FprEN IEC 62471-7:2022

Photobiological safety of lamps and lamp systems - Part 7: Light sources and luminaires primarily emitting visible radiation

Sécurité photobiologique des lampes et des appareils utilisant des lampes - Partie 7: Sources de lumière et luminaires qui émettent principalement un rayonnement visible

Fotobiološka varnost sijalčnih sistemov - 7. del: Svetlobni viri in svetilke, ki oddajajo predvsem vidno sevanje

General Information

• Status: Not Published
• Publication Date: 25-Jun-2023
• ICS: 29.140.01 - Lamps in general
• ICS: 31.260 - Optoelectronics. Laser equipment
• Technical Committee: CLC/TC 34 - 'Lamps and related equipment
• Drafting Committee: IEC/TC 34 - IEC_TC_34
• Current Stage: 5060 - Voting results sent to TC, SR - Formal Approval
• Start Date: 20-Jan-2023
• Completion Date: 20-Jan-2023
• Ref Project: kSIST FprEN IEC 62471-7:2022 - Photobiological safety of lamps and lamp systems - Part 7: Light sources and luminaires primarily emitting visible radiation

Standards Content (Sample)

SLOVENSKI STANDARD
oSIST prEN IEC 62471-7:2022
01-julij-2022
Fotobiološka varnost sijalčnih sistemov - 7. del: Svetlobni viri in svetilke, ki
oddajajo predvsem vidno sevanje

Photobiological safety of lamps and lamp systems - Part 7: Light sources and luminaires

primarily emitting visible radiation

Sécurité photobiologique des lampes et des appareils utilisant des lampes - Partie 7:

Sources de lumière et luminaires qui émettent principalement un rayonnement visible

Ta slovenski standard je istoveten z: prEN IEC 62471-7:2022
ICS:
29.140.01 Žarnice na splošno Lamps in general
oSIST prEN IEC 62471-7:2022 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN IEC 62471-7:2022
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oSIST prEN IEC 62471-7:2022
34/916/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 62471-7 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2022-05-20 2022-08-12
SUPERSEDES DOCUMENTS:
34/885/CD, 34/907A/CC
IEC TC 34 : LIGHTING
SECRETARIAT: SECRETARY:
United Kingdom Mr Petar Luzajic
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:
TC 76
Other TC/SCs are requested to indicate their interest, if
any, in this CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY

SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING

Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.

This document is still under study and subject to change. It should not be used for reference purposes.

Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of

which they are aware and to provide supporting documentation.
TITLE:

Photobiological safety of lamps and lamp systems - Part 7: Light sources and luminaires

primarily emitting visible radiation
PROPOSED STABILITY DATE: 2025
NOTE FROM TC/SC OFFICERS:

Copyright © 2022 International Electrotechnical Commission, IEC. All rights reserved. It is permitted to

download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National

Committee positions. You may not copy or "mirror" the file or printed version of the document, or any part of it, for

any other purpose without permission in writing from IEC.
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oSIST prEN IEC 62471-7:2022
IEC CDV 62471-7 © IEC 2022 2 34/916/CDV
1 CONTENTS

2 FOREWORD......................................................................................................................... 3

3 INTRODUCTION ................................................................................................................... 5

4 1 Scope ............................................................................................................................ 6

5 2 Normative references .................................................................................................... 6

6 3 Terms and definitions .................................................................................................... 7

7 4 Optical radiation hazards of light sources and luminaires .............................................. 13

8 5 Actinic UV hazards exposure for skin and eye (200 nm to 400 nm) ............................... 14

9 5.1 General ............................................................................................................... 14

10 6 UV-A hazard assessment for the eye lens (315 nm to 400 nm) ..................................... 15

11 6.1 General ............................................................................................................... 15

12 6.2 UV-A light source and luminaire assessment ....................................................... 15

13 7 Retinal blue light hazard assessment (300 nm to 700 nm) ............................................ 15

14 7.1 General ............................................................................................................... 15

15 7.2 Blue light hazard assessment for light sources ..................................................... 15

16 7.3 Blue light hazard assessment for luminaires ........................................................ 16

17 7.4 Retinal blue light hazard assessment - small source (300 nm to 700 nm) .............. 18

18 8 Retinal thermal hazard assessment (380 nm to 1 400 nm) ............................................ 18

19 8.1 General ............................................................................................................... 18

20 8.2 Retinal thermal hazard for light source assessment .............................................. 18

21 8.3 Retinal thermal hazard assessment for luminaire ................................................. 19

22 8.4 Retinal thermal hazard assessment – weak visual stimulus (780 nm to

23 1 400 nm) ........................................................................................................... 19

24 9 Infrared radiation hazard assessment for the eye (780 nm to 3 000 nm) ........................ 19

25 9.1 General ............................................................................................................... 19

26 9.2 Light source and luminaire assessment ............................................................... 19

27 10 Thermal hazard assessment for the skin (380 nm to 3 000 nm) ..................................... 20

28 10.1 General ............................................................................................................... 20

29 10.2 Light source and luminaire assessment ............................................................... 20

30 Annex A (informative) Information on emission limits for light sources and luminaires .......... 21

31 Annex B (informative) Information on UV hazards exposure (200 nm to 400 nm) ................. 23

32 Annex C (informative) Information on retinal hazards (300 nm to 1 400 nm) ........................ 24

33 Annex D (informative) Information on IR-hazard (380 nm to 3 000 nm) ................................ 26

34 Annex E (informative) Example of a complete luminaire assessment of a LED office

35 luminaire ..................................................................................................................... 27

36 E.1 UV assessment ................................................................................................... 27

37 E.1.1 Actinic UV (Clause 5) ................................................................................... 27

38 E.1.2 UV-A (Clause 6) ........................................................................................... 27

39 E.2 Retinal hazard assessment .................................................................................. 27

40 E.2.1 Blue light hazard (Clause 7) ......................................................................... 27

41 E.2.2 Thermal retinal hazard (Clause 8) ................................................................ 27

42 E.3 IR assessment (Clause 9) ................................................................................... 27

43 Bibliography ....................................................................................................................... 28

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47 INTERNATIONAL ELECTROTECHNICAL COMMISSION
48 ____________
50 PHOTOBIOLOGICAL SAFETY OF LAMPS AND LAMP SYSTEMS –
52 Part 7: Light sources and luminaires primarily emitting visible radiation
56 FOREWORD

57 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

58 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international

59 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and

60 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,

61 Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their

62 preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with

63 may participate in this preparatory work. International, governmental and non-governmental organizations liaising

64 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for

65 Standardization (ISO) in accordance with conditions determined by agreement between the two o rganizations.

66 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international

67 consensus of opinion on the relevant subjects since each technical committee has representation from all

68 interested IEC National Committees.

69 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National

70 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC

71 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any

72 misinterpretation by any end user.

73 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications

74 transparently to the maximum extent possible in their national and regional publications. Any divergence between

75 any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.

76 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity

77 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any

78 services carried out by independent certification bodies.

79 6) All users should ensure that they have the latest edition of this publication.

80 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and

81 members of its technical committees and IEC National Committees for any personal injury, proper ty damage or

82 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and

83 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC

84 Publications.

85 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

86 indispensable for the correct application of this publication.

87 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent

88 rights. IEC shall not be held responsible for identifying any or all such patent rights.

89 IEC 62471-7 has been prepared by IEC technical committee 34: Lighting. It is an International

90 Standard.
91 The text of this International Standard is based on the following documents:
Draft Report on voting
34/XX/FDIS 34/XX/RVD

93 Full information on the voting for its approval can be found in the report on voting indicated in

94 the above table.

95 The language used for the development of this International Standard is English.

96 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in

97 accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available

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98 at https://www.iec.ch/members_experts/refdocs. The main document types developed by IEC

99 are described in greater detail at https://www.iec.ch/standardsdev/publications.

100 A list of all parts in the IEC 62471 series, published under the general title Photobiological

101 safety of lamps and lamp systems, can be found on the IEC website.

102 The committee has decided that the contents of this document will remain unchanged until the

103 stability date indicated on the IEC website under webstore.iec.ch in the data related to the

104 specific document. At this date, the document will be
105 • reconfirmed,
106 • withdrawn,
107 • replaced by a revised edition, or
108 • amended.
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109 INTRODUCTION

110 The terms "lamps and lamp systems" are used in the title of the IEC 62471 series however, in

111 the title of this Part 7, the terms "light sources and luminaires" are used. The reason for this is

112 that due to the introduction of new LED technologies the characteristics of the light-generating

113 components have changed. Therefore, the terms "electrical light sources" and "luminaires" are

114 nowadays used in TC 34 instead of "lamps and lamp systems". "Electric light source" is the

115 generic term for products which are producing light; the term "lamp" (light source with a lamp

116 cap-holder system) is thereby included. The terms "light sources and luminaires" are used in

117 this Part 7, but in the other parts of the IEC 62471 series, the terms "lamps and lamp systems"

118 are used.

119 "Luminaire" is the basic term (see IEC 60050-845:2020), for a product that includes all

120 necessary accessories and describes a device that distributes, filters, or transforms the light

121 produced from at least one source of optical radiation and which includes, except the sources

122 themselves, all the parts necessary for fixing and protecting the sources and, where necessary,

123 circuit auxiliaries together with the means for connecting them to the power supply.

124 When luminaires are designed and constructed in accordance with the requirements of this

125 document, they are presumed to function safely, from a photobiological safety perspective,

126 under normal use and not to present a photobiological hazard to persons or the environment.

127 Conformity of luminaires is verified by performing the evaluation of this document with the

128 specified acceptable light sources (one or more).

129 The light sources can be interchangeable or an integral part of the luminaire. If the light source

130 is an integral part of the luminaire, the luminaire could also be considered a light source system

131 (corresponding to a lamp system).

132 Most electrical light sources and luminaires within the scope of this document will not present

133 a photobiological hazard due to the spectra used, the light levels, and the natural aversion

134 responses – people do not usually stare into high luminance sources, for example. Normally

135 light sources and luminaires are safe and do not pose photobiological hazards except under

136 unusual exposure conditions. There remain, however, some light sources and luminaires, which

137 have the potential to pose adverse health effects from the emitted optical radiation. Exposure

138 limits for a range of photobiological hazards associated with broad-band optical radiation

139 sources have been developed and published by the International Commission on Non-Ionizing

140 Radiation Protection (ICNIRP).

141 This document introduces a new assessment procedure to address the various lighting

142 applications in which the intended purpose is the illumination of objects and scenes. This new

143 approach uses revised time bases related to the intentional or unintentional view into the

144 luminaire (leading to emission limits) and assessment distances depending on application.

145 These emission limits are based on the exposure limits of ICNIRP.

146 In this document, a complete procedure is used to cover all photobiological hazards in the range

147 of 200 nm to 3 000 nm as implemented in IEC 62471.

148 This procedure, based on a product- and application-related assessment, leads to a single

149 pass/fail result for a specific product in that given application.
150
151
152
153
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154
155 PHOTOBIOLOGICAL SAFETY OF LAMPS AND LAMP SYSTEMS –
156 Part 7: Light sources and luminaires primarily emitting visible radiation
157
158
159
160 1 Scope

161 This part of IEC 62471 specifies an assessment of the photobiological safety of electrical light

162 sources and luminaires in normal use. The assessment is applied for electrical light sources

163 and luminaires that emit radiation predominantly in the visible spectral range (380 nm to

164 780 nm) and are used to illuminate spaces or objects or used for signalling.

165 Electrical light sources and luminaires designed for emitting radiation in the visible range can

166 also emit radiation in the ultraviolet (UV) and infrared (IR) regions depending on the technology

167 applied. The photobiological safety assessment in this document, therefore, includes the blue

168 light-, thermal-, UV-, UV-A-, IR- and skin thermal- hazards (includes optical radiation over the

169 wavelength range 200 nm to 3 000 nm).

170 Electrical light sources and luminaires that are designed to predominantly emit radiation outside

171 the visible spectral range (380 nm to 780 nm) (e.g. UV sterilizers or industrial heaters) are not

172 within the scope of this document.

173 Electrical light sources for illumination are considered to emit continuous light (pulse width

174 modulation (PWM) are included), therefore emission levels of continuous light (continuous wave

175 (CW)) are applied.

176 When there is no limitation on the presence of people, this document can be applied to electrical

177 light sources and luminaires which emit visible light, but the main purpose is not illumination or

178 signalling (e.g. horticulture).

179 This document also applies to laser products when the conditions of IEC 60825-1:2014, 4.4 are

180 met and they are used for illumination or signalling.
181 Note: See IEC 60825-1:2014 for other requirements of laser products.

182 This document is intended to be referenced by TC 34 product standards for the assessment of

183 applicable photobiological safety aspects. Additional details for the photobiological safety

184 assessment and data presentation are specified in the product standards.
185 2 Normative references

186 The following documents are referred to in the text in such a way that some or all of their content

187 constitutes requirements of this document. For dated references, only the edition cited applies.

188 For undated references, the latest edition of the referenced document (including any

189 amendments) applies.
190 IEC 60598-1:2020, Luminaires — Part 1: General requirements and tests
191 IEC 62471:2006, Photobiological safety of lamps and lamp systems

192 IEC 62504, General lighting — Light emitting diode (LED) products and related equipment —

193 Terms and definitions
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194 3 Terms and definitions

195 For the purposes of this document, the terms and definitions given in IEC 62471 and IEC 62504

196 apply with the following additions.

197 ISO and IEC maintain terminology databases for use in standardization at the following

198 addresses:
199 • IEC Electropedia: available at https://www.electropedia.org/
200 • ISO Online browsing platform: available at https://www.iso.org/obp
201 3.1
202 blue light hazard
203 BLH

204 potential for a photochemically induced retinal injury (photic maculopathy) resulting from optical radiation

205 exposure at wavelengths primarily between 400 nm and 500 nm.

206 Note 1 to entry: This damage mechanism dominates over the thermal damage mechanism for exposure duration exceeding

207 10 s.

208 Note 2 to entry: The action spectrum extends into the UV-A for persons without a normal UV-A absorbing lens.

209 [SOURCE: IEC 60050-845:2020, 845-26-055.]
210 3.2
211 exposure limit

212 maximum level of exposure of a surface, usually the eye or skin, that is not expected to result in adverse

213 biological effects

214 Note 1 to entry: Exposure limits for human safety of optical radiation, HL, are normally recommended by the International

215 Commission on Non-Ionizing Radiation Protection (ICNIRP).

216 Note 2 to entry: Exposure limits are often based on irradiance (e.g., for the skin), but where relevant, can also be based on

217 radiance (e.g., the blue light hazard of extended sources).
218 [SOURCE: IEC 60050-845:2020, 845-26-072.]
219 3.3
220 emission limit

221 specified maximum emission level of a source of optical radiation that is not expected to result in adverse

222 biological effect for a specific application

223 Note to entry: Evaluation of sources to the emission limits can be based upon reasonably foreseeable conditions of time-weighted

224 exposure. It incorporates both the concept of exposure duration and exposure distance and is derived from exposure limits.

225 [SOURCE: IEC 60050-161:1990, 161-03-012, modified – The domain has been deleted, the definition

226 has been adapted in relation to optical radiation and the Note to entry has been added.]

227 3.4
228 field of view
229 FOV

230 solid angle as "seen" by the detector (acceptance angle), e.g. of a radiometer or spectroradiometer, out

231 of which the detector receives radiation

232 Note 1 to entry: The field of view should not be confused with the angular subtense of the apparent source, α.

233 Note 2 to entry: A plane angle is sometimes used to describe a circular symmetric solid angle field of view.

234 Note 3 to entry: The field of view is expressed in steradian (sr).
235 [SOURCE: IEC 60050-845:2020, 845-25-077.]
236 3.5
237 illuminance
238 E

239 density of incident luminous flux with respect to area at a point on a real or imaginary surface

𝑑𝛷
240 𝐸 =
𝑑𝐴

241 where Φ is luminous flux and A is the area on which the luminous flux is incident

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242 Note 1 to entry: Illuminance can be derived from the spectral irradiance distribution by

243 𝐸 = 𝐾 ∫ 𝐸 (λ) · 𝑉(λ) · 𝑑(λ)
v m e,λ

244 where K is maximum luminous efficacy, E (λ) is the spectral irradiance at wavelength λ and V(λ) is spectral luminous efficiency.

m e,λ

245 Note 2 to entry: The corresponding radiometric quantity is "irradiance". The corresponding quantity for photons is "photon

246 irradiance".
247 Note 3 to entry: The illuminance is expressed in lux (lx = lm · m ).
248 [SOURCE: IEC 60050-845:2020, 845-21-060.]
249 3.6
250 infrared radiation
251 IRR

252 optical radiation for which the wavelengths are longer than those for visible radiation

253 Note 1 to entry: For infrared radiation, the range between 780 nm and 1 mm is commonly subdivided into:

254 IR-A: 780 nm to 1 400 nm, or 0,78 μm to 1,4 μm;
255 IR-B: 1,4 μm to 3,0 μm;
256 IR-C: 3 μm to 1 mm.

257 Note 2 to entry: A precise border between "visible radiation" and "infrared radiation" cannot be defined because visual sensation

258 at wavelengths greater than 780 nm can be experienced.

259 Note 3 to entry: In some applications the infrared spectrum has also been divided into "near," "middle," and "far" infrared; however,

260 the borders necessarily vary with the application.
261 [SOURCE: IEC 60050-845:2020, 845-21-004.]
262 3.7
263 irradiance

264 density of incident radiant flux with respect to area at a point on a real or imaginary surface

𝑑𝛷
265 𝐸 =
𝑑𝐴

266 where Φe is radiant flux and A is the area on which the radiant flux is incident

267 Note 1 to entry: The corresponding photometric quantity is "illuminance". The corresponding quantity for photons is "photon

268 irradiance".

269 Note 2 to entry: The irradiance is expressed in watt per square metre (W · m ).

270 [SOURCE: IEC 60050-845:2020, 845-21-053.]
271 3.8
272 electric light source

273 primary light source with the means for connecting to the power supply and usually designed to

274 be incorporated into a luminaire

275 Note 1 to entry: In IEC standards, “light source” and "lamp" are commonly used with the same meaning.

276 Note 2 to entry: An electric light source can be an electric lamp, or LED module designed to be connected by terminals, connectors,

277 or similar devices.

278 Note 3 to entry: For products that have the same physical characteristics as electric light sources for general lighting but that are

279 built to emit optical radiation (IEV 845-21-002) mainly in the IR or UV spectrum, the term "IR lamp" or "UV lamp" is often used.

280 [SOURCE: IEC 60050-845:2020, 845-27-004, modified – In Note 1 to entry, "and "lamp" are" has been

281 added and Note 3 to entry has been added.]
282 3.9
283 luminaire

284 apparatus which distributes, filters or transforms the light transmitted from at least one source of optical

285 radiation and which includes, except the sources themselves, all the parts necessary for fixing and

286 protecting the sources and, where necessary, circuit auxiliaries together with the means for connecting

287 them to the power supply

288 Note 1 to entry: A luminaire with integral non-replaceable lamps is regarded as a luminaire, except that the tests are not applied

289 to the integral lamp or integral self-ballasted lamp.

290 Note 2 to entry: In this document, evaluation of a luminaire is understood to mean evaluation of a luminaire with the intended

291 normal use of the specified light sources or with the light sources installed. For the selection of light sources IEC 60598-1:2020,

292 Annex B shall be served.
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293 [SOURCE: IEC 60050-845:2020, 845-30-001, modified – The Notes to entry have been added.]

294 3.10
295 optical radiation

296 electromagnetic radiation at wavelengths between the region of transition to X-rays (λ ≈ 1 nm) and the

297 region of transition to radio waves (λ ≈ 1 mm)

298 Note 1 to entry: Ultraviolet radiation in the wavelength range below 180 nm (vacuum UV) is strongly absorbed by the oxygen

299 in air. For the purposes of this document the wavelength band of optical radiation is limited to wavelengths greater than 200 nm.

300 Further, the eye transmits optical radiation to the retina between 300 nm and 1 400 nm. Thus, this wavelength range requires

301 special consideration in determining the photobiological safety of the retina.

302 [SOURCE: IEC 60050-845:2020, 845-21-002, modified – The note to entry has been added.]

303 3.11
304 radiance
305 L
306 L

307 density of radiant intensity with respect to projected area in a specified direction at a specified

308 point on a real or imaginary surface, expressed by
𝑑𝐼 1
309 𝐿 = ·
𝑑𝐴 𝑐𝑜𝑠𝛼

310 where I is radiant intensity, A is area, and  is the angle between the normal to the surface at

311 the specified point and the specified direction

312 Note 1 to entry: In a practical sense, the definition of radiance can be thought of as dividing a real or imaginary surface into an

313 infinite number of infinitesimally small surfaces which can be considered as point sources, each of which has a specific radiant

314 intensity, I , in the specified direction. The radiance of the surface is then the integral of these radiance elements over the whole

315 surface.

316 The equation in the definition can mathematically be interpreted as a derivative (i.e., a rate of change of radiant intensity with

317 projected area) and could alternatively be rewritten in terms of the average radiant intensity 𝐼 as:

𝐼 1
318 𝐿 = lim · ·
𝐴→0 𝐴 𝑐𝑜𝑠𝛼

319 Hence, radiance is often considered as a quotient of averaged quantities; the area, A, should be small enough that uncertainties

𝐼̅ 1

320 due to variations in radiant intensity within that area are negligible, otherwise, the quotient 𝐿 = · gives the average

𝐴 𝑐𝑜𝑠𝛼

321 radiance and the specific measurement conditions have to be reported with the result.

322 Note 2 to entry: For a surface being irradiated, an equivalent formula in terms of irradiance, E , and solid angle, , is

𝑑𝐸 1

323 𝐿 = · , where  is the angle between the normal to the surface being irradiated and the direction of irradiation. This

𝑑𝛺 𝑐𝑜𝑠𝜃

324 form is useful when the source has no surface (e.g., the sky, the plasma of a discharge).

𝑑

325 Note 3 to entry: An equivalent formula is 𝐿 = , where  is radiant flux and G is geometric extent.

𝑒 e
𝑑𝐺

326 Note 4 to entry: Radiant flux can be obtained by integrating radiance over projected area, Acos , and solid angle,  :

327  = ∬ 𝐿 · 𝑐𝑜𝑠𝛼 · 𝑑𝐴 · 𝑑
𝑒 𝑒

328 Note 5 to entry: Since the optical extent, expressed by G · n , where G is geometric extent and n is refractive index, is invariant,

329 the quantity expressed by Le · n is also invariant along the path of the beam if the losses by absorption, reflection and diffusion

330 are taken as 0. That quantity is called "basic radiance".

331 Note 6 to entry: The equation in the definition can also be described as a function of radiant flux,  . In this case, it is

332 mathemati
...