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National Measurement Institute

Photometry and Radiometry Capabilities

Optical radiometry involves measurements of electromagnetic radiation in the ultraviolet, visible and infrared spectral regions by means of detectors of radiation that produce a measurable physical quantity directly related to the amount of radiation absorbed by the detector. Standards are maintained for spectral measurements and broad-band measurements of source and detector quantities. Calibration of reference material properties such as reflectance and transmittance, and of optical properties such as optical rotation, may be undertaken on special request. Colorimetric quantities may also be calculated on request. Where the measurement is one of spectral data at a number of wavelengths, the data may be made available in machine-readable form on magnetic disk.

Radiometric and Photometric Standards in Australia, JL Gardner, CIE 22nd Session – Division 2, pp 5–8, 1991

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Radiometric Quantities 


NMI has established a scale of absolute radiometric power and hence detector responsivity by both electrical substitution radiometry and silicon radiometry. In electrical substitution radiometry, the temperature rise of the detector due to the incident radiation is equated to electrical heating, thereby relating radiometric power to the known standards of electric voltage and resistance. In silicon radiometry, silicon photodiodes of well known characteristics are used. The radiometric power incident on the photodiode is calculated from the photocurrent and the wavelength of the incident radiation, relating the radiometric power to the standards of voltage and resistance, and to the known physical properties of gases.

Sensitive spectrally non-selective bolometers are used to extend the responsivity scale over the entire wavelength range covered. For the total radiation from a lamp filament operating at a known distribution temperature, or for some other suitable radiation, measurements are made of the responsivities of spectrally non-selective thermal detectors. The radiation is usually incident along the normal to the receiving surface. For other directions of irradiation, correction factors may be obtained. Spectral measurements of responsivity, for both thermal and non-thermal detectors, may be made using either an incoherent source of variable wavelength or a coherent source at specific laser wavelengths. For detectors of sufficiently high spatial uniformity, spectral responsivity measurements may be made in absolute units. Absolute calibration of detectors with non-uniform spatial responsivity will require an additional measurement in a field of uniform irradiance. Responsivities may also be measured to radiation from selected type s of lamps with broad spectral band emission or multiple spectral lines. Measurements at conditions other than those indicated in the table below may be undertaken by arrangement.


The spectral energy distributions of sources, used either as radiance standards or as irradiance standards, are derived from a black-body radiator operating at a known temperature. The temperature is determined by a pyrometric technique, using filters of known transmittance and detectors of known spectral responsivity.

Sources are accepted for radiometric calibration against these secondary standards only if they can be shown to have adequate short- and long-term stability. For measurements of continuous-spectrum sources, such as incandescent lamps, the wavelength spacing of the sampled bands of radiation may be chosen to suit the application. When the radiation contains spectral emission lines, it is necessary to ensure that contiguous bands of radiation are sampled during the measurement. Subsequent calculations may be made to enable the spectral distribution to be tabulated at a convenient wavelength interval greater than that used during the measurement. Measurements may be made of integrated irradiance for particular discharge lamps of known spectral power distribution.

Photometric Quantities

Photometry is radiometry, weighted by the response function of the eye. The different photometric quantities relate to measurements with different geometries. NMI's photometric scale derives from measurements of irradiance with an electrical substitution radiometer combined with a filter that has been designed to produce a radiometric response equivalent to that of the CIE standard photometric observer for photopic vision. The scale is maintained by a set of lamps calibrated as secondary standards of illuminance at a stated distance from the lamp. When this scale is transferred to lamps of suitable design, they become standards of luminous intensity. NMI's scale of luminous flux has been derived by using a photocell that has calibrated luminous responsivity and is mounted on the rotating arm of a goniophotometer, to integrate the flux on a spherical surface around a suitable lamp.

Photometric Detectors

For a given illuminant, the luminous responsivity of a photocell may be calculated from the measured values of spectral responsivity for the photocell. When a precise knowledge of the spectral responsivity is not available, a lamp calibrated as a working standard of either illuminance or luminous intensity enables a value of luminous responsivity to be measured specifically for that photocell and source. Luminance meters may also be calibrated.

Photometric Sources

Tungsten-filament lamps that are intended for use as working standards of illuminance at a stated distance (luminous intensity) or as standards of luminous flux should be of good construction. They are calibrated only if they have been found to be photometrically stable. For a lamp to be considered suitable for calibration, the rate of change of its photometric properties with time must be sufficiently low to provide a useful number of hours as a working standard. For this reason, each lamp must be aged before calibration until an acceptable rate of change is achieved. When a lamp fails to meet the stability requirements, it is rejected and the work performed to that time is charged for.

For photometric work, it is usually preferable to operate lamps on direct current with a fixed polarity. For all lamps, it is preferable to set the operating current. The particulars of electrical operation and/or distribution temperature should be specified when lamps are submitted for calibration.

When discharge lamps are to be calibrated, they should be submitted with the inductive ballast that will be used with each lamp during subsequent periods of operation. It is necessary to know the spectral energy distribution for each type of discharge lamp submitted. For mercury-vapour fluorescent lamps the preferred ambient air temperature should be specified.

Measurement of luminous flux may be made for suitable types of lamps by comparison with luminous flux standards using an integrating sphere, or by use of a goniophotometer. The suitability of lamps for sphere tests should be established by enquiry.