Introduction to Photometry

Example of visible light and photometryPhotometry is the science of light measurement. Light is measured in terms of its perceived brightness to human eye. Photometry is different than other measurements of light within the field of optics like radiometry, which is the science of measurement of electromagnetic radiation including visible light. Radiant power is weighted by a luminosity function in photometry. Also, luminous function models account for human sensitivity to brightness.

Photometry 101

Ultra violet (UV), visible and infra-red spectrum are the three ways electromagnetic waves are emitted by light sources. Radiometry measures these ways and photometry is a branch of radiometry that measures only visible light from the individual’s needs and pespective. The effect of total heating of infrared radiation, measured by thermometers, leads to the development of radiometric units in terms of total energy and power. The human eye is the detector, which then leads to photometric units and weighted by the eye’s response characteristic.

Photometric Units

Many people think that there are too many different units of measure that are being used in photometry. Conversions are often needed between units that can’t be converted. This is the case for lumens and candelas for example. Adjectives are also very common for expressing the measurement of light. For example the adjective “heavy” may refer to density or even weight that are, as we know, altogether different. Also, the adjective “bright” may refer to high luminous flux that is measured in lumens, to a light source that concentrates its luminous flux into a narrow beam, or to a light source that is seen against a dark background.

That said, this table of photometry units from Wikipedia, is a useful overview of the most important photometry measurements:

Photometry units

You can also view the photometric data for Lumax’s full line of commercial and industrial lights here.

Light can propagate through three-dimensional space spreading out, concentrating into small pockets, and reflecting off different kinds of surfaces such as shiny or matte. Because of these forms and the fact that the light consists of different wavelengths, various kinds of light measurement are available. Therefore, there are many numbers of quantities and units that represent these measurements.

Let’s take an office for example. These spaces are “brightly” illuminated in most cases by many recessed CFLs in order to achieve a combined high luminous flux. A laser point, on the other hand, has a very low luminous flux, but it is very bright in one direction. In other words, the luminous intensity of a laser point is very high in that one direction.


While there are many ways of describing light levels in photometry, these four are some of the most fundamental:

• Luminous Intensity (measured in candelas)
• Luminous Flux (measured in lumens)
• Illuminance (measured in lux)
• Luminance (candelas /m2)

Additionally, the term efficacy is used when describing lamps and how efficient they convert electricity to the light we can see. We measure efficacy of lighting in lumens per watt. But, when referring to energy efficiency within the lighting systems, the term system efficacy is used. This term includes the losses of any control system that is incorporated in the lamp.

Photometry is a complex science. Lumax Lighting’s resellers are well-versed in the principles of lighting design including photometry to provide you with the most appropriate lights for your commercial or industrial lighting application. Check out our Sales Rep Locator to find one near you.