A colorimeter is a device used in colorimetry that measures the absorbance of particular wavelengths of light by a specific solution). It is commonly used to determine the concentration of a known solute in a given solution by the application of the Beer-Lambert law, which states that the concentration of a solute is proportional to the absorbance). The essential parts of a colorimeter include a light source (often an ordinary low-voltage filament lamp), an adjustable aperture, a set of colored filters, a cuvette to hold the working solution, a detector (usually a photoresistor) to measure the transmitted light, and a meter to display the output from the detector). Changeable optics filters are used in the colorimeter to select the wavelength which the solute absorbs the most, in order to maximize accuracy. The usual wavelength range is from 400 to 700 nm). There are two different types of colorimeters: color densitometers, which measure the density of primary colors, and color photometers, which measure color transmission and reflection. The output from a colorimeter may be displayed by an analogue or digital meter and may be shown as transmittance (a linear scale from 0 to 100%) or as absorbance (a logarithmic scale from zero to infinity) ). Colorimeters are used in various fields such as manufacturing of paints, textile and food industry, quantitative analysis of proteins, glucose, and other biochemical compounds, testing water quality, and determining the concentration of hemoglobin in the blood.