Abstract: A modulated light source emits light to illuminate a target surface which possesses a certain color and shade of color. Light that is reflected from the target surface is detected by an optical detector. The output from the optical detector is processed and fed back to the optical detector to compensate for any shift caused by ambient light, temperature or other external factors, and is differentially amplified to generate an output signal indicative of the color and shade of the target surface. The output signal from the differential amplifier is then demodulated by a synchronous detector to produce a substantially steady DC voltage that is indicative of the color or shade of color at the target surface. Where the target surface color shade is indicative of a certain measurable quantity or quality (such as an analyte concentration), the steady DC voltage is converted using a look-up table or mathematical formula into a corresponding quantity or quality measurement.