Some talk and inklings of a DIY spectrosensitometer from 5 years ago; did anyone ever get around to doing it?
So let's break down what the optical path might look like. It's hard to tell from the diagrams what exactly is happening in the 3rd dimension.
So we have a light source and a lens (simple meniscus?) to concentrate the light onto a slit. The slit will presumably create a thin "bar" of light that next must go to a diffraction grating. I think the "science class" type gratings that are mounted in 35mm slides would be ideal, as has been noted above.
Now, does the angle of the grating affect how the light is diffracted, and how much it fans out?
How important is the collimator in front of the grating? It looks like the purpose of that is to get the light to come at it perfectly parallel. But wouldn't a horizontal bar of light (from the slit) hitting the grating still make a reasonable projection of the spectrum?
Then the diffracted beam of light encounters the step-wedge, placed in front of our test film.
What I'm having trouble imagining is the interaction between the projected spectrum and the step wedge. Most step wedges that we know are arranged in a long thin strip, not wider than a centimeter or two. How on earth are we supposed to project the spectrum onto this and achieve a 21-step gradation at all spectral frequencies? We need each spectral region (400, 500, 600, etc.) to go through the whole range of steps. This seems obviously impossible with the thin step wedges that we're all used to.
Now, Kirk, you say you have a spherical mirror with a grating in it; how would that design work? It sounds like that would be fundamentally different from the above diagram posted by Emulsion.