Radiometry is the measurement of radiation in the electromagnetic spectrum. This includes ultraviolet (UV), visible and infrared (IR) light.

Electromagnetic radiation is characterized by its frequency of oscillation. The frequency determines the “color” of the radiation (see Figure 1). The speed of light is a constant, and frequency is related to wavelength by the relationship:

  • C = λγ
  • C = speed of light
  • γ = wavelength
  • λ = frequency

Radiometric-Wavelength-Chart-UDT-InstrumentsThe preferred units of measure for wavelength are nanometers (nm) and micrometers (μm or “microns”).

The visible region of the electromagnetic spectrum can divide into the basic colors of the rainbow: red, orange, yellow, green, blue, indigo and violet. Red light has the longest wavelength in the visible region (780 nm). Violet has the shortest (380 nm).

This Excerpt is Taken from the The Ultimate Guide to Radiometry (Free Download)

Ultraviolet light is shorter in wavelength than visible light. It extends approximately from 10 nm to 400 nm. And like other colors of the visible region, UV can be subdivided into 3 smaller regions: UVA, VUV and UVC. The UVA region ranges from 400 nm down to 320 nm and is the least harmful of UV radiation. The vacuum-ultraviolet (VUV) and UVC regions are shorter and important to the study of cancer.

Infrared light extends from 700 nm to 100 microns. Its regions are known as near-IR, mid-IR and far-IR.

Measurements of optical radiation require specific methods to obtain accurate measurements. UDT Instruments, a division of Gamma Scientific, supplies calibrated detector/filter combinations that cover from 200-1800 nm (0.2 to 1.8μm). To obtain accurate measurements, one must understand the light source (i.i.e. laser, lamp, LED); the optical medium (i.i.e. air, water, optics); and the particular response characteristics of the detector.

Important Radiometry Terms

In order to accurately describe an optical source, one must use the correct units and know how these units apply to detector-based radiometry. In practical light measurement applications, the receiver of optical radiation is a detection device that converts optical radiation to electrical current according to a known relationship.

The Radiometric Quantities and Units chart is a short breakdown of radiometric terms and their corresponding units of measure.


Radiant Energy

Radiant energy refers to the amount of power reaching a given point accumulated over time. This is referred to as joules (watt-second).

Radiant Flux

Radiant flux is the fundamental unit in detector-based radiometry. It is defined as the total optical power of a light source, and is expressed in watts.

To measure radiant flux, the detector must collect all emitted light. Examples of typical flux measurements are shown in Figure 2. Focused lasers and fiber optic cables require only the proper sensor head because the source and detector can be configured so that all radiation is incident within the active area of the sensor. Diverging light sources, such as LEDs and lamps, may require an integrating sphere to capture light radiating in several directions.


Irradiance is the amount of radiant flux incident on a known surface area. Its international unit of measure is watt/m2. However, because many sensor heads have a 1-cm2 detector area, it is simpler to use watt/cm2.

There are two ways to control the size of the detector area. The first is to use a sensor head with a known detector area. The second is to place an aperture with a known area between the source and the detector. When source radiation does not completely fill the detector, an aperture is the only reliable method of controlling detector area.

Radiant Exitance


Radiant Exitance, a property of the light source, is the total radiant flux from the source divided by the surface area of the source. Its unit of measure is watt/m2, simplified as watt/cm2. This type of measurement only applies to extended light sources and is useful for making efficiency measurements of different light source materials.

To make radiant existence measurements, one must know the surface area of the source and then measure the total radiant flux leaving the source.

Radiant Intensity

Radiant Intensity is the amount of flux emitted through a known solid angle. It is measured in watts/steradian.

To measure radiant intensity, start with the angle subtended by the detector at a given distance from the source (see Figure 4). Then divide the amount of flux by that solid angle.

Radiant Intensity is a property of the light source and may not be relevant if the spatial distribution of radiation from the source is non-uniform. It is appropriate for point sources (and for close approximations, such as an LED intensity measurements), but not for collimated sources.


Radiance is the radiant intensity emitted from a known unit area of a source. Units of radiance are used to describe extended light sources, such as a CRT or an EL/O Panel unit for characterizing point sources.

To measure radiance, you need to define the area of the source to be measured, and also the solid angle received (see Figure 5). This is usually simulated using an aperture and a positive lens in front of the detector. It is expressed as watts/cm2·ster.

Radiometer Systems from Gamma Scientific

UDT Instruments, a Gamma Scientific company, offers a line of radiometer and photometer components to meet the industrial need for simple, practical, and reliable optical measurements. This line includes a range of optometers, photosensors and photosensor accessories. UDT Instruments offers high-accuracy photometer and radiometer systems based on our TIA-3000 transampedance amplifier technology. These systems deliver state of the art performance to the most exacting clients in research and advanced metrology.

Photometry Resources from Gamma Scientific


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