IEC TR 62471-4:2022
(Main)Photobiological safety of lamps and lamp systems - Part 4: Measuring methods
Photobiological safety of lamps and lamp systems - Part 4: Measuring methods
IEC 62471-4:2022, which is a Technical Report, provides manufacturers, test houses, safety personnel and others with practical guidance on methods to perform radiometric and spectroradiometric measurements to determine the level of accessible optical radiation emitted by lamps and lamp systems in accordance with IEC 62471.
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IEC TR 62471-4 ®
Edition 1.0 2022-09
TECHNICAL
REPORT
colour
inside
Photobiological safety of lamps and lamp systems –
Part 4: Measuring methods
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IEC TR 62471-4 ®
Edition 1.0 2022-09
TECHNICAL
REPORT
colour
inside
Photobiological safety of lamps and lamp systems –
Part 4: Measuring methods
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.140.01, 31.260 ISBN 978-2-8322-5734-0
– 2 – IEC TR 62471-4:2022 © IEC 2022
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 10
4 Application . 10
4.1 General . 10
4.2 Safety precautions . 10
4.3 Hazard assessment overview . 10
4.4 Selection of hazards . 11
4.5 Assessment levels . 11
4.6 Initial filtering . 12
4.7 Measurement quantities . 12
4.7.1 Emission wavelengths . 12
4.7.2 Irradiance . 13
4.7.3 Radiance . 14
4.7.4 Source size and location . 16
4.7.5 Temporal emission . 18
4.8 Measurement uncertainty . 18
5 Test conditions . 19
5.1 General . 19
5.2 Dark room (level A) . 19
5.3 Environmental conditions (level A) . 19
5.4 Power supply . 19
5.5 Product configuration . 19
5.5.1 General . 19
5.5.2 Warm up . 20
5.5.3 Measurement distance . 20
5.6 Optical alignment . 22
6 Performance characteristics: level A instruments . 22
6.1 General . 22
6.2 Spectral irradiance and radiance . 23
6.2.1 Spectral analysis . 23
6.2.2 Entrance optics . 25
6.2.3 Calibration standards . 26
6.3 Imaging devices . 27
6.4 Temporal emission . 27
6.5 Source size and location . 28
7 Performance characteristics: level B instruments . 28
7.1 General . 28
7.2 Irradiance or radiance . 28
7.2.1 General . 28
7.2.2 UV lines . 29
7.2.3 Narrow band sources . 29
7.2.4 Known spectral distribution . 29
7.2.5 Luminance-based . 30
7.3 Apparent source location and subtense . 31
7.4 Temporal emission . 31
Annex A (informative) Hazard selection . 32
Annex B (informative) Instrumentation description . 33
B.1 Double monochromators . 33
B.2 Single monochromators . 33
B.3 Array spectrometers . 33
B.4 Detectors . 33
B.5 Entrance optics . 34
B.6 Measurement geometries . 36
B.6.1 Irradiance . 36
B.6.2 Radiance . 36
B.7 2D imaging detector . 39
Annex C (informative) Extrapolation of spectral irradiance for thermal radiators . 41
Annex D (informative) Temporal emission measurement . 43
D.1 General . 43
D.2 Pulse duration . 43
D.3 Averaged irradiance and averaged radiance . 44
Annex E (informative) Uncertainty analysis . 47
Annex F (informative) Application examples . 48
F.1 General . 48
F.2 Example 1 – LED flashlight . 48
F.3 Example 2 – Infrared tungsten filament lamp . 49
F.4 Example 3 – Compact fluorescent lamp (CFL) . 51
F.5 Example 4 – LED bulb . 53
Annex G (informative) Stray radiation . 54
Annex H (informative) Report . 56
H.1 General . 56
H.2 Report. 56
Annex I (informative) Relationship between "true" source radiance and spatially
averaged radiance . 58
Bibliography . 62
Figure 1 – Schematic representation of irradiance measurement . 14
Figure 2 – Consideration of filling of FOV . 15
Figure 3 – Example of a direct measurement of radiance using a lens and aperture. 15
Figure 4 – Indirect measurement of radiance . 16
Figure 5 – Example of a rectangular source . 18
Figure 6 – Example of the non-uniform radiance distribution . 18
Figure 7 – Example of the emission profiles . 22
Figure B.1 – Examples of the diffuser optics . 35
Figure B.2 – Schematic representation of irradiance measurement . 36
Figure B.3 – Geometry of radiance measurement with a single thin lens . 37
– 4 – IEC TR 62471-4:2022 © IEC 2022
Figure B.4 – Geometry of a general radiance measurement . 38
Figure B.5 – Setup of the aperture stop behind the lens . 38
Figure B.6 – Setup of the aperture stop in front of the lens . 39
Figure B.7 – Example of a 2D imaging detector . 40
Figure D.1 – Example of temporal pulse wave . 44
Figure D.2 – Example of a colour-tunable white LED lamp . 44
Figure D.3 – A single pulse wave . 45
Figure D.4 – Example of a spectrally variable pulse . 46
Figure F.1 – Example of a LED flashlight . 48
Figure F.2 – Example of a radiance profile . 49
Figure F.3 – Spectral radiance distribution . 49
Figure F.4 – Example of an infrared tungsten filament lamp . 50
Figure F.5 – Example of a radiance profile . 50
Figure F.6 – Spectral radiance and irradiance distributions . 51
Figure F.7 – Radiance profile of the lamp . 51
Figure F.8 – Example of a compact fluorescent lamp (CFL) . 51
Figure F.9 – Example of a radiance profile . 52
Figure F.10 – Spectral radiance and irradiance distribution . 52
Figure F.11 – Example of a radiance profile . 53
Figure F.12 – Example of a LED bulb . 53
Figure I.1 – Usual measurement conditions for the determination of (time integrated)
radiance. 59
Figure I.2 – B(λ)-weighted radiance distribution of a phosphor-coated white LED
component . 60
Table 1 – Optical radiation hazards considered by IEC 62471 . 11
Table 2 – Recommended wavelength accuracy . 24
Table 3 – Recommended bandwidths . 24
Table A.1 – Examples of potential risk categories . 32
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOBIOLOGICAL SAFETY OF LAMPS AND LAMP SYSTEMS –
Part 4: Measuring methods
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 62471-4 has been prepared by IEC technical committee 76: Optical radiation safety and
laser equipment. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
76/654/DTR 76/707/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.
A list of all the parts in the IEC 62471 series, under the general title Photobiological safety of
lamps and lamp systems, can be found on the IEC website.
– 6 – IEC TR 62471-4:2022 © IEC 2022
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
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INTRODUCTION
Most lamps and lamp systems are safe and do not pose photobiological hazards except under
unusual exposure conditions, whilst a full photobiological safety assessment requires
sophisticated instrumentation and detailed analysis.
In order to provide a framework for the application of detailed measurement only where such is
necessary, this document introduces two measurement approaches. Level A encompasses high
accuracy, laboratory-based techniques whilst level B represents an estimation of the accessible
emission using readily available instrumentation.
– 8 – IEC TR 62471-4:2022 © IEC 2022
PHOTOBIOLOGICAL SAFETY OF LAMPS AND LAMP SYSTEMS –
Part 4: Measuring methods
1 Scope
This part of IEC 62471, which is a Technical Report, provides manufacturers, test houses,
safety personnel and others with practical guidance on methods to perform radiometric and
spectroradiometric measurements to determine the level of accessible optical radiation emitted
by lamps and lamp systems in accordance with IEC 62471.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62471:2006, Photobiological safety of lamps and lamp systems
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62471 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
accessible emission
level of radiation determined at a given distance and under measurement conditions defined in
IEC 62471
Note 1 to entry: The accessible emission is compared with the accessible emission limits to determine the
applicable risk group.
3.1.2
angular response
detector output signal as a function of input beam angle
3.1.3
aperture stop
opening that defines the area over which average optical emission is measured
3.1.4
entrance pupil
image of the aperture stop as seen through the object space in an optical system
Note 1 to entry: The entrance pupil defines the acceptance cone in the object space.
Note 2 to entry: If there is no lens in front of the aperture stop, the location and size of the entrance pupil are
identical to those of the aperture stop. Optical elements in front of the aperture stop can either magnify or diminish
the image and modify the location of the entrance pupil with respect to the physical aperture stop.
Note 3 to entry: The entrance pupil is important since the amount of optical radiation collected from the source
depends on this cone angle.
3.1.5
exit pupil
image of the aperture stop as seen through the image space in an optical system
Note 1 to entry: The exit pupil defines the acceptance cone in the image space.
Note 2 to entry: If there is no lens behind the aperture stop, the location and size of the exit pupil are identical to
those of the aperture stop.
Note 3 to entry: The exit pupil is important since the amount of optical radiation falling on the detector depends on
this cone angle.
Note 4 to entry: The cone angle is held constant in the luminance or radiance measurement at different object
distances.
3.1.6
field stop
opening that defines the solid angle over which average optical emission is measured
3.1.7
level A assessment
accurate determination of the accessible emission using sophisticated instruments in laboratory
conditions, performed by a trained operator
3.1.8
level B assessment
estimation of the accessible emission using readily available broadband radiometers or
photometers with minimum training
Note 1 to entry: Level B assessment can be used as a screening method to determine where further detailed
analysis is required without leading to the burden of measuring all sources.
3.1.9
measurement distance
distance between the (apparent) source or the closest point
of human access of the source under test and the entrance pupil
Note 1 to entry: If projection optics generate a virtual image of the emitter, the radiance measurement system
images the plane of this apparent source and not the plane of the closest point of human access.
3.1.10
measurement distance
distance between the (apparent) source or the closest point
of human access of the source under test and the aperture stop
3.1.11
spectral weighting function
function of the relative spectral effectiveness of optical radiation for a specified photobiological
effect in consideration of calculation of a weighted quantity, such as weighted radiance or
weighted irradiance
3.1.12
weighted irradiance
radiometric quantity obtained by multiplying spectral irradiance by a spectral weighting function
and integrating over the limits of the weighting function
– 10 – IEC TR 62471-4:2022 © IEC 2022
3.1.13
weighted radiance
radiometric quantity obtained by multiplying spectral radiance by a spectral weighting function
and integrating over the limits of the weighting function
3.2 Abbreviated terms
CCD charge-coupled device
CCT correlated color temperature
CMOS complementary metal–oxide–semiconductor
CW continuous wave
FOV field of view
GLS general lighting service
HID high-intensity discharge
LED light-emitting diode
NMI national metrology institute
PMT photomultiplier tube
4 Application
4.1 General
This document is intended to be used as a reference guide by (but not limited to) manufacturers,
testing laboratories, safety officers and officials of industrial or governmental authorities. It
contains interpretations of IEC 62471 and supplementary information relating to the practical
realization of radiometric measurements of lamps and lamp systems.
The procedures described in this document are adequate to meet the measurement
requirements of IEC 62471 where measurements are deemed to be required. The existence of
other equivalent measurement techniques, yielding results as valid as those described in this
document, is acknowledged.
In many cases, measurement may not be necessary. Compliance with the requirements of
IEC 62471 can be determined from an analysis of reported characteristics of the source and
the design of the product.
4.2 Safety precautions
Optical radiation emitted from a test lamp or lamp system may be potentially hazardous to the
operator's eyes and skin during the measurement. The existence of these hazards may be
unknown in advance of the measurement results, especially for UV and infrared sources. If in
doubt, it is recommended that the operator wear personal protection equipment to avoid
damage to eye or skin during measurement, e.g. gloves, goggles, protective clothing or masks.
In the special case of vacuum-UV sources, the formation of ozone in the measurement path
may produce additional hazards, so the test room should be appropriately ventilated.
It should be noted that the optical emission from the lamp or lamp system may be sufficiently
intense to cause damage to instrumentation and black (absorbing) material in the laboratory.
The risk of deterioration of non-metallic safety critical components, such as electrical wires, due
to long term exposure to UV, should be assessed.
4.3 Hazard assessment overview
IEC 62471 provides an assessment and classification scheme for the photobiological safety of
all electrically powered lamps and lamp systems emitting optical radiation in the wavelength
range from 200 nm to 3 000 nm. The assessment of optical radiation hazards considers
exposure of the skin, the anterior elements of the eye (cornea, conjunctiva and lens) and the
retina as detailed in Table 1. Lasers are excluded from this scope except laser products
designed to function as conventional light sources that meet the relevant criteria in
IEC 60825-1:2014, Clause 4.4.
Each hazard is accompanied with a measurement type, spectral range and, where applicable,
a hazard weighting function to account for the wavelength dependence of the hazard.
Table 1 – Optical radiation hazards considered by IEC 62471
Hazard Action Wavelength Measured Symbol for Units
spectrum range (nm) quantity emission level
−2
Actinic UV S (λ) 200 to 400 Irradiance E
W⋅m
UV s
−2
Near UV N/A 315 to 400 Irradiance E
W⋅m
UVA
−2 −1
Blue light B(λ) 300 to 700 Spatially- L
W⋅m ⋅sr
B
averaged
radiance
−2
Blue light, B(λ) 300 to 700 Irradiance E
W⋅m
B
small source
−2 −1
Retinal thermal R(λ) 380 to 1 400 Spatially- L
W⋅m ⋅sr
R
averaged
radiance
−2 −1
Retinal thermal, R(λ) 780 to 1 400 Spatially- L
W⋅m ⋅sr
IR
weak visual averaged
stimulus radiance
−2
IR radiation, N/A 780 to 3 000 Irradiance E
W⋅m
IR
eye
NOTE The naming of hazards differs between IEC 62471 and IEC 62471-5.
The methods employed to measure these quantities (described in Clause 5) are designed to
account for biophysical phenomena, including averaging over apertures or fields of view (FOVs)
which may be considered inappropriate for general radiometric measurements. Without taking
these measurement conditions into account, hazards may be overestimated.
This assessment may be performed on both lamps and lamp systems. IEC 62471 provides
guidance on the conditions under which the risk group classification may be transferred from
lamps to lamp systems.
4.4 Selection of hazards
Given prior knowledge of the source type, one can select which of the hazards presented in
Table 1 should be assessed for a particular product. The potential hazard categories relating
to a range of typical sources are listed in Table A.1 in Annex A.
If in doubt, the full spectral range of emission can be measured before performing more detailed
analysis.
4.5 Assessment levels
Most lamps and lamp systems are safe and do not pose photobiological hazards except under
unusual exposure scenarios. There remain, however, some lamps and lamp systems for which
the emitted optical radiation has the potential to pose adverse health effects. It follows that the
most detailed measurement need only be performed in the latter case. The assessment
approach recommended in this document is based on two assessment levels, termed level A
and level B.
– 12 – IEC TR 62471-4:2022 © IEC 2022
Level A represents the most accurate determination of the accessible emission using
sophisticated spectroradiometric equipment and can be used in all cases. The instrumentation
description is given in Annex B.
Level B measurements are made with readily available instruments, such as broadband
radiometers or photometers with minimum training. Level B may then be considered as an initial
screen to determine where detailed analysis is required. The application of level B is not suitable
for those sources known to be potentially hazardous (including UV lamps) nor for consideration
of the retinal thermal hazard, where required.
As with all measurements, a consideration of measurement uncertainty is important, particularly
in the case of a level B assessment if the estimated accessible emission is close to the
accessible emission limit. If a level B assessment does not clearly designate the risk group,
then proceeding to a level A assessment is recommended.
Examples of the risk group determination of several lamps are shown in Annex F.
4.6 Initial filtering
4 −2
In the case of sources emitting white light, if the luminance of the source is below 10 cd∙m ,
detailed analysis is not required. The luminance of the source can be obtained by measurement
or calculation of reported parameters.
A luminance meter, with the FOV selected to be smaller than the emission area of the source,
will report the value directly.
Luminance can be estimated from the illuminance, E , measured at a distance, D, from the
v
source and the estimated luminous area of the source, A.
ED⋅
v
L = (1)
v
A
Using data sheet values, luminance can be computed from luminous intensity, I (cd) by
v
Equation (2) and from luminous flux, Φ (lumen), luminous area, A and beam half emission
v
angle, θ, from Equation (3).
I
v
L = (2)
v
A
Φ
v
L = (3)
v
2π ⋅⋅A (1− cosθ )
4.7 Measurement quantities
4.7.1 Emission wavelengths
Given the wide spectral range over which photobiological safety hazards are considered and
the wavelength dependence of those hazards, spectral analysis of the emission of lamps and
lamp systems is recommended. In the case of the UV, blue light and retinal thermal hazards for
which the spectral weighting functions are strongly wavelength dependent, detailed spectral
measurements may be required.
Whilst the spectral range of application of IEC 62471 is 200 nm to 3 000 nm, spectral
measurements beyond 1 400 nm are not required, but may be used.
The range of hazards considered for a particular product can be reduced given prior knowledge
of the source type.
The potential hazard categories relating to a range of typical source types are listed in Table A.1.
4.7.2 Irradiance
The following description applies both to broadband irradiance and spectral irradiance
measurements.
Measurement of the irradiance produced at a given distance by a lamp or lamp system is
required to assess the following hazards:
• actinic UV hazard, E ;
S
;
• near UV hazard, E
UVA
• retinal blue light hazard (small source), E ;
B
• IR radiation eye hazard, E .
IR
The instrument should:
a) have a plane circular entrance aperture of diameter between 7 mm and 50 mm;
b) accept radiation within a circular cone whose centreline is normal to the plane of the
entrance aperture;
c) have an angular spatial response varying as the cosine of the angle (up to the acceptance
angle) from the normal to the detector area.
Whilst in general radiometry, the irradiance geometry requires a hemispherical cosine response,
in consideration of biophysical phenomena the full acceptance angle (γ) is limited to 1,4 rad.
Where a source subtends an angle greater than 1,4 rad at the measurement distance, a field
stop should be placed at the source to ensure measurement under 1,4 rad. Where the source
cannot be accessed, the aperture can be moved toward the entrance aperture, as in Figure 1.
The entrance aperture of the optic serves as an averaging aperture and should be selected in
consideration of the spatial uniformity of irradiance in the measurement plane. For sources that
do not produce a spatially uniform irradiance, for example narrow beam reflector lamps, the
peak irradiance may be significantly higher than that obtained by measurement using a wide
entrance aperture. In such cases, a 7 mm entrance aperture diameter should be used.
– 14 – IEC TR 62471-4:2022 © IEC 2022
Figure 1 – Schematic representation of irradiance measurement
4.7.3 Radiance
The following description applies both to broadband radiance and spectral radiance
measurements.
Measurement of the spatially averaged radiance in a defined FOV and at a given distance of a
lamp or lamp system is required to assess the following hazards:
• retinal blue light hazard, L ;
B
• retinal thermal hazard, L ;
R
• retinal thermal hazard (weak visual stimulus), L .
IR
This measurement accounts for biophysical phenomena including pupil constriction and eye
movements. With increasing exposure time, the image produced on the retina is spread over a
larger area, defined by a time-dependent angular subtense of the area of the retina irradiated.
The retinal irradiance for a given exposure time can be obtained from a radiance measurement
in a FOV that matches the time-dependent angular subtense.
Specific FOVs are specified in IEC 62471 according to hazard and risk group and for pulsed
sources.
The measurement FOV is independent of the size of the source.
For sources that subtend an angle smaller than the specified FOV, as depicted in Figure 2 b),
the spatially averaged radiance value might be smaller than the radiance of the source. This
biologically effective value of spatially averaged radiance is the appropriate value to be
compared to the emission limits. Since the spatially averaged radiance is dependent on the
overlap of the measurement FOV with the spatial radiance profile in the plane of the source,
care should be taken when measuring at a different distance than the assessment distance.
Further guidance is provided in 5.5.3, and further information in Annex I.
a) Source size larger than the FOV (over-filled) b) Source size smaller than the FOV (under-filled)
Figure 2 – Consideration of filling of FOV
Radiance can be measured in one of two ways: using a direct imaging technique or indirectly
through a measurement of irradiance under defined geometry to set the FOV.
Direct radiance measurements are performed with an optical system similar to that in Figure 3.
The instrument should:
• have a plane circular entrance pupil of diameter between 7 mm and 50 mm;
• image the radiant source onto a detector;
• have a circular field stop of diameter, d, to establish the specified averaging FOV;
• have a means of accommodating the image distance, H, according to measurement
distance.
The entrance pupil of the optic serves as an averaging aperture and should be selected in
consideration of the spatial uniformity of irradiance in that plane. For sources that do not
produce a spatially uniform irradiance, for example narrow beam reflector lamps, the peak
radiance may be significantly higher than that obtained by measurement using a wide entrance
pupil. In such cases, a 7 mm entrance aperture pupil should be used.
The entrance pupil might be a real or virtual image when the aperture stop is located behind
the imaging lens. See B.6.2 for further details.
Figure 3 – Example of a direct measurement of radiance using a lens and aperture
– 16 – IEC TR 62471-4:2022 © IEC 2022
Indirect radiance measurements are performed using an irradiance entrance optic as described
in 4.7.2 and B.6.1, the FOV being defined by placing an aperture (serving as field stop)
sufficiently close to the apparent source to produce the required FOV (see Figure 4).
The diameter, f, of the field stop is computed from:
γ
f= 2⋅⋅D tan ≈ D⋅γ , for small γ (4)
Where
D is the measurement distance;
γ is the specified averaging FOV (in rad).
The radiance, L, is then computed from the measured irradiance, E, from:
E
L = (5)
Ω
Where the solid angle, Ω, corresponding to the FOV planar angle, γ, is computed from:
ππ⋅⋅f γ
Ω ≈
, for small γ (6)
4D
This method can only be applied for narrow FOVs (as is the case in the IEC 62471 assessment)
and where the (apparent) source can be accessed directly, otherwise the physical aperture acts
as an aperture stop rather than a field stop.
Figure 4 – Indirect measurement of radiance
4.7.4 Source size and location
4.7.4.1 General
Measurement of the (apparent) source size at the measurement distance and location with
respect to the closest point of human access may be required to assess the following hazards:
• retinal blue light hazard, L ;
B
=
• retinal thermal hazard, L ;
R
• retinal thermal hazard (weak visual stimulus), L .
IR
In consideration of the blue light hazard, if the source is entirely over-filled by the 0,011 rad
FOV at the given measurement distance, the simpler irradiance-based blue light small source
criteria should be applied. This does not require specific measurement of the source size.
In consideration of the retinal thermal hazard, the angular subtense of the source at the
measurement distance is required to compute the accessible emission limit.
If projection optics generate a virtual image of the emitter, the size of the apparent source
should be considered, and the radiance measurement system should image the plane of this
apparent source.
4.7.4.2 Determination of source location
An imaging technique should be applied to determine the position of the apparent source with
respect to the closest point of human access, achieved by fixing the camera focus and
translating the camera between two positions to image the apparent source or the closest point
of human access.
4.7.4.3 Determination of source size
An imaging technique should be applied to determine the (apparent) source size and location,
ensuring the resulting image is not saturated.
The average angular subtense, α, is computed from the dimension of the 50 % of the peak
emission points of the source (physical or apparent) and the assessment distance, subject to
limiting angular subtense, α and α , for continuous wave and pulsed sources.
min max
Where the source subtense is less than α it should be set to equal α ; likewise, where the
min min
source subtense is greater than α it should be set to equal α .
max max
Where the source is spatially uniform, the dimensions of the 50 % emission points can be
determined and the angular subt
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