Classification and quantitative optical analysis of liquid and solid samples using a mobile phone as illumination source and detector

Abstract

A mobile phone has been used both as illumination source and image detector for quantitative optical analysis of colored liquid samples (4 different colorants) and solid samples (printed color patterns, plastic beads and colored sand grains). Even though the measurement conditions were far from ideal, because the light source was strongly polychromatic and the illumination was not a collimated light beam with homogeneous light intensity, a logarithmic concentration dependence, in accordance with the Beer-Lambert law, described the data of the colored liquids quite well. By utilizing blue-blue (420-510 nm), green-green (480-590 nm) and red-red (575-695 nm) illumination/detection combinations, each sample could be assigned a unique color signature for classification that agreed with reference absorbance spectra measured with a spectrometer. Quantification of validation samples within a few percent of the actual values was achieved. Also the long-term repeatability of the measurements was investigated and was surprisingly good for such a simple system. Analysis of the colored solid samples was more complex with results being dependent on the morphology and colorimetric properties of the samples

Two measurement modes for mobile phone optical sensing

“…Illuminations were performed with the three primary colors red (R), green (G) and blue (B) and recorded in the red, green and blue channels of the camera, respectively. The spectral properties of the screen and the camera have been described previously [16]. The open source integrated design environment NetBeans was used to design and implement software in the form of a MIDLet (Mobile Information Device applet) which controlled the display as light source and simultaneously acquired images to obtain reflectance intensities from the samples.…”

“…This means that a data point from Area 1 (Area 2) is the mean value of 300 000 (80 000) pixel values. More information about the video recording and the image processing can be found in [16].…”

“…Although this is not an optimal transmission measurement, data from this area resemble those of standard transmission measurements through a liquid as long as scattering contributions are low and is very sensitive to absorption. Since the absorption by colored solutions is wavelength dependent, the data from Area 1 of colored liquid solutions can be used to both classify sample identities and quantify sample concentrations [16]. This area may also be influenced by scattering of light and fluorescence in the liquid.…”

“…In this example the RGB measurement thus enables both classification (direction in RGB space) and quantification (distance from the origin). More details about this measurement mode are available in [16].…”

“…In addition, light that is diffuse reflected by the measurement vessel, containing the liquid, contributes to the intensity in the whole image. In a recent publication we showed that this light, which is transmitted through the liquid, contains information about the wavelength dependence of the absorption by colored transparent liquids [16]. For a large absorption range the concentration dependence is well described by the Beer-Lambert law allowing quantification of concentrations.…”