FIELD OF INVENTION
The invention relates to a light source used to measure cell efficiency of a solar cell.
BACKGROUND OF THE INVENTION AND PRIOR ARTS
For measurement of the electrical performance of solar cell under light and dark conditions, there is a tool called IV (Current –Voltage) tester. Voltage is applied to the solar cell and corresponding current is measured which is plotted in the computer by software tools. It consists of many integral components like probe assembly, mounting platform, vacuum jig, illumination source, temperature controller, cooling fan etc (BOT1).
Sun itself can’t be used as a standard light source because there are variations in atmospheric conditions, availability of sun light, climatic changes etc. and therefore, there is a need to deploy an artificial light source. The properties of an artificial light source that simulates the sun should have following features
1. Spectrum matching to that of the sun
2. Spatial uniformity over the illumination area.
3. Stability over time

As measuring solar cells requires a stable light source that closely matches the properties of sunlight namely its intensity and its spectrum, a light source has been designed using halogen lamps and Switched Mode Power Supply (SMPS) which is described herein. It is a low cost, easy to fabricate technique for light source which can be developed in the lab itself. It can replace the expensive light sources available globally.
US20130328587A1 (2013-12-12) titled “LED based solar simulator” comprises of an array of quarter panels below a test plane. Each quarter panel includes multiple close pitch LEDs of different wavelengths in an array, In order to match with solar spectrum more closely, a number of LEDs for different wavelengths are taken per quarter panel. It is different from present innovation because we are using 9 halogen lamps, all equally matching with solar spectrum.
US20130194564A1 2013-08-01 titled “Method and apparatus for measuring photovoltaic Cells” deals with a solar simulator which is developed with a test chamber for receiving a photovoltaic device for testing, In this case the light source used for illuminating the photovoltaic device produces a test signal which is used by a spectrophotometer for providing a measurement of the spectral distribution of the output of the light source. A database containing spectral response information of monitor cell, reference device and DUT (Device Under Test), and a computation device for receiving said test signal and said

measurement, wherein the computation device converts said test signal into a test value based on said measurement.
US8138782B2 2012-03-20 titled “Photovoltaic cell solar simulator” deals with a solar simulator to measure module of a solar cell production line. In one embodiment the solar simulator receives a solar cell module in a horizontal position and reorients the module into a vertical position. A light source is oriented to emit a flash of light in a substantially horizontal orientation toward the vertically oriented solar cell module. There is no details available regarding light source in this case.
US5623149A 1997-04-22 titled “High fidelity dual source solar simulator” deals with a high fidelity dual source solar simulator that provides a higher fidelity spectral match to the Air Mass Zero (AM0) solar spectrum and is derived from ultraviolet (UV) and visual light from a xenon arc lamp which is band pass filtered by a hot reflective mirror and slope filtered by a color glass filter, with the infrared (IR) light from a tungsten lamp which is band pass filtered by a cold reflective mirror and slope filtered by a thin film filter, so that, when the xenon and tungsten light components are combined as spectral segments, the spectra of the simulator matches both the curve shape and integral energy of the AM0 spectra while attenuating optical spikes so as to provide high fidelity over the

entire AM0 spectrum. The patents in general, deal with method and apparatus for measuring solar cell. No details of light source has been given in any of them.
SUMMARY OF THE INVENTION
One of the most important area in solar cell characterization techniques is the measurement of cell efficiency. To measure the cell efficiency and to compare it across the globe accurately, some standards have been defined for parameters like temperature, illumination, spectrum etc. Solar cell is kept on a temperature controlled vacuum chuck which is illuminated uniformly by the light source whose spectrum matches with that of the sun. The illumination of the light source should be 100 mW/cm2. The invention teaches a low cost and simple light source has been fabricated to serve the purpose using readily available halogen lamps and SMPS.
OBJECTS OF THE INVENTION
An object of the invention is to develop a light source for solar cell IV tester using halogen lamps and Switched Mode Power Supply (SMPS).
Another object of the invention is to provide a light source whose spectrum matches with the sun.

Yet another object of the invention is to provide a simple design to measure cell efficiency.
Yet another object of the invention is to provide a light source that maintains spatial uniformity over the illumination area.
A still further another object of the invention is to provide a light source that maintains stability over time.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 – Shows basic arrangement of an IV tester in accordance to the
invention.
Fig.2 – Shows design of light source using halogen lamps and SMPS in
accordance to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
In solar cell characterization techniques is the measurement of cell efficiency is very important. Standardized testing allows the comparison of devices manufactured at different companies and laboratories with different technologies. The standards for cell testing are:

1. Air mass 1.5 spectrum (AM1.5) for terrestrial cells and Air Mass 0 (AM0) for for space cells.
2. Intensity of 100 mW/cm2 (1 kW/m2, one-sun of illumination).
3. Cell temperature of 25 °C.
4. Four point probe to remove the effect of probe/cell contact resistance.
Pictorial depiction of IV tester along with its components is shown in Fig 1.
The invention deals with the fabrication of a light source, adjusted to such a height that it gives one sun illumination on a solar cell (11) placed on the vacuum chuck. Nine halogen lamps (1) are taken and arranged in 3 X 3 array to illuminate 8” X 8” area uniformly where solar cell (11) of 6” X 6” is kept for measuring electrical properties.
A halogen lamp produces a continuous spectrum of light, from ultraviolet to infrared. Since the lamp filament can operate at a higher temperature than a non-halogen lamp, the spectrum is shifted towards blue, producing light with a higher effective colour temperature and higher power efficiency. This makes halogen lamps an option as a consumer light source with Black-body radiation spectrum similar to that of the sun and most suitable for the eye. All the nine halogen lamps (1) are powered from mains through nine SMPS (one for each lamp). Any change in voltage changes the spectrum of light source. Therefore, it is very necessary to keep the constant power flow to the lamps

fixed. As SMPS is an electronic power supply that uses switching regulator to
convert electrical power efficiently with little or no power loss, it regulates the
voltage and absorbs all fluctuations so that lamps are illuminated at a fixed
intensity.
The halogen lamps work at high temperature and heat the surroundings as well.
To dissipate the heat, cooling fans (2) are installed on the roof of casing of light
assembly.
Fig 2 depicts the light source i.e. the nine halogen lamps (21) which are continuously powered through SMPS. The bulb assembly is kept above the solar cell at a fixed height. The solar cell mounts on a temperature controlled chuck (25). The halogen lamps are cooled by means of a cooling fan (2). The use of SMPS ensures fixed spectrum and elongates lamp life. The whole set up of light assembly is easy to fabricate and is very cost effective. The design is so simple that it can be fabricated in the lab itself. The light sources available use xenon arc source which are very expensive as their lamps require special and complex power supply.

WE CLAIM
1. An apparatus to measure cell efficiency of a solar cell, the apparatus
comprising:
- a temperature controlled chuck (25);
- at least a solar cell (22) mounted on the said temperature controlled chuck (25);
- a light source (21) for emitting light, capable of emitting light Matching to solar spectrum;
wherein the light is kept incident on the solar cell;
- a cooling fan (23) disposed above the light source (21);
wherein a Switched Mode Power Supply (SMPS) supplies voltage to the light source to sustain the luminosity of the light source (21).
2. An apparatus as claimed in claim 1, wherein the light source comprises of nine halogen lamps.
3. An apparatus as claimed in claim 1, wherein the SMPS is an electronic power supply to provide uninterruptible and fluctuation proof voltage.
4. An apparatus as claimed in claim 1, wherein the cooling fans are installed on the top of the light assembly.

5. An apparatus as claimed in claim 1, wherein the intensity of the light source is similar to the intensity of the sun.
6. An apparatus as claimed in claim 1, wherein the height of the lighting assembly is adjustable.
7. An apparatus as claimed in claim 1, wherein as illustrated in the accompanying drawings.