Question: Quantitative analysis using paper-based kits

Can anyone answer some questions for me about quantitative analysis using paper based kits? I'm interested in the idea of using cameras (or a spectrometer) to provide quantitative data by transmitting light though paper rather than solution.

For example, the following url is a link to a 2009 paper from Analytical Chemistry: "Quantifying Colorimetric Assays in Paper-Based Microfluidic Devices by Measuring the Transmission of Light through Paper" https://gmwgroup.harvard.edu/pubs/pdf/1065.pdf

I am part of a group of people interested in measuring agricultural contamination of waterways. The key chemical parameters that I am interested in are nitrate and phosphorus, with measurement accuracy needing to be ppm-ppb. The following quote from the paper suggests that paper-based kits may be something we could use. I realise that the application in the paper is quite different, but I'd appreciate a chemist having a look with our application in mind and receiving their views on the matter.

"This prototype transmittance colorimeter is inexpensive, rugged, and fully self-contained, and thus potentially attractive for use in resource-limited environments and developing countries" (Ellerbee et al. 2009). - Sounds like us!