Photometry (optics)

With photometry or photometry ( to ancient Greek φῶς 'light' and μετρεῖν measure ') measurement methods are referred to in the wavelength range of ultraviolet and visible light by means of a photometer.


The photometry is originally a branch of physics or chemistry, astronomy and photography, but now a regular engineering. It is for example in photovoltaics or in the production of displays for industrial measurement technology for quality assurance and quality control constantly evolving. For the development of optical technologies, such as laser technology, it is also part of how the related colorimetry for hand tools.

In addition, the photometry is used especially in the ( bio) chemical and medical analysis. It permits the qualitative and quantitative detection as well as the tracking of the dynamic processes of chemical radiation absorbing chemical compound.

A measurement of absorbance on different wavelengths is referred to as spectroscopy, such as UV / VIS spectroscopy, or infrared spectroscopy. A recorded measurement at different wavelengths is referred to as spectrum. UV fluorescence in a sample can be irradiated light in unfiltered lead to measurement errors in the visible range of radiation, so a filter, prism or diffraction grating can be used to limit the wavelength range of the incident light. Likewise it is to know important radiation functions and spectral dependencies of materials. For this reason, the spectral measurements are carried out. The generalization of the photometry of the entire electromagnetic spectrum ( radio to gamma radiation ) is called radiometry.

Transmission measurements

Absorption and color of a liquid or transparent solid body depend on the material composition and concentration. With the photometry, the concentrations are determined by colored solutions using the visible light. The measurement is in a special sample vessel, the so called cuvette, made.

Irradiation of the solution of an absorbing substance with light passing through the intensity depends on (requires a possible linearly operating detector ) of the generally wavelength-dependent absorption properties of the substance, the concentration and the length of the light path in the solution. This law is described by the Lambert -Beer law. To apply this law, which measured in a narrow range of wavelengths for various known and unknown concentrations intensity signal I is logarithmically plotted against the concentration c. The result is a straight line, where the unknown concentration can be read.

A photometer does this interpolation calculation: The intensities are determined by the intercept I ( c = 0) divided (→ transmittance ) and logarithmic (→ extinction). The absorbance is proportional to the concentration.

Are more absorbing species in the solution, such a wavelength range is selected, which is absorbed by the species to be determined, but not by other constituents. Some substances that show little or no absorption can be converted by chemical means in well absorbing substances. For example, can be combined with formaldoxime the concentration of numerous metal ions photometrically determined. Light having the selected wavelength generated with filters, monochromators, or lasers.

Reflectance measurements

Photometric studies relate here primarily the color evaluation of surfaces for quality control in coloring. It calibrated, measured by means of filters at multiple wavelengths photosensors are used.

From possibly wavelength-dependent diffuse reflection can also be closed to the surface structure (eg DRIFTS ).

Review of light sources

The photometric measurement of light sources by means of light- related variables such as light intensity, luminous flux, illuminance and luminance. The sensitivity of the human eye is taken into account by means of light sensitivity curves. Through the V- lambda curves for photopic and scotopic vision photometric units can be calculated from radiometric units. Based on the light intensity as the basic unit of photometry whose definition provides, however no reference to the spectral luminous efficiency function.

Properties such as color rendering index, color temperature and light color also serve the photometric measurement of light sources. In addition, mainly producing radiation and efficiencies of lights, bulbs and LEDs a useful evaluation parameter dar.


In astronomy, there are other photometric systems that do not lean against the sensitivity curve of the eye, but to physical properties of the stellar spectra.

Astronomy used for historical reasons, as the unit magnitude.

Photometric sizes

The following photometric quantities are derived from the corresponding radiometric quantities. The difference is that in photometry, the sensitivity of the observer is included by the radiometric quantities are multiplied by the spectral luminous efficiency curve. A convention, Attached to the sizes of the index v ( not in italics ) for " visually " stands for the respect to visible light, so typically the region of the spectrum from 380 to 780 nm

Other sizes and values:

  • Exposure Hv (Lux times second, lx · s ), the product of illuminance and time
  • Luminous efficacy ( lumens per watt, lm / W)
  • Light sensitivity (dimensionless)
  • Correlated color temperature (Kelvin ) = Actual measurement of the temperature


The Visual photometry is forerunner of today's photometry.