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This design would require a photodiode array or linear CCD. Is that the reason why it is not used here?
Could white light be shone through the sample, and then split into the individual wavelengths? Wouldn't the absorption be the same? This would make the testing much faster and eliminate the need for a motor. Or would it be too hard to calibrate a baseline for each wavelength?
A simple DIY spectrophotometer
Why would you not use an RGB LED instead of the stepper motor and diffraction grating? It would be solid state and more controllable.