There are several DIY optical spectroscopes projects around, most too simple and fragile (even made of cardboard), just good enough for educational purposes. Others I can find are quite sophisticated, requiring custom optics or specialized sensors.
I want to make a spectroscope that just works, cheap and good enough to tell if a particular LED light source will be effective to grow plants underneath. Think of a reliable version of the simple cardboard one.
There are also professional tools to measure PAR energy, but they are in the several hundred of euros range and some depend on specific, known light sources.
any with a camera port will do
Raspberry PI camera without the IR filter
The easiest way to use a known, characterized CMOS sensor that is available and cheap
DVD-R to cut a difraction network
It has to be a DVD-R, not a CD, not a BlueRay, not other types of DVD