Mobile phones as photometers

I am using a colorimetric method for measuring nitrate concentrations in streams. The final part requires the user to compare the colour that develops in the sample vial to a colour chart, but our volunteers don’t like the subjective nature of this last step. I am wondering whether a mobile phone could be used as a photometer to measure the light absorbance in the sample vial (cuvette)at the relevant wavelength? I imagine a small piece of hardware being attached to the phone that splits a light beam from the phone so that part goes through the sample cuvette and part goes through a blank cuvette. Both beams are reflected back into the camera of the phone and an app on the phone compares the intensity of light from each beam (i.e. measures light absorbance in the beam passing through the sample) at the relevant wavelength. Does anyone know if this could be possible, and whether anything like this exists? Thanks