Principles of Photometry
Photometry is the measurement of electromagneticradiation weighted by the human eye's response. This response changes with wavelength, and to an extent, from person to person. Internationally-agreed standard observer functions are therefore used in order to provide a consistent measurement base for photometry; the two most widely used are the V(λ) function, which applies for photopic vision (typical day-time light levels) and the V'(λ) for scotopic vision (low lighting levels). At intermediate light levels (mesopic or ‘twilight’ levels, such as found on lit roads at night), the CIE system of mesopic photometry is used to provide a smooth transition between these two functions. In photometry, the word 'luminous' is used to indicate that measurements have been made using a detection system (called a photometer) that 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 other quantities, such as luminance from luminous intensity standards. NPL has extensive facilities 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.
Luminous intensity measures the luminous output from a source ina specific direction into unit solid angle. The candela (one of the SI base units, abbreviation cd), is the unit of luminous intensity and is maintained at NPL using 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).