Abstract
Smartphones provide an ideal platform for colorimetric measurements due to their low cost, portability and image quality. As with any imaging-based colorimetry system, ambient light and device variations introduce error which must be dealt with. We propose a novel processing method consisting of a one-time calibration stage to account for inter-phone variations, and an innovative use of ambient light subtraction with image pairs to account for variation in ambient light. Data collection is kept very simple, making it particularly useful for use in the field, since nothing additional is required in the images. Ambient subtraction is first demonstrated for a range of colors and phones (Samsung S8 and LG Nexus 5X), and the Subtracted Signal to Noise Ratio (SSNR) is defined as a metric for assessing whether an image pair is appropriate at the time of image capture. The experimentally determined SSNR threshold below which to suggest retaking the images is 3.4. The classification accuracy for results using the proposed calibration pipeline is then compared to the simplest image metadata-based alternative and is found to be greatly superior. Finally, a custom colorcard is shown to improve the accuracy of device-independent results for known smaller ranges of colors over a standard colorcard, making this a possible application-specific modification to the overall processing pipeline.
Highlights
IntroductionQuantifying colorimetric urine tests for measurements of pH and glucose [1,2,3,4] or determining saliva alchohol concentration [5]
The desire for quantitative measurements of color exists across many fields
We suggest the Subtracted Signal to Noise Ratio (SSNR) as a suitable metric, given by the signal to noise ratio of the flash only pixel values obtained after subtraction
Summary
Quantifying colorimetric urine tests for measurements of pH and glucose [1,2,3,4] or determining saliva alchohol concentration [5]. As well as test strips, within medicine there are further applications which aim to quantify colorimetric biomarkers of the human body to detect conditions such as jaundice [6,7,8], anaemia [9] and the eye condition anterior blepharitis [10]. Applications continue beyond medicine, for example in testing water quality [11] or improving the rigour of marine monitoring [12]. Commercial devices for contact measurements of color exist which are becoming more affordable, for example the X-Rite CAPSURE [13], but for many of the above applications a contact measurement is not possible.
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