How are solar spectra simulators used and are they used to test photo-diodes and image sensors?
A solar simulator is use to approximate the spectrum of the sun but do so with some control. The biggest reason is if you wanted to simulate cells in all sun conditions, you would have to go to the equator, where the sun is brightest at ~1000W/m^2. A solar simulator allows you to vary the power and take the measurements you need (and you don't have to wait the course of a day either for the sun to dim you can control the lights, or worry about atmospheric conditions.)

When testing satellite solar panels, I used a solar simulator to simulate the power of the sun in space. This ensured that the panels were wired correctly and gave the appropriate output power before sending it up into space. You only need to approximate the spectrum of the sun and get the overall shape right.

I would think more accurate sources are used for photodiode/imagesensors like tunable or uniform light sources.