Abstract: A compact and portable colorimeter, including an integrally-formed probe having both a solid-state source of relatively monochromatic light and an optically coupled photodetector mounted therein. A power oscillator gating means is connected to the light source for pulsing same, to provide a chopped optical input. A demodulating circuit receives the output from the photodetector. The power oscillator gate signal is provided to a synchronous detector at the demodulating circuit, to synchronize detection with the chopping. The output from the demodulator is converted to a signal and read-out indicative of the concentration of that component of the sample which has absorbance characteristics correlated with the wavelength of the monochromatic source.