A photometer is an instrument for absorption measurements that consist of a source, a filter, and a photoelectric detector. A colorimeter differs from a photometer in the respect that the human eye serves as the detector. The photometer offers the advantage of greater precision and the ability to discriminate between color. The main advantages of a colorimeter are simplicity, low cost, and the fact that no power supply is needed. Diode array spectrometers detect the entire spectral range essentially simultaneously and can produce a spectrum in less than a second.
Spectrophotometers require several minutes to perform the same task.
Accordingly the diode array instruments can be used to monitor processes that occur on very fast time scales. Conventional spectrophotometers require several minutes to perform the same task. Typically the halogen is iodine. As WI encounters the hot filament, the molecule decomposes and tungsten is redeposited on the filament. Light scattered or lost from the sample has the net effect of making the absorbance appear greater.