus adjustment picture having a bright line with
wide widths in vertical and horizontal directions and displayed
on said screen of the video display apparatus having the
cathode-ray tube is detected by slightly displacing a focus of
each of lens forming said rod lens array from said screen of the
video display apparatus.
11. A focus measuring method according to claim 7, wherein said optical low-pass filter is a tape-like member bonded to a light receiving surface of said line sensor and having a light diffusion function.
12. A focus measuring method according to claim 7, wherein said optical low-pass filter is disposed in an optical path from said screen of the video display apparatus having the cathode-ray tube to said line sensor.
13. A focus measuring method according to claim 7, wherein a predetermined focus evaluation membership function is employed to thereby obtain a focus quantitative value from a calculated value of the diameter of said beam spot based on a result of determination whether the condition is the under-focussed condition or the over-focussed condition.
14. An apparatus for measuring the focus substantially as herein described with reference to and as illustrated in the accompanying drawings.