Photometers / Spectrophotometers

A photometer is used for the measurement of photometric values, such as luminosity. Common applications of a photometer are the determination of stellar brightnesses in astronomy and the measurement of light that is absorbed by liquids or gases, indicating the concentration of certain substances. In analytics, a photometer is primarily used for determining concentrations. The measuring principle of most photometers is based on the fact that the brightness in the measuring point is inverse proportional to the square of its distance from the light source.

Functionality of a photometer

The sample to be analysed is in a cuvette through which light emitted by a light source is passing. This generates a signal in the photosensitive receiver which is proportional to the intensity of the light entering the cuvette. The signal is amplified in the photometer by an amplifier and displayed in a specific display unit. The attenuation of light by the sample in the cuvette is dependent on the concentration of the examined substances in the sample. The substances dissolved in the liquid absorb light of a certain colour. In most photometers, the sample is illuminated by monochromatic light, generated with the help of a prism, a spectral filter or a very special light source. Often a sample is dyed with reagents to determine the concentration. The intensity of the colouring is therefore a measure of the concentration, acquired with the help of a photometer.

Spectrophotometer

A spectrophotometer determines the remission values. This is accomplished by illuminating the measured surface across the whole spectrum of visible light. This measurement serves to determine the exact colours and therefore to calculate the colour deviations from one measured value to the next. The spectrum of visible light is separated by a spectrophotometer into narrow bands and measuring channels. For each channel, a digitised luminosity value is gained. A spectrophotometer measures the reflectance of the sample after performing both white and black balance. From these values a remission characteristic can be constructed for the measured colour. A kind of unique colour 'fingerprint' results from that. Polychromatic light is diffracted with the help of a filter or a prism, to produce the required mono-chromatic light. The luminance reflected by the reference and the sample is determined by a photometer. From these values, the ratio is formed. If the sample light path as well as the reference light path are measured, the photometer is called a double beam photometer.

Literature

• http://de.wikipedia.org/w/index.php?title=Photometer&oldid=87037076 (called: 30.03.11).