TN 1421 - Chapter 1: Introduction

A National Measurement System for Radiometry, Photometry, and Pyrometry Based upon Absolute Detectors

I. Introduction

The National Institute of Standards and Technology's (NIST) measurement procedures employed to characterize the spectral characteristics of light sources have been based upon the well established physics of blackbody sources [1]. The photometric units were traditionally established using optical source based methods and utilizing the accepted International Commission on Illumination (CIE) defined response for ordinary human vision, the spectral luminous efficiency, V(λ) [2]. This Technical Note will outline the new NIST methods of maintaining radiometric, photometric, and pyrometric units based upon the use of absolute detector systems and will advocate a shifting of methodology for these measurements that takes advantage of the new opportunities afforded by detector based measurements.

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.

**Table 1. This table gives the radiometric and photometric quantities, their usual symbols and their metric unit definitions.**
Radiometric Quantity	symbol	units	units	symbol	Photometric Quantity
Radiant Energy	Q	J	lm s	Q_v	Luminous energy
Radiant Flux (power)	P, Φ	W	lm	Φ_v	Luminous Flux
Irradiance	E	W/m²	lm/m²	E_v	Illuminance
Radiance	L	W/(m² sr)	lm/(m² sr)	L_v	Luminance
Radiant Intensity	I	W/sr	lm/sr	I_v	Luminous Intensity

J=joule, W=watt, lm=lumen, m=meter, s=second, sr=steradian

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, T = 1234.93 K, were to be maintained using optical techniques [10]. This change results in the temperature unit for temperatures above the silver freezing point to depend upon similar optical measurement technology that is used to define the radiometric and photometric units. As a consequence the NIST radiation temperature unit for a well characterized blackbody source can be referenced to absolute detectors.

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 540 × 10¹² Hz (about 555 nm) and with an intensity of 1/683 W/sr. This optical power based definition, and the recognition of the CIE V(λ) function, provided the opportunity to base photometric units on detector measurements [11]. Many national laboratories have implemented this approach for developing their photometric units and after a substantial development program NIST recently completed its efforts to place the calibrations for the candela and the lumen directly upon an absolute detector [12,13]. An impetus for the change to a detector approach for radiometry and photometry is the advent of the availability of high accuracy cryogenic electrical substitution radiometers. These devices can have a relative combined standard uncertainty of less than 0.01% for optical power measurements and can be conveniently utilized in a well equipped modern radiometric laboratory.* A brief review of these devices will be given in a following section of this Technical Note.

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].