Using the mobile phone as Munsell soil-colour sensor: An experiment under controlled illumination conditions

Soil colour has been determined in most cases by using Munsell soil-colour charts, sometimes with spectrometers, and occasionally with digital cameras. The objective here is to assess whether a mobile phone, which has all the requirements to capture and process digital images, might also be able to provide an objective evaluation of soil colour under controlled illumination. For this, we have developed an Android application that takes a picture of a soil sample, allowing the user to select the region of interest and then, after a RGB image-processing and a polynomial process transform between colour spaces, the Munsell (HVC) and CIE (XYZ) coordinates appear on the screen of mobile phone. In this way, a commercial HTC smartphone estimated the colour of 60 crumbled soil samples between 2.9YR and 2.3Y with a mean error of 3.75 ± 1.81 CIELAB units, taking as a reference the colour measurements performed with a spectroradiometer. The Munsell hue had the worst estimates (mean error of 2.72 ± 1.61 Munsell units) because of its geometric mismatch with the RGB colour space and for being defined to illuminant C, different of the D65 source under which the phone camera took the pictures. Because the measuring errors were lower than those described in the literature for the visual determination of soil colour, and the application also worked successfully in a different smartphone than the one used in its development, we think that current experimental results encourage the expectations of using smartphones in the field as soil-colour sensors.