I am trying to get light similar to sunlight especially in terms of the heating part of it. So, I am confused among bulbs such as halogen, incandescent, CFL and LED bulbs. Please suggest with appropriate reference, if possible.
Our Sun is pretty good imitation of a black body, with nice and smooth spectral content, most easily parametrized by its surface temperature around 6500 K. This fact alone suggests that the ordinary light bulbs, operating with hot tungsten, might be most appropriate. Yet the tungsten melting point is "only" 3422 C, thus the imitation is far from perfect. If you want to have mostly thermal radiation, then the high power halogen bulb should be preferred. Other sources you mention produce light with sharp peaks in visible and, sometimes, UV range later converted to visible range. They are designed to make impression of a white light, with negligible content of long wavelength radiation. On the other hand high pressure mercury lamps are often used as a source of IR radiation.
Yes, you would have to heat up a black body to 6500K and then filter what our atmosphere is filtering. Practicably close you can get by using one of the many FL and LED products. FL have mostly still some peaks in the spectrum but there are ready to use products (LifeLite, True-Light,...). There are also some LED products trying that but mostly using a cold white led and a amber or red one. They often lack in the turquoise part of the spectrum. If you want to get as close as possible you have to use 5 different color-LEDs and mix the light well. We did that in a research project, i can tell you more details if you need.
Measuring solar cells requires a stable light source that closely matches the conditions of sunlight. Not only the intensity but also the spectrum must be matched to a standard. An obvious option is to simply use the sun itself. In locations where there is little cloud this is a good solution but there are still variations in atmospheric conditions that require correction to compare measurements over time. The spectrum also changes through out the day and this further limits the time for testing.
The most common solution is to use an artificial light source that simulates the sun. The ideal illumination source would have following features;
- a spatial non uniformity of less than 1%.
- a variation in total irradiance with time of less than 1%,
- filtered for a given reference spectrum to have a spectral mismatch error of less than 1%.
These requirements are essential in obtaining an accuracy of better than 2%
Testers are classified according to three criteria:
Spectral match
1) Irradiance inhomogeneity 2) Spatial uniformity over the illumination area
3) Temporal Instability, stability over time.
The most common light source is a Xenon arc lamp with filters installed to approximate the AM1.5G spectrum (AM: Air Mass) . Simple testers often just use a halogen lamp with a dichroic filter. The lamp filament is much lower than the sun's 6000 K so it produces much more infrared light and much less UV. The reflector on the bulb is selective so that the visible and UV is reflected towards the cell but most of the infrared radiation isn't reflected and passes out the back of the bulb. Halogen lamps have the advantage of greater temporal stability compared to Xenon arc lamps.
It is difficult to make a light source that exactly matches the AM1.5 spectrum and with the necessary illumination intensity. There is often a considerable amount of variation between the spectrum of the lamp and the required AM 1.5 spectrum. There are two approaches for correcting for the differences between the AM1.5 spectrum and the actual spectrum from a solar simulator.
Calibration cell with the same spectral response.
The approach taken by most in-house testers is to use a calibration cell that has the same spectral response as the cell under test. The light intensity of the tester is adjusted so that the cell Isc matches the Isc as measured at an external testing laboratory. However, slight changes in cell processing (e.g. the doping profile of the emitter, variation of anti-reflection coatings) cause changes in spectral response and the need for a new calibration standard.
Measure Spectral Response
Primary calibration labs use light sources that are much closer to the standard however differences still occur. To compensate for the differences, calibration labs measure the spectral response of the device under test and then use that to correct for the known difference between the spectrum of the light source and the standard spectrum. Such corrections are time consuming and prone to error. In the early 90's an analysis of test error lead to an improved standard and also the downgrading of some record efficiencies by up to 1% absolute.
By the way, there is an online calculator to determine the level of spectral mismatch from a light source at "PV Lighthouse".
http://www.pvlighthouse.com.au/calculators/spectral%20mismatch%20calculator/spectral%20mismatch%20calculator.aspx
It includes the ability to correct for an arbitrary spectrum.
Good luck sunlovers,
Frank
There are very cheap xenon lamps available at the hardware store.
Would a bunch of those xenon lights, together, provide the right spectrum (over the visible and infrared) and intensity, maybe not the illumination uniformity?
Silly questions: if that was the case, why spend a lot of money on a solar simulator?
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