The ILT 770 is a radiometer. Other types of light meter are photometers, colorimeters, spectrometers and spectroradiometers, which all measure the amount of light but they operate in fundamentally different ways and have advantages and disadvantages.

Radiometers, Photometers & Colorimeters

Radiometers and photometers consist of a photodetector, an electrical readout and a calibration. The spectral response of a photodiode varies with the wavelength of the incident radiation. This means that 1 W/m² of irradiance at 350nm in the UV will produce a different electrical current compared to 1 W/m² of red light at 650nm.

The photodetector would typically be fitted with an optical filter which modifies the inherent spectral response of the sensor. For a radiometer to measure the amount of monochromatic light, you only need to know the calibrated spectral response of the photodiode at the relevant wavelengths. If the light source is anything other than monochromatic (which is the case for all light sources except lasers), a radiometer with bare photodiode cannot yield absolute radiometric measurements. A “radiometric” filter can be fitted to the photodiode that normalises the response over a limited wavelength range, typically ±10% from 450-950nm, which provides a partial solution.

A radiometer can also be equipped with a filter that limits or adapts the spectral response and these are typically used in safety or medical applications where certain wavelengths are more dangerous or more efficacious than others. Examples of this are measuring the safety of UVC light sources, measuring the erythemal effectiveness of a UVB source and measuring blue phototherapy lights used for the treatment of bilirubin (jaundice in new-borns).

A photometer is similar to a radiometer, but the photodiode is equipped with a special filter that modifies the spectral sensitivity of the detector/filter combination so as to match as closely as possible that of the human vision system, defined at the CIE spectral luminous efficiency for photopic vision (“photopic” response). The photopic response of the human eye strongly favours green coloured light, with blue and red light being perceived as less intense. Thus, a photopic sensor ranks the brightness of light sources in close agreement to how the human vision system would perceive them.

A colorimeter is a type of photometer that combines three of four photodiodes each with a filter that closely matches the tristimulus response of the eye (the XYZ or X_RX_BYZ tristimulus colour matching functions). A colorimeter measures the emitted colour of the light source under test, and reports this as tristimulus values, as CIE chromaticity coordinates and as correlated colour temperature (CCT) values or as a dominant wavelength. The Y filter of a colorimeter is the same as the photopic filter in a photometer, hence a “tristimulus” colorimeter can also function as a photometer.

Spectrometers & Spectroradiometers

Whereas radiometers, photometers and colorimeters employ photodiodes with special filters, a spectroradiometer measures what is called the spectral power of the light source, which is the amount of light at each wavelength. The spectral response of the spectroradiometer is calibrated at each wavelength, which therefore avoids the error that arises from measuring the irradiance of broadband light sources with unfiltered radiometers (or even radiometers equipped with normalising filters).

From the measured spectral power, the desired radiometric, photometric or colorimetric metrics can be calculated. A spectroradiometer avoids the potentially significant errors that filtered photometers and colorimeters suffer from where the spectral response of the filtered sensor doesn’t exactly match that of the target observer, for example the photopic response of the eye. This is an important consideration; a high-quality photometer may have a average spectral mismatch (the f₁’ factor) to the photopic observer of 5% over the 380-780nm visible light band, but can still yield errors of 50% or more when used to measure blue or red LEDs, for example.

A spectroradiometer is the name given to a spectrometer that is equipped with appropriate collection optics and an absolute calibration. A spectrometer is an optical instrument that employs a diffraction grating that physically separates the incident light into its component wavelengths. Each discrete wavelength is imaged onto an array photodetector, which allows for the recording of a spectral power distribution instantaneously.

Datasheet: ILT 770 UV Radiometer Datasheet [Pending]

Manual: ILT 770 UV Radiometer Manual [Pending]