In the broadest sense, a photometer is any instrument used to measure light intensity. As applied in industrial photometry,   "photometer" is the general term covering instruments for detecting

  • scattered light intensity
  • absorption
  • fluorescence

These detection principles are used for measuring the turbidity value in liquids and gases, absorption, and fluorescence intensity.

The objective is the same in all cases: to expose the medium in question to light and then to measure the intensity of the light produced by the respective phenomenon. The following sketch shows the way the single-beam method is used for absorption measurement.

Single-beam method for absorption measurement

The light source (L) transmits a light beam through the medium in the flow cell (M) and the photoreceiver (Ph) measures the intensity of the remaining light. After being amplified in the amplifier (V), an electrical signal is delivered as the absorption reading.

The purpose of the measurement is to detect how much the light intensity is weakened by the substance in the flow cell. Besides this sought variable, however, the reading will also be affected by the luminosity of the light source (L) and the sensitivity of the photoreceiver (Ph). Because the properties of these components vary as a result of aging and fluctuations of the supply voltage and the ambient temperature, single-beam photometers are inherently unstable and require frequent recalibration.

This may be feasible for intermittent measurements in the laboratory, but it is clearly unacceptable for industrial duty and continuous measurement. One way to eliminate the effect of light source fluctuations is to use the dual-beam method.

Dual-beam method with two photodetectors

With this configuration, a semitransparent mirror produces two beams: a measurement beam (M), which passes through the sample and strikes the first photoreceiver, and a reference beam (V) that goes straight to the second receiver.

Because the reading represents the ratio of the two, fluctuations of the light source's luminosity have no effect on the results. But the changing sensitivity of the two photodetectors remains as source of error. It makes these dual-beam photometers, too, unstable over the long term and necessitates frequent recalibration.

The optimal solution is to use two beams and just one photoreceiver. This is the alternating-light, dual-beam configuration used in Sigrist photometers.

Alternating-light, dual-beam method with one only photodetector

Here again the reading is arrived at by forming the ratio between the measurement and reference beams. A rotating disk, or "chopper", is inserted to let pass the two beams alternately to the same photoreceiver. As a result, both the luminosity fluctuations of the light source and any changes in photodetector sensitivity are eliminated as sources of error. Sigrist alternating-light, dual-beam photometers are stable over their entire service lives and require no recalibration.