A colorimeter requires a light source, a filter, a sample with a fixed path length, and a detector.  In Orion colorimeters, the light source uses Light Emitting Diodes (LED's).  The advantage of using LED's is longer battery life and its temperature stability which does not shift wavelength with changes in temperature.  The filter removes all light except the wavelengths used for the analysis.  The detector determines how much light was transmitted through the sample for correlation to concentration.  Learn more about colorimetry theory with the file below.