1. A colorimetric processing method for paper based sensors, comprising: using a camera and taking a picture of a colorimetric paper sensor for colorimetric reactions; identifying the type of paper sensor by image processing; identifying one color channel in which both a dye and reference color have near zero absorption and wherein the color channels in which both the dye and the reference color have absorption; identifying the known absorption ratio between the dye and the reference color in each color channel; separating each test area in the channel from the non-test areas; normalizing each absorption channel by the near zero absorption channel to remove spatial variation; calibrating the test area reading with the reference color and a substrate white in the composite image; and, reporting the concentration of a test substance analyte based on the calibrated reading in each test area.