According to DIN 38404, the coloration and organic loading of water can be determined by absorption measurement (SAK). Depending on the wavelength used, absorption detects visible color (e.g. yellow coloration at 390 nm in APHA-Hazen units) or invisible substances (in the UV range, 254 nm).
Because any turbidity substances present will also affect the absorption reading, Sigrist recommends the dichromatic method, which eliminates this error by measuring alternately at two different wavelengths. And because this method also makes it possible to display the turbidity value as second result, it opens the way to a cost-effective version of the photometer: the combined SAK/turbidity instrument.