The calculated molecular absorption of the products [10], [11], [12] and [25] increased with methamphetamine concentrations of up to 2.5 mg mL−1 and above that, became constant (Fig. 4). Although these relationships were similar to the results from spectrophotometric methods, the sensitivity (slope of the calibration graph) of the digital image–RGB method for the red channel was lower (0.23 ± 0.01 a.u. mL mg−1 against 0.38 ± 0.01 a.u. mL mg−1). The linear range of this method was also shorter (0.1–2.5 mg mL−1 compared with 0.1–5.0 mg mL−1). The lowest absorption was obtained from the blue channel due to its higher reflectivity resulting in the appearance of this color as the test result.