SP250-95 Photometry.pdf

NIST Special Publication 250-95

NIST Measurement Services

Photometric Calibrations

Yuqin Zong

Maria E. Nadal

Benjamin K. Tsai

C. Cameron Miller

This publication is available free of charge from:

https://doi.org/10.6028/NIST.SP.250-95

NIST Special Publication 250-95

NIST Measurement Services

Photometric Calibrations

Yuqin Zong

Maria E. Nadal

Benjamin K. Tsai

C. Cameron Miller

Sensor Science Division

Physical Measurement Division

National Institute of Standards and Technology

100 Bureau Drive

Gaithersburg, MD 20899

This publication is available free of charge from:

https://doi.org/10.6028/NIST.SP.250-95

July 2018

U.S. Department of Commerce

Wilbur L. Ross, Jr., Secretary

National Institute of Standards and Technology

Walter G. Copan, NIST Director and Under Secretary of Commerce for Standards and Technology

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 250-95

Natl. Inst. Stand. Technol. Spec. Publ. 250-95, 147 pages (July 2018)

CODEN: NSPUE2

This publication is available free of charge from:

https://doi.org/10.6028/NIST.SP.250-95 i

T h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

-9

Preface

The calibration and related measurement services of the National Institute of Standards and

Technology (NIST) are intended to assist the makers and users of precision measuring instruments in achieving the highest possible levels of accuracy, quality, and productivity.

NIST offers over 300 different calibrations, special tests, and measurement assurance services.

These services allow customers to directly link their measurement systems to measurement systems and standards maintained by NIST. These services are offered to the public and private organizations alike. They are described in NIST Special Publication (SP) 250, NIST

Calibration Services Users Guide.

The Users Guide is supplemented by a number of Special Publications (designated as the

“SP250 Series”) that provide detailed descriptions of the important features of specific NIST calibration services. These documents provide a description of the: (1) specifications for the services; (2) design philosophy and theory; (3) NIST measurement system; (4) NIST operational procedures; (5) assessment of the measurement uncertainty including an uncertainty budget; and (6) internal quality control procedures used by NIST. These documents will present more detail than can be given in NIST calibration reports, or than is generally allowed in articles in scientific journals. In the past, NIST has published such information in a variety of ways. This series will make this type of information more readily available to the user.

This document, SP250-95 (2017), NIST Photometric Calibrations, is a revision of SP250-37

(1997). It covers the calibration of standards of luminous intensity, luminous flux, illuminance, luminance, luminous exposure, and color temperature (test numbers 37010C–37070C and

37020S-37130S in SP250, NIST Calibration Services Users Guide). Inquiries concerning the technical content of this document or the specifications for these services should be directed to one of the authors or to one of the technical contacts cited in SP250.

NIST welcomes suggestions on how publications such as this might be made more useful.

Suggestions are also welcome concerning the need for new calibrations services, special tests, and measurement assurance programs.

Gregory F. Strouse James K. Olthoff

Associate Director for Measurement Services Director

Physical Measurement Laboratory ii h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Abstract

The National Institute of Standards and Technology provides calibration services for submitted artifacts for luminous intensity, illuminance, color temperature, total luminous flux, luminous exposure and luminance. Additionally, the National Institute of Standards and Technology issues calibrated standards of luminous intensity, luminance, and color temperature. The procedures, equipment, and techniques used to perform these calibrations are described. Detailed estimates and procedures for determining uncertainties of the reported values are also presented along with the internal quality control procedures.

Key words

Calibration; Candela; Color temperature; Illuminance; Lumen; Luminance; Luminous exposure; Luminous flux; Luminous intensity; Lux; Photometry; Standards; Total luminous flux.

iii h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Table of Contents

Introduction

1.1. Photometry, physical photometry, and radiometry

1.2. Photometric quantities and units

1.2.1. Photometric quantities

1.2.2. Relationship between the SI units and English units

1.3. NIST photometric units

1.3.1. Luminous intensity unit

1.3.2. Luminous flux unit

1.3.3. Luminous exposure unit

2. Outline of the calibration services

3. Luminous intensity and illuminance responsivity calibrations

3.1. Photometry, physical photometry, and radiometry

3.2. Design of the NIST standard photometers

3.3. Realization of the NIST illuminance responsivity unit

3.3.1. Calibration of the NIST standard photometers

3.3.2. Correction for the photometer temperature

3.3.3. Correction for illuminance responsivity geometry

3.3.4. Linearity of the NIST standard photometers

3.3.5. Uncertainty budget for NIST illuminance responsivity unit

3.4. Realization of the NIST candela and luminous intensity unit

3.4.1. Photometry bench

3.4.2. Alignment of the NIST standard photometers

3.4.3. Spectral mismatch correction factor (SMCF)

3.4.4. Uncertainty budget for NIST illuminance unit

3.4.5. Uncertainty budget for NIST candela unit

3.5. Luminous intensity calibrations

3.5.1. Lamp standards for luminous intensity

3.5.2. Long-term stability of NIST standard photometers

3.5.3. Electrical power supply

3.5.4. Deviation from inverse square law

3.5.5. Uncertainty budget for luminous intensity calibrations

3.5.6. Calibration and quality control procedures for candela calibrations

3.6. Illuminance responsivity calibrations

iv h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

3.6.1. Types of photometers and illuminance meters

3.6.2. Operation and handling of photometer heads and illuminance meters

3.6.3. Illuminance responsivity calibration and quality control procedures

3.6.4. Uncertainty of illuminance responsivity calibration

3.7. Calibration of high-illuminance photometers

3.7.1. High-illuminance facility, calibration procedure and quality control

3.7.2. Uncertainty budget for high-illuminance calculations

4. Total luminous flux calibrations

4.1. Principles of the absolute integrating sphere method

4.2. Design of the NIST 2.5 m Integrating Sphere Facility

4.3. Uncertainty of the NIST sphere photometer system responsivity

4.3.1. Temperature dependence

4.3.2. Sphere photometer system SMCF

4.3.3. Spatial non-uniformity correction factor

4.3.4. Incident angle correction factor

4.3.5. Illuminance on the reference plane of the aperture and photometer wheel

4.3.6. Average illuminance correction factor

4.3.7. Uncertainty budget for the NIST sphere photometer system responsivity

4.4. Total luminous flux and artifacts for calibration

4.4.1. Incandescent lamps – Typical budget

4.4.2. Fluorescent lamps – Typical budget

4.4.3. Reflector lamps – Typical budget

5. Luminance calibrations

5.1. NIST luminance unit and uncertainty budget

5.2. Calibration of luminance meters

5.3. Calibration of high-luminance meters

5.4. Calibration of luminance sources

5.5. Artifacts for calibration

5.6. Calibration of diffusers for luminance coefficient

5.6.1. Calibration procedure for opal glass luminance coefficient

5.6.2. Calibration procedure for reflectance plaque luminance coefficient

6. Color temperature calibrations

6.1. General Description

6.2. NIST color temperature scale

v h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

6.3. Artifacts for calibration

6.4. Equipment and instruments for calibration

6.5. Calibration procedures

6.6. Uncertainty of correlated color temperature calibration

7. Flash photometer calibrations

7.1. General description

7.2. Methods for realization of the luminous exposure unit

7.2.1. Electrical method

7.2.2. Pulsed photometry method

7.3. Flash standard photometers

7.4. Calibration scheme for flash photometers

7.5. Uncertainty analysis of luminous exposure and flash photometer calibrations

References

Appendix A: List of symbols used in document

Appendix B: Sensitivity coefficients

Appendix C: Samples of calibration reports vi h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

List of Tables

Table 1. Quantities and units used in photometry and radiometry

Table 2. English units and definitions Table 3. Conversion between English units and SI units Table 4. Uncertainty budget for NIST illuminance responsivity unit Table 5. Uncertainty components as correction factors for NIST illuminance unit Table 6. Uncertainty budget for NIST illuminance unit

Table 7. Uncertainty budget for NIST candela unit Table 8. Uncertainty components as correction factors for luminous intensity calibration. .. 35

Table 9. Uncertainty budget for typical luminous intensity calibration

Table 10. Uncertainty components as correction factors for illuminance responsivity calibration Table 11. Uncertainty budget for a typical illuminance responsivity calibration Table 12. Uncertainty components as correction factors for high-illuminance unit

Table 13. Uncertainty budget for the NIST high-illuminance unit Table 14. Uncertainty components for high-illuminance calibration factor

Table 15. Uncertainty budget for a typical high-illuminance calibration factor determination.

Table 16. Uncertainty components for sphere photometer

Table 17. Uncertainty components for sphere external illuminance

Table 18. Uncertainty budget for a sphere external illuminance determination Table 19. Uncertainty budget for a NIST sphere photometer system responsivity Table 20. Uncertainty components for total flux calibration

Table 21. Uncertainty budget for total luminous flux calibration of a typical incandescent lamp

Table 22. Uncertainty components for total flux calibration Table 23. Uncertainty budget for total luminous flux calibration of a typical fluorescent lamp.

Table 24. Uncertainty budget for total luminous flux calibration of a typical reflector lamp. 67 Table 25. Uncertainty components for NIST luminance unit

Table 26. Uncertainty budget for NIST luminance unit Table 27. Uncertainty components for luminance meter calibration Table 28. Uncertainty budget for a typical luminance meter calibration

Table 29. Uncertainty components for high luminance meter calibration Table 30. Uncertainty budget for a typical high-luminance meter calibration Table 31. Uncertainty components for luminance source calibration Table 32. Uncertainty budget for a typical luminance source calibration Table 33. Uncertainty components for NIST reference opal glasses

Table 34. Uncertainty budget for a NIST reference opal glass calibration Table 35. Uncertainty components for reflectance plaque

Table 36. Uncertainty budget for a typical reference plaque luminance coefficient calibration.

Table 37. Uncertainty budget for the offset calibration of the transfer spectroradiometer at

2856 K Table 38. Typical offsets and standard uncertainties of the offsets

Table 39. Uncertainty budget for the correlated color temperature calibration at 2856 K vii h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Table 40. Results of the flash standard photometers calibration using the two methods Table 41. Uncertainty budget for luminous exposure responsivity using the electrical method.

Table 42. Uncertainty budget for the luminous exposure unit using the electrical method. 100 Table 43. Uncertainty budget for luminous exposure responsivity using the pulsed photometric method Table 44. Uncertainty budget for calibration of luminous exposure responsivity using the pulsed photometric method viii h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

List of Figures

Fig. 1. CIE V(λ) function

Fig. 2. Realization and maintenance of the NIST photometric units Fig. 3. Schematic of the NIST Primary Optical Watt Radiometer (POWR) Fig. 4. Geometry for the detector-based candela realization Fig. 5. Design of the NIST standard photometer Fig. 6. Temperature dependence of the photometers’ illuminance responsivity

Fig. 7. Linearity of one transimpedance amplifier for NIST standard photometers Fig. 8. Schematic of the NIST photometric bench

Fig. 9. Alignment of the bi-post base socket using a jig

Fig. 10. Polynomial fit for the SMCFs Fig. 11. Appearance of luminous intensity standard lamps and their electrical polarity Fig. 12. Aging characteristics of a typical Airway Beacon type lamp at 2856 K Fig. 13. Aging characteristics of a selected FEL type lamp at 2856 K

Fig. 14. Spatial non-uniformity of a typical FEL type lamp Fig. 15. Drift of illuminance responsivity of NIST standard photometers

Fig. 16. Results of check standard lamp over a four month period Fig. 17. Configuration of the high illuminance calibration facility Fig. 18. Calculated spectral power distribution of the source combined with the filters

Fig. 19. Spatial non-uniformity of illuminance at 30 cm from the front lens

Fig. 20. Arrangement of NIST 2.5 m integrating sphere for detector-based total luminous flux calibrations Fig. 21. Results of the AC/DC measurement of the NIST integrating sphere with a 1000-W tungsten lamp operated in the sphere Fig. 22. Mapping of NIST 2.5 m integrating sphere responsivity (normalized SRDF)

Fig. 23. Circuit diagram for the calibration of test fluorescent lamps Fig. 24. Typical normalized linear fluorescent tube light output as a function of ambient temperature

Fig. 25. Arrangement for NIST luminance unit realization Fig. 26. Configuration for the realization of opal glass luminance coefficient

Fig. 27. Configuration for the realization of reflectance plaque luminance coefficient Fig. 28. Configuration for color temperature calibration Fig. 29. Color temperature correction values for the NIST diode-array spectroradiometer. .. 87

Fig. 30. Derivation of the luminous exposure unit using the electrical method Fig. 31. Derivation of the luminous exposure unit using the pulsed photometry method Fig. 32. The normalized luminous exposure responsivity [V/(lx·s)] of the three NIST flash photometers that maintain the unit Fig. 33. Arrangement for the calibration of flash photometers

Fig. 34. The spectral power distributions of the xenon flashing-light source and filtered red flashing-light source used in this calibration h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Introduction

This document supersedes the NIST Special Publication 250-37 (1997) and NBS Special

Publication 250-15 (1987). In 1992, a new candela was realized based on an absolute cryogenic radiometer, and the old NIST gold-point blackbody-based unit [1] was replaced by the new detector-based unit [2]. A group of eight calibrated standard photometers based on the cryogenic radiometer maintains the capability to realize the NIST candela, and replaces the lamp-based standards formerly used. Further, the photometric calibration procedures have been revised to utilize the detector-based methods [3]. Since publication of the previous version (1997), the facilities for total luminous flux were improved significantly with the introduction of detector-based calibration procedures [4]. A new service for calibration of flash photometers was developed. There have been many improvements in other existing calibration facilities and procedures.

This document describes the updated photometric calibration procedures for luminous intensity (candela; cd), illuminance (lux; lx), total luminous flux (lumen; lm), luminance

(cd/m2), color temperature (Kelvin; K), and luminous exposure (lx·s, cd·s). Throughout this document, uncertainty statements follow the NIST policy given by Taylor and Kuyatt [5], which prescribes the use of an expanded uncertainty with a coverage factor k = 2 for uncertainties of all NIST calibrations.

Descriptions for the individual standards and calibrations available from NIST covered by this document, as of October 2017, are listed and explained in Section 2. Updated information about calibration services and prices are published periodically in the NIST

Calibration Services Users Guide and Fee Schedule [6].

The material presented in this document describes photometric calibration facilities and procedures as they existed at the time of publication. Further improvement of photometric calibration facilities and procedures are underway. Additional documents will cover new photometric calibration procedures for average LED intensity (candela; cd), LED total luminous flux of LEDs (lumen; lm), colorimetry of LEDs, and spectrally integrated total radiant flux of LEDs (watt; W), along with total spectral radiant flux calibration procedures for incandescent light sources.

1.1. Photometry, physical photometry, and radiometry

The primary aim of photometry is to measure visible optical radiation, light, in such a way that the results correlate with what the visual sensation is to a normal human observer exposed to that radiation. Until about 1940, visual comparison techniques of measurements were predominant in photometry, whereby an observer was required to match the brightness of two visual fields viewed either simultaneously or sequentially. This method of photometry is so-called visual photometry and is seldom used today.

In modern photometric practice, measurements are made with photodetectors. This is referred to as physical photometry. In order to achieve the aim of photometry, one must take into account the characteristics of human vision. The relative spectral responsivity of the human vision system was first defined by the CIE (Commission Internationale de l’Éclairage) in 1924 [7], and redefined as part of colorimetric standard observers in 1931 [8].

It is called the spectral luminous efficiency function for photopic vision, or the V() function, h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2 defined in the domain from 360 nm to 830 nm, and is normalized to unity at its peak, 555 nm

(Fig. 1) [9]. This model gained wide acceptance, was republished by CIE in 1983 [10] and was published by CIPM (Comité International des Poids et Mesures) in 1982 [11] to supplement the 1979 definition of candela. The tabulated values of the function at 1 nm increments are available in References [10-12]. In most cases, the region from 380 nm to

780 nm is used for calculation with negligible errors because the V() function falls below

10-4 outside this region. Thus, a photodetector having a spectral responsivity matched to the

V() function replaced the role of human eyes in photometry.

Fig. 1. CIE V(λ) function.

Radiometry concerns physical measurement of optical radiation as a function of its wavelength. As specified in the definition of the candela by CGPM (Conférence Générale des Poids et Mesures) in 1979 [13] and CIPM in 1982 [11], a photometric quantity XV is defined in relation to the corresponding radiometric quantity Xe,λ by the equation, nm830 nm360 λe,mv )( dVXKX . (1)

The constant, Km, relates the photometric quantities and radiometric quantities, and is called the maximum spectral luminous efficacy (of radiation) for photopic vision. The value of Km is given by the 1979 definition of candela, which defines the spectral luminous efficacy of light at the frequency 540 × 1012 Hz (at the wavelength 555.016 nm in standard air) to be

683 lm/W. Although the value of Km is calculated as 683 × V(555.000 nm)/V(555.016 nm) =

683.002 lm/W [10], it is normally rounded to 683 lm/W with negligible errors [11]. Various photometric and radiometric quantities are described in the next section.

h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

It should be noted that the V() function is based on the CIE standard photometric observer for photopic vision, which assumes additivity of sensation and a 2° field of view at relatively high luminance levels (higher than 1 cd/m2). The human vision in this level is called photopic vision. The spectral responsivity of human vision deviates significantly at very low levels of luminance (less than 10-2 cd/m2). This type of vision is called scotopic vision. Its spectral responsivity, peaking at 507 nm, is designated by the V’() function, which was defined by CIE in 1951 [14], recognized by CIPM in 1976 [15], and republished by CIPM in

1982 [11]. Human vision in the region between photopic vision and scotopic vision is called mesopic vision. While active research is being conducted [16, 17], there is no internationally accepted spectral luminous efficiency function for the mesopic region yet. In current practice, almost all photometric quantities are given in terms of photopic vision, even at low light levels, except for special measurements for research purposes. This document, therefore, does not deal with quantities specified in terms of scotopic or mesopic vision.

Further details of definitions outlined in this section are given in Reference 10.

To better understand the international metrology system, it is useful to know the relationship among such organizations as CGPM (Conférence Générale des Poids et Mesures), CIPM

(Comité international des poids et mesures), CCPR (Comité Consultatif de Photométrie et

Radiométrie), BIPM (Bureau International des Poids et Mesures), and CIE (Commission

Internationale de l´Eclairage). These abbreviations are based on their French names. In

English, these organizations are: CGPM (General Conference of Weights and Measures), CIPM (International Committee for Weights and Measures), CCPR (Consultative Committee of Photometry and Radiometry), BIPM (International Bureau of Weights and Measures), and

CIE (International Commission on Illumination). All SI units (Système international d'unités or International System of Units) are officially defined by the CGPM, which is the decision-making body for the Treaty of the Meter (Convention du Mètre), signed in 1875. The decisions of the CGPM legally govern the global metrology system among those countries signatory to the Treaty of the Meter or agreeing to its usage. The CIPM is a committee under the CGPM, charged with the management of the SI and related fundamental units, consisting of many subcommittees for each technical field. The CCPR is a subcommittee under the

CIPM that discusses and recommends units in photometry and radiometry. It consists of representatives of interested national standardizing laboratories. The CCPR also holds international intercomparisons of photometric units and radiometric scales. The BIPM is a metrology laboratory under the supervision of the CIPM, with staff and facilities in Paris.

The CIE, on the other hand, is originally an academic society in the field of lighting science and was organized to promote uniformity and quality in optical measurements. Many definitions developed by the CIE, such as the V() function, the color matching functions, and the standard illuminants, have been adopted by the CGPM and by the ISO (International

Organization for Standardization) as international standards. The CIE has recently been recognized officially by the ISO as a standards-creating body in the field of optical radiation.

The NIST staff actively participates in CCPR and CIE activities.

1.2. Photometric quantities and units

1.2.1. Photometric quantities

The base unit of all photometric quantities is the candela. The first realization of the candela was performed circa 1860 using candles made from sperm whale fat, known as spermacetti.

In 1898, the candles were replaced by gas lamps that were wickless and used a mixture of h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2 pentane and air. In 1909, the first internationally recognized unit was established collaboratively between the National Bureau of Standards (currently NIST), the National

Physical Laboratory in England, and the Laboratoire Central d’Electricité in France. The unit was based on the pentane lamp, but was maintained on carbon filament lamps. In 1948 the ‘candle’ became the candela and was defined by CGPM such that ‘the magnitude of the candela is such that the luminance of a full radiator at the temperature of solidification of platinum is 60 candelas per square centimeter.’ It became one of the base SI units when SI was established in 1960. Most recently, the candela was redefined by CGPM in 1979 as follows.

“The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 Hertz and that has a radiant intensity in that direction of (1/683) Watt per steradian [11].”

Table 1 lists photometric quantities and their corresponding radiometric quantities side by side, with units and symbols. The precise definition of each quantity is given by CCPR [12] and CIE [18].

Table 1. Quantities and units used in photometry and radiometry.

Photometric

Quantity

Unit Relationship to lumen

Radiometric Quantity Unit

Luminous flux lm (lumen) Radiant flux W (Watt)

Luminous intensity cd (candela) lm sr-1 Radiant intensity W sr-1

Illuminance lx (lux) lm m-2 Irradiance W m-2

Luminance cd m-2 lm sr-1 m-2 Radiance W sr-1 m-2

Luminous exitance lm m-2 Radiant exitance W m-2

Luminous exposure lx s lm m-2 s Radiant exposure W m-2 s

Luminous energy lm s Radiant energy J (Joule)

Color temperature K (Kelvin) Radiance temperature K

Although the candela is defined as an SI base unit, luminous flux (lumen) is perhaps the most fundamental photometric quantity, as the four other photometric quantities are defined in terms of lumen with appropriate geometric factors. Luminous flux (Φv) is the time rate of flow of light as weighted by V(λ). It is defined as dVK )(e.λmv , (2) h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2 where Φe,λ is the spectral concentration of radiant flux in (W/nm) as a function of wavelength λ in nm.

Luminous intensity (Iv) is the luminous flux (from a point source) emitted per unit solid angle in a given direction. It is defined as d d v vI , (3) where dΦv is the luminous flux leaving the source and propagating in an element of solid angle dΩ containing the given direction.

Illuminance (Ev) is the density of the luminous flux incident on a given point of a surface or a plane. It is defined as

A

E d d v

, (4) where dv is the luminous flux incident on an element dA of the surface containing the point.

Luminance (Lv) is the luminous flux from an element of a surface surrounding a given point, emitted into a small solid angle containing the given direction, per unit area of the element projected on a plane perpendicular to that given direction. It is defined as cosΔΔ

Δ lim v

0Δ,0Δ v

A L

A

, (5) where v is the luminous flux emitted (reflected or transmitted) by an elementary beam passing through the given point and propagating in the solid angle containing the given direction; A is the area of a section of that beam containing the given point; is the angle between the normal to that section and the direction of the beam.

Luminance coefficient (q) is measured for opal glass and is defined as the ratio luminance to

Lv illuminance Ev, as defined by

E q . (6) h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Luminous exitance (Mv) is the density of luminous flux leaving a surface at a point. Eq. (6) is the same as Eq. (4), with dv meaning the luminous flux leaving a surface. This quantity is rarely used in the general practice of photometry. However, with the development and application of organic light emitting diodes, this quantity may become more popular.

Luminous exposure (Hv) is the time integral of illuminance Ev(t) over a given duration t, as defined by t dttEH )(vv . (7)

Luminous energy (Qv) is the time integral of the luminous flux (v) over a given duration t, as defined by t dttQ )(vv . (8)

Color temperature (Tc) is the temperature of a Planckian radiator with radiation of the same chromaticity as that of the light source in question. However, the chromaticity coordinates of most lamps do not fall on the Planckian locus, and in actual lamp calibrations, either distribution temperature or correlated color temperature is used. “Color temperature” is often used informally for the correlated color temperature in the United States.

Distribution temperature (TD) is the temperature of a blackbody with a spectral power distribution closest to that of the light source in question, and it is a useful concept for quasi-

Planckian sources.

Correlated color temperature (Tcp) is a concept used for sources with a spectral power distribution significantly different from that of Planckian radiation, for example, discharge lamps. Correlated color temperature is the temperature of the Planckian radiator whose perceived color most closely resembles that of the light source in question.

General information (definitions, symbols, and expressions) on many other physical quantities and units including photometric and radiometric quantities are given in

Reference 19.

1.2.2. Relationship between the SI units and English units

Under NIST policy [20], results of all NIST measurements are reported in SI units.

However, the English units shown in Table 2 are still rather widely used. For all the photometric measurements and calculations, use of the SI units shown in Table 1 is recommended, and use of non-SI units is discouraged [21]. The definitions of the English units are described below for conversion purposes only.

h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Table 2. English units and definitions.

Unit Quantity Definition foot candle (fc) illuminance lumen per square foot (lm ft-2) foot-Lambert (fL) luminance 1/π candela per square foot (π-1 cd ft-2)

It should be noted that the definition of foot-Lambert is such that the luminance of a perfect diffuser is 1 fL when illuminated at 1 fc. In SI units, the luminance of a perfect diffuser would be 1/ (cd/m2) when illuminated at 1 lx. For convenience of changing from English units to SI units, the conversion factors are listed in Table 3. For example, 1000 lx is the same illuminance as 92.9 fc, and 1000 cd/m2 is the same luminance as 291.9 fL. Conversion factors to and from some other units are given in Reference 22.

Table 3. Conversion between English units and SI units.

To obtain value in Multiply value by By lx from fc fc 10.764 fc from lx lx 0.09290 cd/m2 from fL fL 3.4263 fL from cd/m2 cd m-2 0.29186 m (meter) from feet feet 0.30480 mm (millimeter) from inch inch 25.400

1.3. NIST photometric units

1.3.1. Luminous intensity unit

Until 1991, the NIST luminous intensity unit was derived from the NIST spectral irradiance scale [23, 24], which was based on a gold-point blackbody, and therefore, was dependent on the temperature scale. In 1990, the international temperature scale was revised [25], thereby revising gold point temperature from 1337.58 K to 1337.33 K. Due to the change in the temperature scale, the NIST luminous intensity unit increased in magnitude by 0.35 %.

In 1992 at NIST, a new luminous intensity unit (candela) was realized based on the absolute responsivity of detectors (using 100 % Quantum Efficient silicon detectors [2] and subsequently a cryogenic electrical substitution radiometer [3]). The old luminous intensity unit was replaced with the new unit in 1992.

h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

Fig. 2. Realization and maintenance of the NIST photometric units.

The new candela is realized and maintained on the NIST standard photometers (a group of eight photometers that are not temperature controlled) which are calibrated for illuminance responsivity in A/lx. These standard photometers also embody the NIST illuminance unit, and allow luminous intensity to be determined from measured illuminance and distance. The realization and maintenance of the photometric units at NIST are shown in Figure 2. A new

NIST reference cryogenic radiometer, Primary Optical Watt Radiometer (POWR) [26], has been constructed and replaced the previous reference radiometer [27] in 2005. POWR realizes and maintains the optical power unit (watt, W), which serves as the basis for all radiometric and photometric units and scales realized at NIST. The POWR, including the detector module, has been completely designed by NIST. The radiometer is cooled by liquid helium to 5 K, and works on the principle of electrical substitution. The construction of the

POWR is shown in Figure 3. Based on laser-beam power measurements with the POWR at several wavelengths, the NIST detector spectral responsivity scale is maintained on silicon photodiode light-trapping detectors. The new radiometer is designed to have the versatility to grow with NIST's needs, to embrace new technologies, and to be able to provide optical

Absolute spectral response transfer

Distance

Aperture area

Absolute integrating sphere method

Illuminance distance

Absolute spectral response transfer

Aperture area measurement calculation based on candela definition

Optical Power

[W]

(POWR cryogenic radiometer)

Temperature

[K]

(High temperature blackbody)

Spectral Responsivity

[A/W]

(Silicon photodiodes)

Spectral Irradiance

[W/m2]

(Standard lamps)

Illuminance Responsivity

[A/lx]

(Standard photometers)

Color Temperature

[K]

(Reference lamps)

Luminance Intensity

[cd]

(Standard photometers)

Luminance

[cd/m2]

(Reference lamps)

Total Luminous flux

[lm]

(Transfer lamps) h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2 power measurements with expanded uncertainties of 0.02 % (k = 2), which has been verified by a recent intercomparison with two other cryogenic radiometer facilities at NIST [26].

Fig. 3. Schematic of the NIST Primary Optical Watt Radiometer (POWR).

The spectral responsivity scale is transferred to other detectors using the Spectral Comparator

Facility (SCF) [28], where the absolute spectral responsivity s(λ) (A/W) of each of the NIST standard photometers is determined. The illuminance responsivity [A/lx] of each photometer is then calculated from s(λ), the area of the aperture, and other correction factors. The relative expanded uncertainty of the illuminance responsivity determination is 0.39 % [2].

The standard photometers are recalibrated annually utilizing the detector spectral responsivity scale. The details of the candela realization are described in Section 3.1 and in

Reference 2. As the result of the candela realization in 1992, the magnitude of the NIST luminous intensity unit increased by approximately 0.3 %. With the effect of the change of the international temperature scale in 1990 included, the magnitude of the NIST candela is larger (measured values are smaller) by approximately 0.6 % than that reported before 1990.

At the latest CCPR international key comparison for luminous intensity (CCPR-K3.a) [29] in

1998, the NIST candela was 0.12 % smaller (measured values are larger) than the Key

Comparison Reference Value (similar to what was called the world mean value). The 1998 intercomparison showed that the candela standards disseminated in different countries varied within about ± 1 %. The most recent status of the differences in the magnitude of h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2 photometric units for different countries in the world is published on the BIPM website http://www.bipm.org/.

1.3.2. Luminous flux unit

Until 1994, the NIST luminous flux unit was derived from the previous luminous intensity unit which was based on blackbody radiation. The previous luminous flux unit was last realized in 1985 by goniophotometric measurements [1], and was maintained on a group of six incandescent standard lamps. The unit was periodically transferred to groups of working standard lamps used for routine calibrations.

In 1995, a new NIST luminous flux unit was derived, based on the detector-based candela introduced in 1992, with a new method using an integrating sphere and an external source.

The basic principle of this method (Absolute Integrating Sphere Method) is to measure the total flux of a lamp inside the sphere compared to a known amount of flux introduced into the sphere from a source outside the sphere.

This method was first studied theoretically using a computer simulation technique [30], then experimentally verified [31] using a 0.5 m integrating sphere. Utilizing this method with a

2 m integrating sphere, the new NIST luminous flux unit was established in 1995 [32, 33]. A new total luminous flux calibration facility using a 2.5 m sphere was developed at NIST

[4, 34]. Primary standard lamps are calibrated periodically against the NIST illuminance unit in order to monitor the luminous flux unit. The 2.5 m absolute integrating sphere is used to provide routine calibrations. The details of the luminous flux unit realization are described in

Section 4.1.

The realization of the 1995 luminous flux unit has resulted in a change (increase) of the magnitude of NIST luminous flux unit by approximately 1.1 % compared to older realizations. The measured lumen values reported by NIST are smaller by that percentage than those previously reported. The new luminous flux unit has been disseminated in NIST calibrations since January 1, 1996. At the latest CCPR international key comparison for total luminous flux (CCPR-K4) [29] in 1998, the NIST lumen was larger (measured value was smaller) by 0.21 % than the key comparison reference value. This intercomparison showed that the lumen standards disseminated in different countries varied within about ± 1 %.

1.3.3. Luminous exposure unit

The photometric measurement of flashing light is essential in the evaluation of various flashing-light sources used in many applications including photography and signaling in transportation. Among them, a need for accurate measurements of aircraft anticollision flashing lights is addressed by the Federal Aviation Administration [35]. The Federal

Aviation Administration has specified the requirements for the intensity of anticollision flashing lights [36] and is enforcing the maintenance of the anticollision flashing lights on all commercial airplanes.

In the specification of aircraft anticollision flashing lights and other vehicle signals using flashing lights, the term effective intensity (unit: cd) is widely used. Effective intensity is the luminous intensity of a fixed (steady) light, of the same relative spectral distribution as the flashing light, which would have the same luminous intensity level as the flashing light under identical conditions of observation [37]. Effective intensity Iv is calculated from Jv, the time-h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2 integrated luminous intensity [cd·s], and a model based on visual perception. The variable, Jv (= ∫ 𝐼v(𝑡) 𝑇 𝑑𝑡), is defined as the time integral of instantaneous luminous intensity Iv(t) of a flash over the flash duration T.

Four effective intensity models are available: the Allard method [38], the Blondel-Rey equation [39], the Form Factor method [40], and the Modified Allard method [41]. The

Blondel-Rey equation, developed in 1911, is most commonly used. The use of one single method has not been agreed upon internationally. All four methods give the same result for a very narrow, single pulse, as one emitted by a xenon flash tube. For such a very narrow single pulse, the effective intensity can be approximated to be five times the time-integrated luminous intensity. The integrated luminous intensity Jv (= Hv×d2) can also be obtained by measuring the luminous exposure, Hv [lx·s] (see Eq. 7), and the photometer-to-source distance, d.

In order to calibrate photometers designed for measurement of the flashing lights, NIST established a luminous exposure scale in 1997 [42].

2. Outline of the calibration services

This section provides a list and brief description of the photometric calibration services currently available at NIST and covered in this SP250. In addition to the photometric calibration services in this SP250, additional documents will cover new photometric calibration procedures for average LED intensity (candela; cd), LED total luminous flux of

LEDs (lumen; lm), colorimetry of LEDs, and spectrally integrated total radiant flux of LEDs

(watt; W), along with new procedures for the total spectral radiant flux calibrations of incandescent light sources. For the updated information, please check the NIST photometry calibration service website https://www.nist.gov/calibrations/photometric-measurements-calibrations#37010C

All the calibration services listed here, including the Special Tests, are provided routinely.

Fixed services (Test Numbers ending in the letter C) are those in which NIST issues calibrated artifacts to customers. Special Tests (Test Number ending with the letter S), on the other hand, are those in which NIST calibrates artifacts submitted by customers. The fees for the fixed services are listed in the NIST Calibration Services Users Guide (SP250) [6].

The fees for Special Tests depend on the type of artifacts, number of artifacts, measurement range requested, etc. Calibrations on Special Test items or under special conditions, other than listed, may be available after consultation as Special Photometric Tests 37100S. The details of the artifacts and measurement procedures for calibration are described in Sections 3 through 7.

37010C - Luminous Intensity and Correlated Color Temperature Standard Lamp

NIST will issue to the customer a 1000 W modified FEL (a lamp-type designation, but not an acronym, of the American National Standards Institute or ANSI) quartz-halogen lamp calibrated for luminous intensity (candela) and correlated color temperature (Kelvin). This lamp has a double coil filament and a clear bulb, and is potted on a medium bipost base. The lamp is operated at approximately 7.3 A/85 V dc, at a correlated color temperature of

2856 K. The relative expanded uncertainty of the luminous intensity of this lamp is less than h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

0.6 % (k = 2) and the expanded uncertainty of the correlated color temperature is 8 K (k = 2) at 2856 K.

37020S - Special Tests for Luminous Intensity and Correlated Color Temperature of Submitted

Lamps

NIST will calibrate the luminous intensity and correlated color temperature of incandescent lamps submitted by customers. The inside frosted lamps, the airway beacon lamps, and the

1000 W FEL lamps previously issued by NIST can be submitted for recalibration. Customers can specify either the lamp current or the correlated color temperature of the lamp (normally

2856 K) for calibration. The relative expanded uncertainty of the luminous intensity of these lamps is typically 0.6 % (k = 2) and the expanded uncertainty of the correlated color temperature is 8 K (k = 2) at 2856 K.

37030C - Correlated Color Temperature Standard Lamp

NIST will issue to the customer a 1000 W modified FEL quartz halogen lamp as described in

37010C calibrated for correlated color temperature. The lamp is usually calibrated for a correlated color temperature of 2856 K. The expanded uncertainty of the correlated color temperature of these lamps is 8 K (k = 2) at 2856 K.

37040C - Each Additional Correlated Color Temperature for 37030C

The correlated color temperature standard lamp issued for 37030C can be calibrated for additional correlated color temperature points in a range from 2000 K to 3200 K. The expanded uncertainty of this calibration is 5 K to 11 K (k = 2) in the range from 2000 K to

3200 K.

37050S - Special Tests for Correlated Color Temperature of Submitted Lamps

NIST will calibrate the correlated color temperature of incandescent lamps submitted by customers. The inside frosted lamps, the airway beacon lamps, and the 1000 W FEL lamps previously issued by NIST can be submitted for recalibration. The range of correlated color temperature points is from 2000 K to 3200 K. The expanded uncertainty of this calibration is

5 K to 11 K (k = 2) in the range from 2000 K to 3200 K.

37060S - Special Tests for Total Luminous Flux of Submitted Incandescent and Fluorescent

Lamps

NIST will calibrate the total luminous flux (lumen) of incandescent lamps and fluorescent lamps submitted by customers. The standard lamps previously issued by NIST can be submitted for recalibration. Miniature lamps may also be accepted. Customers should contact

NIST before submitting lamps. The relative expanded uncertainty of this calibration is typically 0.8 % (k = 2) for incandescent lamps and 2.0 % (k = 2) for fluorescent lamps, which depend upon the reproducibility of the test lamps.

37070C - Opal Glass Luminance Coefficient Standards

NIST will issue a flashed opal glass plate, 51 mm x 51 mm, calibrated for luminance coefficient (ratio of luminance/illuminance, unit: sr–1) for CIE Illuminant A (2856 K source).

The glass plate, masked with a circular aperture 25 mm in diameter, is calibrated for the luminance coefficient within a circular area of 1 cm in diameter in the center of the aperture.

The relative expanded uncertainty of this calibration is 0.5 % (k = 2).

h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

37080S - Special Tests for Submitted Luminance Sources and Transmitting Diffusers

NIST will calibrate luminance (cd/m2) of submitted sources or the luminance coefficient

(sr-1) of submitted transmitting diffusers, including opal glass previously issued by NIST.

Customers should contact NIST before sending sources or diffusers. The relative expanded uncertainty of luminance calibration is typically less than 0.8 % (k = 2).

37090S - Special Tests for Submitted Photometers, Illuminance Meters, and Luminance Meters

NIST is capable of calibrating photometers, illuminance meters, and luminance meters submitted by customers. Calibration is normally performed using a CIE Illuminant A source

(2856 K incandescent source).

37091S - Special Test for Submitted Illuminance Head or Illuminance Meter for Illuminance

Responsivity

NIST will calibrate illuminance photometers and illuminance meters submitted by customers for illuminance responsivities. Calibration is performed at illuminance levels from 0.1 lx to

3000 lx. Calibrations at higher illuminance levels (up to 100 klx) are also available under special arrangements. The relative expanded calibration uncertainty (k = 2) is typically 0.5 %, which may increase at low illuminance levels close to 0.1 lx or high illuminance levels close to 100 klx. As an option, NIST can also measure the spectral responsivity of a submitted illuminance photometer and calculate the spectral mismatch correction factor for a source with a known spectral power distribution.

37092S - Special Test for Submitted Luminance Head or Luminance Meter for Luminance

Responsivity

NIST will calibrate luminance photometers and luminance meters submitted by customers for luminance responsivities. Calibration is performed at luminance levels from 0.1 cd/m2 to

8000 cd/m2. The relative expanded calibration uncertainty (k = 2) is typically 0.7 %, which may increase at low luminance levels close to 0.1 cd/m2.

37100S - Special Photometric Tests

NIST can provide special tests for sources, detectors, and photometric instruments other than those stated above under limited conditions by special arrangements with NIST. Customers should contact NIST for consultation.

37110S - Special Tests for Submitted Flash Photometers

NIST will calibrate submitted flash photometers designed for measuring luminous exposure

[lx·s] or effective intensity [cd]. Calibration is normally performed with white light

(6500 K) from a Xenon strobe light in the range of 1 lx·s to 100 lx·s and with red light using an Aviation Red filter covering the Xenon strobe light. The relative expanded uncertainty of calibration is 1 % (k = 2) or larger depending on the performance of the flash photometer under test.

37130S - Special Tests for Luminous Intensity and Luminous Flux of LEDs

NIST is capable of calibrating various light emitting diodes (LEDs) submitted by customers including low-power, mid-power, and high-power LED packages, LED arrays or modules, h is p u b lic a tio n is a v a ila b le fre e o f c h a rg e fro m : h ttp s ://d o i.o rg /1

.6

/N

IS

T .S

P .2

LED light engines, LED lamps, following measurement standards such as IES 79 and CIE S

025 for measurement of LED lamps and LED luminaires, IES 85, CIE 225:2017, and CIE

226-2017 for measurement of high-power LED packages at a specified junction temperature, and CIE 127:2007 for measurement of low power LED packages.

37131S - Special Test for Submitted LEDs for Luminous and/or Radiant Intensity and Color

(Optional)

NIST can calibrate submitted LEDs for Averaged LED Intensity under the CIE Standard

Conditions A and B (defined in CIE 127:2007), true luminous/radiant intensity as defined in

CIE 225:2017, and color characteristics.