I've made a really primitive spectrophotometer using an RGB LED and a transimpedance amplifier, and I'd like to fine tune it a bit. Most importantly, I was wondering if it was possible to map RGB values to a specific wavelength so that I could graph output voltage values against wavelength values. I found this online tool: Link here, but I'm not sure if it's accurate for an LED. Also, I found that most colors appear uniform with the LED, but mixing equal red and blue is always distinctly red and blue; i.e., it doesn't appear purple. Any suggestions/input?
Figure 1. Apparent colour mixing when adjusting intensities of RGB LEDs.
We tend to think that by mixing RGB intensities that we can generate any colour or wavelength. This is, in fact, not true. What happens is that due to the way the eye is designed that we have the perception of the new wavelength.
The human eye can distinguish three broad wavelength bands - short, medium and long - with the three different cone types. These sensors correspond roughly to red green and blue.
The perception of colour is due to the relative signal strength detected by each cone.
Most importantly, I was wondering if it was possible to map RGB values to a specific wavelength so that I could graph output voltage values against wavelength values.
No. The wavelengths emitted by your RGB LED won't change - only their relative intensities. (They might shift slightly due to temperature at higher brightnesses but not as much as your question suggests.)
An RGB LED produces only three frequencies, (in different mixtures) so with an RGB LED as source you can measure at only those three (rather broad) frequency peaks.
So you have a spectrophotometer that produces only three data points per assay, I think "rather primitive" is an understatement,