Radiometry is the measurement of radiation in the electromagnetic spectrum. This includes ultraviolet (UV), visible and infrared (IR) light. For additional information on radiometry we have written a post as an Introduction to Radiometry and a Guide on Radiometry which is available as a free PDF download.

When characterizing a spectral radiometer system selecting a properly calibrated radiometric sensor head and the right readout device are important in obtaining accurate results.

The sensor head converts electromagnetic radiation into an electrical signal, while the readout device receives this signal and interprets it. A properly calibrated measurement system will measure the light source and display the measurement in the appropriate optical units.

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

The readout unit should be selected according to its features, and the detector head should be selected according to its power measurement range, wavelength calibration and size. The two matched together will accurately measure the source in the correct optical units.

Consider Your Source

Collimated light sources, such as lasers, are typically characterized by radiant flux measurements (Figure 6). A beam that overfills a sensor head may be characterized by its power density (irradiance) in watts/cm2. An integrating sphere can also be used in a radiant flux measurement in order to attenuate the laser power to be within the limits of the sensing device in units of radiant intensity, provided the spatial distribution is uniform. This type of measurement is easily achieved with an aperture and baffled tube to define the solid angle of detector acceptance.

Spectral-Radiometer-Configuration-Figure-6Measurements must always be made with a consistent solid angle. This constant solid angle may be defined by the detector’s area and its distance from the point source. A point source may also be defined by units of radiant flux, provided all the radiation is captured in an integrating sphere.

Uniform extended sources such as lamps may be characterized by radiant flux or irradiance measurements. Uniform extended sources such as flat-panel LCD displays, are best characterized by units of radiance.

Energy Measurements

Measurements of pulsed sources require special considerations. The standard unit of optical energy is the joule (watt-second). Integrating the signal over a known time period makes energy measurements. When making energy measurements of pulsed sources using silicon and germanium detectors, one must consider the effects of peak power on the detector. Saturation of the detector will cause the detector to behave non-linearly and will result in measurement error.

Wavelength and Optical Filters

Optical filters can be designed to allow certain wavelengths to pass through, while screening out others. A filter can be selected to modify a detector’s response in order to limit the bandpass to match some desired response curve or to attenuate the input signal by a known amount. Many filter and detector combinations must be calibrated to ensure measurement accuracy.

A detector/filter combination that achieves a spectrally flat response (Figure 7) is especially useful for measuring broadband sources or sources where the peak wavelength is uncertain or may vary. UDT Instruments’ newest flat filters are accurate between 450 and 950 nm to within ±5%.


Detector/filter combinations that allow a specific broadband transmission are well suited for measuring arc lamp distribution peaks and spectral content (Figure 8).

Narrow bandpass filters are usually utilized for laser power measurements. This type of detector/filter combination assures that only the monochromatic radiation from the laser reaches the detector’s active surface.

An important consideration to make before specifying a sensor head is how much power will be measured. In addition to having a well-defined wave-length range, they should also have a well-defined power handling capacity.

Silicon InGaAs and Germanium Materials

Spectral-Radiometer-Configuration-Figure-8Silicon, InGaAs and germanium are especially well suited to measure light. These materials are specially processed to convert incident radiation to an electrical signal by the photoelectric effect. The conversion ratio or detector responsivity is linear over the sensor’s input range. For a silicon sensor, this range spans 12 decades. For a germanium sensor it spans 9 decades. The sensor’s response is also uniform over the active surface, making it ideally suited for both power and power density measurements.

InGaAs sensors are frequently used in the telecommunications and fiber industries when high sensitivity, low dark current and high dynamic impedance are needed.

Once the applications and characteristics of the source and receiver have been fully defined, you can properly select a spectral radiometer system for your testing needs.

Spectral Radiometer Systems from Gamma Scientific


UDT Instruments, a Gamma Scientific company, offers a line of spectral 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 photodetectors. UDT Instruments offers high-accuracy photometer and radiometer systems based on our TIA-3000 transimpedance 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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