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In these methods, absorbance varies with analyte concentration and the light path through the sample. A typical instrument directs light through a cuvette to a detector, with a blank used to account for reagent and solvent absorbance. Colorimeters select wavelength regions with broad-band filters; spectrophotometers provide narrower, selectable wavelengths. Reliable measurements depend on suitable cuvettes, calibration against known standards, and interpreting results within the instrument’s linear range. Biochemical applications include glucose, creatinine, enzyme activity, hemoglobin derivatives, and nucleic acid purity assays. Sample turbidity or coloration, cuvette condition and positioning, and temperature can also influence measurements, so quality-control practices are important.