Extensive facilities are available for the photometric measurement of both sources and detectors, including photometers, luxmeters, luminance meters and colour temperature meters. Services include the calibration of luminous intensity, illuminance, luminance, luminous flux and correlated colour temperature.

Most photometric calibrations are carried out for a colour temperature of 2856 K (CIE standard illuminant A), but the photometric calibration of a detector can be combined with a spectral responsivity measurement enabling the calculation of correction factors for other types of source.

NPL can supply calibrated tungsten and tungsten halogen lamps, including a range of lamps designed specifically for use as standards of luminous intensity or luminous flux, as well as arrange for the hire of an NPL designed high performance photometer.

Photometry is the measurement of electromagnetic radiation weighted by the human eye's response. This response changes with wavelength, and to an extent, from person to person. To provide a consistent measurement base for photometry, a standard observer function, V(λ) , has been adopted internationally for photopic vision (medium to high lighting levels) and V'(λ) for scotopic vision (low lighting levels).

In photometry, the word 'luminous' refers to the fact that a detection system, (called a photometer), has a spectral response similar to that of a human eye. The two principal photometric scales maintained at NPL are of luminous intensity and luminous flux. Setting up appropriate geometries permits calibrations of quantities, such as luminance from luminous intensity standards.

Luminous intensity measures the luminous output from a source in a specific direction into unit solid angle. The candela, (one of the fundamental SI units), is the unit of luminous intensity (luminous flux per unit solid angle) and is maintained at NPL in standard photometers and lamps with an uncertainty of ±0.2%. Working reference standards are calibrated against the standard photometers whose calibration derives in turn from the NPL spectral responsivity scale based on the cryogenic radiometer. Luminous intensity measurements are carried out on a photometric bench. They compare the output of test lamps with that of working reference standard lamps using a specially constructed filter-corrected silicon photodiode (photometer).

Illuminance, (measured in lux), at a point of a surface is the quotient of luminous flux incident on an element of the surface, by the area of the element. For a point source and a surface normal to the direction of view, illuminance equals luminous intensity divided by distance squared. Apparatus for illuminance measurements are similar to that for luminous intensity, but, instead of comparing lamps, the distances from light centres to the photometer are carefully measured for each lamp. The illuminance produced at the photometer by the standard is calculated using the inverse square law. The best measurement capability is ± 0.5%. This method can be used to calibrate photometers and luxmeters.

Total luminous flux is a measure of the amount of light emitted from a source in all directions (i.e. the full solid angle of 4π steradians). The NPL total luminous flux scale is derived at the ± 0.3% level from the luminous intensity scale by use of a specially constructed goniometer. The reference goniophotometer has a maximum detector to source distance of 18 m, which can permit very high angular resolution of large sources such as luminaries and spotlights.

Luminance (in cd/m²) weights the spectral power distribution with the V(λ) function to give a value of visual lightness. The measurement scale is maintained in lamps such as 'luminance gauges', which give a uniform field of luminance. The reference system at NPL uses a white diffuser of known reflectance, illuminated normally by a luminous intensity standard at a measured distance. A telephotometer designed to have a response close to the V(λ) function measures light reflected at 45º to the normal. The best measurement capability is ±1.5%. The Photonics Group carries out luminance measurements on complicated display technologies.

Low level photometry is undertaken using a spectrally corrected photomultiplier photometer with a V(λ) response.

Colour temperature (c.t.) is a specification system describing the colour of a source by reference to a Planckian radiator scale. The appearance of coloured materials depends on source colour, so lamp c.t. is quoted in several standards. A common requirement is the use of CIE Source A, defined as a tungsten source with a c.t. of 2856 K. NPL offers a lamp calibration service to determine the operating current of tungsten lamps for a specific c.t. to within ± 10 K of 2856 K. We can also calibrate c.t. meters.

Facilities

• 8 m photometric bench
• 1.7 or 18 m goniometer
• Lamp characterisation system
• Traceable, stabilised power supplies
• High accuracy NPL-designed photometers
• Photometer spectral responsivity calibration facility
• Telephotometer
• Luxmeter calibration facility

Additional special rigs are also constructed to perform special measurements.

Other Facilities

We hire NPL-designed high performance photometers which can be optimised for particular applications. Our services can include the 'ageing' of lamps which is in some cases essential to improve stability. All uncertainties quoted in this document are at the 95% confidence level.

Training and Consultancy

We run hands on training courses in photometric techniques. We can also offer our expetise in consultancy services.

Representation

NPL staff participate in many activities of the International Illumination Council (CIE), and other standards organisations.

For the use of this apparatus in calibration and measurement work (some of our services are UKAS accredited), or to dicuss training or constulancy requirements, please contact the Optical Technologies (OT) group through our general enquiries page.

More information about a selection of the photometric services offered by the OT group:

• Luxmeters and Photometers
• Services for the Lighting Industry