The radiometric and photometric units and quantities usually referenced in
technical discussions are shown in Table 1. The left side of the table
identifies the radiometric quantity, the usual symbol utilized to describe it,
and the corresponding units in the International System of units (SI)
[3]. The right side describes the
photometric units, their symbols and the SI units associated with them. In the
table, the SI unit of the candela (cd) is replaced with the unit of the lumen
(lm) using the defined relationship, cd = lm/sr. Representing the
quantities in terms of the lumen, the photometric equivalent of the watt,
instead of the candela lends symmetry to the table and a convenience for
understanding the often confusing radiometric and photometric terms.
Radiometric Quantity | symbol | units | units | symbol | Photometric Quantity |
---|---|---|---|---|---|
Radiant Energy | Q | J | lm s | Qv | Luminous energy |
Radiant Flux (power) | P, Φ | W | lm | Φv | Luminous Flux |
Irradiance | E | W/m2 | lm/m2 | Ev | Illuminance |
Radiance | L | W/(m2 sr) | lm/(m2 sr) | Lv | Luminance |
Radiant Intensity | I | W/sr | lm/sr | Iv | Luminous Intensity |
The NIST maintenance of irradiance, radiance, and the photometric units has been discussed in a series of special publications available from NIST or the U.S. Government Printing Office [4, 5, 6]. In order to assess the importance of the NIST photometric and radiometric work in the technical community, Kostkowski reviewed the industrial and commercial impact of these activities and the Council for Optical Radiation Measurement (CORM) has produced detailed reports outlining industrial and technical requirements for the U.S. scientific community [7, 8].
The NIST unit of radiation temperature is based upon the radiation output of
blackbody sources and is described in detail in a NIST special publication
[9]. In 1990 the Comité International des Poids et Mesures (CIPM)
decided that temperature measurement for temperatures above the freezing point
of silver,
In 1979 the Conférence Générale des Poids
et Mesures (CGPM) adopted the 1977 CIPM recommendation for the
redefinition of the candela. The new definition for the candela is a source
of monochromatic radiation at a frequency of
Users of optical radiation measurement devices require improved accuracy to meet competitive demands in the marketplace and to improve the quality and efficiency of production facilities. Optical sensor systems for space based Earth observation have an increasing demand for more accurate measurements. This places a burden on the national metrology system to provide calibration support with reduced uncertainties. This Technical Note will review NIST's efforts to improve the accuracy and stability of its radiometric, photometric, and pyrometric units to accommodate these expressed needs of its customers. General aspects of radiometry and photometry will not be reviewed as this task is effectively and broadly covered by the technical literature and recent books [14].