Today I stumbled about another "problem".
I wanted to take a set of spectrum captures where the light source would change the distance to the slit of the spectrometer in order to get data on intensities at different distances (see: inverse square law) and how stable the RGB peaks' wavelengths were.
I took a LED torch light and started at 80 cm distance (about 31.5 inches). Then I reduced distance in 5 cm intervals (about 2 inches) and took spectra.
I noticed that sometimes even when I got closer the intensity would drop instead of rise. That's when I saw the light rings in front of the LED torch light (see image above). Its not so easy to always aim the same part of the light ring at the slit, so sometimes you get a brighter light and sometimes a lower light depending on the angle of the torch light rays.
I'm sure most of you guys use a strong light source with equal light distribution or even a laser (not a cheap LED torch light like mine) but I thought I'd mention the problem anyway. If you don't have equally distributed light you need to make sure that you always get the same amount of light from the light source (in this case here always the same part of the "ring of light") when doing distance/intensity experiments.