In its widest sense, a photometer is an instrument for measuring light intensity or optical properties of solutions or surfaces. Photometers are used to measure:

  • Illuminance
  • Irradiance
  • Light absorption
  • Scattering of light by media
  • Reflection of light
  • Fluorescence
  • Phosphorescence
  • Luminescence


Before electronic light sensitive elements were developed, photometry was done by estimation by the eye. The relative luminous flux of a source was compared with a standard source. The photometer is placed such that the illumination from the source being investigated is equal to that of the standard source as equal illumination can be judged by the eye. The relative luminous fluxes can then be calculated as the illumination decreases proportionally to the inverse square of distance. A well known such photometer consists of a paper with an oil spot, that makes the paper slightly more transparent – when the spot is not visible from either side the illumination from the two sides is equal.

Maintenance Data[]

Listed are recommended maintenance cycles that can be loaded within local computerized maintenance management systems. These cycles can also be used to estimate average monthly cycle inspections, PMs, and calibrations with average technician hours. This device is corrective maintenance only. Disclaimer: It is recommended to consult OEM cycles.

Maintenance cycles (in months) with estimated technician hours (in minutes)
Months Minute(s) Risk Level
Safety Inspection (INSP) 0 0 No Significant
Preventive Maintenance (PM) 0 0 N/A
Calibration (CAL) 0 0 N/A
Schedule Parts Replacement (SPR) 0 0 N/A
Acceptance Inspection (AI) 0 120 N/A

Principle of photometers[]

Most photometers detect the light with photoresistors, photodiodes or photomultipliers. To analyze the light, the photometer may measure the light after it has passed through a filter or through a monochromator for determination at defined wavelengths or for analysis of the spectral distribution of the light.