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IS0
I N T ER N AT1 O N A L ORGAN IZATl O N FOR STA N DARD I ZAT I O N
IS0 RECOMMENDATION
R5
DIFFUSE TRANSMISSION DENSITY
(PHOTOGRAPHY)
ISf EDITION
October 1955
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
This IS0 Recommendation R 5 was prepared by Technical Committee
ISO/TC 42 - Photography, the Secretariat of which is held by the American
Standards Association, Inc. (ASA).
In April 1948, the Secretariat asked the General Secretariat to submit, as
a Draft Proposal, for study by the Technical Committee, the American Standard
Z 38.2.5 - 1946, concerning diffuse transmission density. This Draft Proposal
was distributed on 12 May, 1948, to all the Members of the Technical Committee.
In view of the fact that no observations were received, the Secretariat considered
the Draft Proposal as adopted by the Technical Committee as a Draft IS0
Recommendation.
In January 1951, this Draft IS0 Recommendation was submitted to the
IS0 Member Bodies by the General Secretariat. Out of 29 IS0 Member Bodies,
4
the following 17 sent in their approval:
Australia Mexico Switzerland
Austria Netherlands Union of South Africa
Belgium New Zealand United Kingdom
Czechoslovakia Poland U.S.A.
Finland Portugal Yugoslavia.
Italy Sweden
The following 2 Member Bodies stated that they had no objection to the
approval of the Draft:
Denmark
France
The ISO/TC 42 Secretariat amended the Draft, taking account of the obser-
vations made by various Member Bodies.
The revised text was then submitted by correspondence to the IS0 Council
Memberswho decided,in May 1954, to accept it as an IS0 RECOMMENDATION.
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lSO/R 5 . 1954 (E)
.
I
IS0 Recommendation May 1954
I
DIFFUSE TRANSMISSION DENSITY
(PHOTOGRAPHY)
CONTENTS
Page
Introduction . 5
Discussion of Density and Explanation of Terms . 5
Diffuse Density . 5
Specular Density . 5
Doubly Diffuse Density . 5
Calibration of Practical Densitometers . 6
IS0 Recommendation for Diffuse Transmission Density . 7
................................
1 . Scope 7
2 . General Definition of Density . 7
3 . Totally Diffuse Density . 7
4 . IS0 Recommended Diffuse Density . 7
5 . IS0 Recommended Diffuse Visual Density . 7
6 . IS0 Recommended Diffuse Printing Density . 8
7 . Integrating Sphere Method . 8
7.1 General . ._. . 8
7.2 Apparatus . 9
7.3 Procedure When Inverse Square Law is Used . 9
7.4 Procedure €or Using the Polarization Photometer . 11
8 . Opal Glass Method . 13
8.1 General . 13
............................ 13
8.2 Apparatus
8.3 Procedure . 13
9 . Contact Printing Method . 13
9.1 General . 13
9.2 Apparatus . 14
9.3 Procedure . 15
9.4 Qualifying Tests for Apparatus and Procedure . 17
Appendix . 18
Al Geometric Types of Density . 18
A2 Spectral Types of Density . 18
A3 Complete Density Classification . 20
Table 1 . Spectral Conditions for IS0 Recommended Diffuse Visual Density. Type
Vl-b . 7
Table 2 . 8
Spectral Conditionsfor IS0 Recommended Diffuse Printing Density. Type P2-b
Table 3 .
Relationship Between Specimen Density D. and Relative Light Source
to Test Frame Distance Xo . 17
Fig . 1 . Apparatus for Integrating Sphere Method Using Inverse Square Law . 10
Fig . 2 . 10
Apparatus for Integrating Sphere Method Using Polarization Photometer
Fig . 3 . Martens Polarization Photometer Scale . 12
Fig . 4 . 12
Apparatus for Opal Glass Method .
Fig . 5 . Conversion Chart for Obtaining IS0 Recommended Diffuse Density from
Opal Density .
13
Fig . 6 . 14
Apparatus for Contact Printing Method .
Fig . 7 . D vs log E Curve for a Hypothetical Paper Used in the Contact Printing
Method .
16
Figs :E I Totally Diffuse Density .
............... 19
9a
Figs 9b 1 Doubly Diffuse Density . 19
10a
Figs I Specular Density . 19
Fig . 11 . Totally Diffuse Density . 19
Fig . 12 . Relationship of the Various Types of Transmission Density . 22
I
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ISO/R 5 - 1954 (E)
INTRODUCTION
Discussion of Density and Explanation of are obtained when the incident radiant flux is
perfectly diffuse and the specularly transmitted
Terms. Transmission density is defined in
component is collected and evaluated.
general terms as the common logarithm of the
ratio of the radiant flux incident on the sample
Specular Density results when the radiant
to the radiant flux transmitted by the sample.
flux is incident normally on the sample and only
When radiant flux is incident on an exposed
the normal component of the transmitted flux
and processed photographic film or plate, part
is collected and evaluated.
of the flux is reflected, part is absorbed, and
Doubly Diffuse Density is obtained when the
part is transmitted, the transmitted flux usually
radiant flux incident on the sample is com-
being scattered. In practice the receiver may
pletely diffuse and all of the transmitted flux
collect all or only a portion of the transmitted
is collected and equally evaluated. These three
flux depending on the nature of the receiver and
types of density are discussed more fully in
its position relative to the sample. Similarly,
the Appendix.
as the sample is removed, the receiver may
The present recommendation is concerned
collect all or only a portion of the incident flux.
only with diffuse density. This type of density
Morcover, the incident flux may be diffuse, or
is closely related to many practical applica-
semi-diffuse, or it may be a parallel beam inci-
tions of photographic materials (see Appendix).
dent at an angle.
The characteristics of photographic films are
Considering some of these possible variations
frequently expressed in terms of diffuse den-
in the geometrical arrangement of the optical
sity and the term is often used in photographic
system alone, it is apparent that a multiplicity
literature, but it is seldom defined precisely.
of numerical values of density can be obtained
The present recommendation provides a defi-
for a given sample depending on how the meas-
nition (see Section 3) for this type of density
urements are made. In any specified geometric
and introduces the term “totally diffuse density”
condition the effective value of density will also
as a means of indicating that all of the light
depend on both the color quality of the light
is collected.
and the color sensitivity of the receiver, if the
In totally diffuse density the incident radiant
sample is spectrally selective. To avoid con-
flux is normal to the plane of the sample, and
fusion it is desirable to standardize certain
all of the transmitted flux is collected and
specific methods of measurement.
equally evaluated; or, the incident flux is
The problem of establishing a standard of
perfectly diffuse and only the normally trans-
density can be divided into two parts:
mitted flux is collected and evaluated. Moreover,
(a) Geometric. Specification of the geometric
the term signifies that the effects of reflections
characteristics of the optical system used in
between the sample and any part of the appa-
the measurement.
ratus (cover glasses, surfaces of the receiver
(b) Spectral. Specification of the spectral
or illuminator, etc.) are negligible and that stray
.-
sensitivity of the receiver and the spectral
radiation, room light, etc. are excluded.
energy distribution of the radiant flux incident
When the density of photographic films or
on the sample.
plates is measured, the diffusion requirements
Variations in the geometrical arrangement
are better satisfied if the emulsion side of the
of the optical system give rise to a number of
sample faces the receiver which is to collect
geometric types of density which generally
the transmitted flux. When the incident radia-
lead to different numerical density values.
tion is diffuse, the emulsion side of the sample
Among these are found three distinct and funda-
should face the diffuser.
mental types :
The conditions required for the measurement
(a) Diffuse density,
of totally diffuse density cannot be met per-
(b) Specular density,
fectly, but can be approached very closely, in
(e) Doubly diffuse density.
instruments and apparatus designed for the
purpose.
Diffuse Density is obtained when the radiant
Since the theoretically ideal conditions for
flux is incident normally on the sample and all
of the transmitted flux is collected and equally totally diffuse density are never met perfectly
in practice, the term “IS0 Recommended
evaluated. The phrase “collected and equally
diffuse density” has been chosen to designate
evaluated” means that the effect on the receiver
of all the rays transmitted by the specimen is densities which have been determined under
the practical geometric conditions provided
the same regardless of the angle of emergence.
by the apparatus and methods described in
Experimental studies show that the same results
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~
lSO/R 5 - 1954 (E)
this recommendation. The recommended ap-
the incident radiant flux and indicates that the
paratus and methods are especially appropriate, spectral quality is that of a tungsten lamp opera-
however, since they give values of density so
ting at a color temperature of 3 O00 “K.
closely approaching totally diffuse density that
“Diffuse photoelectric density” and “diffuse
no errors of practical significance arise when the spectral density” * (diffuse density for a single
apparatus is constructed and used as prescribed wavelength) designate other spectral classes of
in this recommendation.
diffuse density which are not covered in detail
For a sample which is spectrally nonselective in the present recommendation. These spectral
the specification of the geometric conditions is
classes of density are, however, important in
sufficient since this specification will lead to a
certain practical work. Their relation to the
unique value of density. However, for samples present recommendation is described in the
which are spectrally selective, it is necessary to
Appendix and illustrated in Figure 12.
consider, in addition, the spectral conditions.
Calibration of Practical Densitometers. With
Variations in the spectral conditions give rise
the establishment of this recommendation it
to a number of spectral types of density for any
becomes practicable to calibrate a number of
given geometric type. Although other important
photographic samples by one of the approved
uses of photographic silver images are found in
measuring methods and to use these as reference
practice, photographic films and plates are
specimens for calibrating ordinary densito-
usually either viewed by the human eye or
meters.
printed on positive photographic materials.
In general only those densitometers which
Therefore, the present recommendation specifies
conform to the geometric and spectral conditions
in detail only two spectral types of diffuse
specified in this recommendation are capable of
density, namely :
giving accurate readings of IS0 Recommended
Diffuse visual density, Type VI-b
diffuse visual density or IS0 Recommended
Diffuse printing density, Type P2-b
diffuse printing density for all types of photo-
graphic materials. However, many simple
The significance of these terms will be clarified
densitometers give readings on different photo-
by reference to the density chart in the Appendix.
graphic materials with sufficient accuracy for
The term “visual” is used to indicate that the
most practical work. The scope of such instru-
receiver of the transmitted flux is either the
ments can be tested by measuring samples which
human eye or has a spectral sensitivity equal
vary in scattering power and in spectral selectiv-
to it. In the first classification the term “Type
ity and comparing these results with those
V1-b” indicates that the spectral conditions have
obtained by the appropriate recommended
been particularized even further. Type V1
method.
refers to the spectral sensitivity of the average
If a nonconforming densitometer is to be used
normal human eye, a representative curve for
for a large amount of routine work in connection
which has been standardized by the International
with a given type of photographic material, it
Commission on Illumination. The “b” in
may be calibrated from reference samples or a
Type V1-b refers to the spectral energy distri-
reference wedge composed of the same material.
bution of the incident radiant flux and indicates
In this way any type of densitometer can be
that the spectral quality is that of a tungsten
IS0 Recommended diffuse
calibrated to read
lamp operating at a color temperature of
visual density or IS0 Recommended diffuse
3 O00 “K.
printing density for any single type of photo-
Similarly, in the second classification, the
graphic material, to a degree of accuracy
term “printing” is used to indicate that the
commensurate with the stability and repro-
receiver of the transmitted flux is either the
ducibility of the instrument itself. In general,
or has a spec-
photographic printing material
a new calibration must be made if accurate
tral sensitivity equal to it. Type 2, used in
readings are desired on a different photographic
connection with printing density, refers to a
material when a nonconforming densitometer
spectral-sensitivity curve representative of
is used.
commonly used photographic printing papers.
This spectral sensitivity is specified in detail in
* See Report of Committee on Colorimetry, Jorcrnal of
this recommendation. The “b” in Type P2-b
the Optical Society of America, 34, 4, 188, Section 8, “Trans-
again refers to the spectral energy distribution of
mittance, Opacity and Density”.
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lSO/R 5 - 1954 (E)
IS0 Recommendation for Diffuse Transmission Density
means and methods. These conditions approach
1. Scope
the ideal conditions for totally diffuse density
1.1 The principal purpose of this recommen-
given in Section 3.1.1 as closely as practical
dation is to define diffuse transmission density
equipment and methods permit.
and specify techniques for its measurement.
4.1.1.1 The integrating sphere method is
1.2 It applies primarily to processed black-and-
described in detail in Section 7.
white photographic films and plates, although
4.1.1.2 The opal glass method is described
it can also be applied to other radiation-
in detail in Section 8.
absorbing media such as cast colloidal carbon-
or filters, filters consisting of dyes
gelatin tablets 4.1.1.3 The contact printing method is
in gelatin, glass filters, or various types of radia-
described in detail in Section 9.
tion-absorbing screens used in photographic
Any spectral conditions may be associated
work where diffuse-density measurements are
with IS0 Recommended diffuse density.
desired.
5. IS0 Recommended Diffuse Visual
2. General Definition of Density
. Density
2.1 Density is defined in general terms as the 5.1 Definition. IS0 Recommended diffuse
logarithm of the ratio of the radiant flux Po visual density, Type V1-b, is a particular
spectral type of IS0 Recommended diffuse
incident on the sample to the radiant flux Pt
transmitted by the sample. density and is defined by the expression in
Section 4.1 when the spectral conditions which
follow are fulfilled :
5.1.1 The product of the relative spectral
sensitivity of the receiver of the radiant flux
3. Totally Diffuse Density
times the relative energy of the incident radiant
flux at each wavelength shall be proportional
3.1 Definition. Totally diffuse density is defined
by the expression in Section 2 when the following to the product of the sensitivity and energy
given in logarithmic form in column 4 of
conditions are fulfilled :
Table 1.
3.1.1 Geometric Conditions
TABLE 1
3.1.1.1 The incident radiant flux shall be
Spectral Conditions for
normal (at an angle of 90 degrees) to the plane
IS0 Recommended Diffuse Visual Density
of the sample and all of the transmitted radiant
Type V1-b
flux shall be collected and equally evaluated;
1 2 3 4
or the incident radiant flux shall be perfectly
_~
diffuse and only the normally transmitted com-
log
(Relative
Wave-
log
ponent shall be collected and evaluated. 1%
Sensitivity
length (Relative (Relative
times
Sensitivity*) Energy)
mu
3.1.1.2 The effects of reflections between
Energy)
~ .~
the sample and parts of the apparatus (cover
glasses, surfaces of the illuminator or collector,
0.00 0.00 0.00
400
etc.) shall be negligible.
1.11
420 1.00 0.11
0.22 1.98
440 1.76
3.1.1.3 Stray radiation shall be negligible.
0.31 2.49
460 2.18
3.1.2 Spectral Conditions. Any spectral con-
2.94
480 2.54 0.40
ditions may be associated with totally diffuse
0.48 3.39
500 2.91
3.81
density. The geometric conditions can usually 520 3.25 0.56
0.64 4.02
540 3.38
be fulfilled independently of the spectral con-
4.11
3.40 0.71
560
ditions. If the sample is spectrally nonselective,
4.11
580 3.34 0.77
the specification of the geometric conditions is
3.20 0.83 4.03
600
sufficient for the unique evaluation of density.
3.86
620 2.98 0.88
0.94 3.58
640 2.64
0.99 3.17
660 2.18
4. IS0 Recommended Diffuse Density
2.66
680 1.63 1.03
1.08 2.09
4.1 Definition 700 1.01
4.1.1 The term “IS0 Recornmended diffuse
density” designates densities determined under * For the purposes of this recommendation the relative
spectral sensitivity of the receiver is defined in general
the practical geometric conditions provided by
terms as the reciprocal of the relative energy necessary
any one of the following three recommended
to produce a given response.
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ISO/R 5 - 1954 (E)
5.1.2 The tolerances on the spectral char- TABLE 2
acteristics of the system shall be such that the
Spectral Conditions for
resulting numerical values of density will not
IS0 Recornmended Diffuse Printing Density
be significantly different from those which
Type P2-b
would be obtained if the spectral requirements
3 4
were perfectly met. *
log
5.1.3 The relative sensitivity values given in
(Relative
Wave-
log
logarithmic form in column 2, Table 1, are those Sensitivity
length (Relative
times
Sensitivity*)
mu
adopted by the International Commission on
Energy)
Illumination, for the average normal human eye
adapted to photopic vision. 340 2.00 0.00 2.00
0.11 4.05
350 3.94
5.1.4 The relative energy values given in
360 4.77 0.22 4.99
logarithmic form in column 3, Table 1, are for a
4.94 0.31 5.25
370
tungsten lamp operating at a color temperature
380 5.00 0.40 5.40
of 3 000 “K. 390 5.00 0.48 5.48
5.54
400 4.98 0.56
5.1.5 These requirements permit the use of
410 4.94 0.64 5.58
filters in combination with various sources and
0.71 5.61
420 4.90
receivers provided that the overall spectral
4.84 0.77 5.61
430
characteristics of the combination conform with 440 4.76 0.83 5.59
0.88 5.54
those specified in column 4, Table 1. 450 4.66
460 4.52 0.94 5.46
0.99 5.34
470 4.35
6. IS0 Recommended Diffuse Printing
480 4.13 - 1.03 5.16
Density 490 3.85 1.08 4.93
4.56
500 3.44 1.12
6.1 Definition. IS0 Recommended diffuse
510 2.81 1.15 3.96
printing density, Type P2-b, is one spectral
520 2.18 1.19 3.37
type of IS0 Recommended diffuse density and
1.55 1.22 2.77
530
is defined by the expression in Section 4.1 when 540 0.00 1.26 1.26
the following spectral conditions given in para-
graph 6.1.1 are fulfilled.
* For the purposes of this recommendation the relative
spectral sensitivity of the receiver is defined in general
6.1.1 The product of the relative spectral
terms as the reciprocal of the energy necessary to produce
a given response. In the case of photographic receivers
sensitivity of the receiver of the radiant flux at
used in this particular recommendation, the spectral sen-
each wavelength times the relative spectral-
sitivity shall be measured in terms of the reciprocal of the
energy distribution of the radiant flux incident energy necessary to produce a reflection density equal to
that used in the photographic photometry of Section 9.3.G
on the sample shall be proportional to the pro-
(null point check). This reflection density corresponds
duct of sensitivity and energy given in loga-
approximately to a point on the density-log exposure curve
of the photographic material where the gradient is a maxi-
rithmic form in the last column of Table 2.
mum (see Fig. 7). This measurement of sensitivity is intended
primarily for use in this recommendation, and may not
6.1.2 The tolerance on the log relative
be applicable in other problems.
sensitivity times energy values in column 4 of
Table 2 shall be such that the resulting numerical transmission of glass optics at short wavelengths
values of density will not be significantly
and the transmission band of silver deposits at
different from those which would be obtained 320 millimicrons.
if the spectral requirements were perfectly met.
6.1.4 The log relative-energy values given in
6.1.3 The relative sensitivity values given in 3 in Table 2 are for a tungsten lamp
column
logarithmic form in column 2 of Table 2 will be operating at a color temperature of 3 O00 OK.
the logarithm of the product of an average of the
6.1.5 The use of filters in combination with
relative spectral sensitivities of commonly used
various sources and receivers is permissible
photographic printing materials times the trans-
provided the overall spectral characteristics of
mission of an ultra-violet absorbing filter which
the combination conform with those given above.
has a sharp cut-off at 360 millimicrons. The
filter has been included in order to minimize
7. Integrating Sphere Method
errors which might arise because of the uncertain
7.1 General. This method is approved because
* When the samples are spectrally nonselective, the
it provides means for measuring density either
product of the energy of the source and the sensitivity
and energy of the receiver and source actually used is not visually or objectively with a high degree of
critical and may depart widely from values given in the
reproducibility and gives IS0 Recommended
column headed “log Relative Sensitivity times Energy”
diffuse density values directly. The integrating
given in Table 1 without significantly affecting the results.
Since photographic films and plates developed in ordinary,
sphere method has been described in the
nonstaining developers are often sufficiently nonselective,
literature. *
the spectral conditions given in Table 1 need not be in close
agreement with the ideal values. However, the measure-
of the effective density of a sharpcutting yellow color * “Standardization of Photographic Densitometry”. CI€-
ment
ton Tuttle and A. M. Koerner, Journal of the Society of
filter, for example, will require relatively close agreement
Motion Picture Engineers XXIX, No. 6, December, 1937.
with the ideal spectral values given in Table 1.
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lSO/R 5 - 1954 (E)
The integrating sphere, made and used law shall include a straight track or photometric
according to the following specifications, pro- bench on which to move the source of radiation
vides the desired geometric conditions. Modu- in a straight line, lying on the optic axis of the
lation of the radiant flux is effected by means sphere (see Fig. 1). The length of the track shall
of either the photometric inverse square law be not less than 2 meters and shall be great
(Fig. 1) or the Martens type polarization photo- enough to permit the measurement of the dis-
meter (Fig. 2). The method is approved for the tance from the source to the sphere with an
measurement of IS0 Recommended diffuse accuracy of 0.2 percent. In no case shall the
density with the following specifications. source be used nearer the sphere than 10 times
the greatest linear dimension of the source or
7.2 Apparatus
the aperture through which the light enters the
7.2.1 The diameter of the sphere shall be sphere. The ratio of the intensity of the com-
greater than 3.5 inches (90 mm). parison source to that of the main source shall
be constant to within f 0.4 percent. In cases
7.2.2 The sum of the areas of the openings
where the eye serves as the receiver, the spectral
in the sphere shall be less than 2 percent of the
quality of the radiation from the comparison
area of the sphere wall.
source shall be such that when it is used with
7.2.3 The aperture at the sample shall be
the translucent diffusing screen as shown in
bounded by knife edges so as not to hinder light
Fig. 1, the spectral quality of the radiation
from reaching the sample area at grazing angles
emitted by the screen will be approximately
of incidence.
equal to that of the radiation emitted by the
7.2.4 The screen used inside the sphere shall
sphere when no sample is present. It is per-
be elliptical and just large enough to shield the missible to use a filter in combination with the
light spot in the sphere from the sample.
translucent screen in order to achieve the
desired equality in spectral composition.
7.2.5 The interior wall of the sphere shall be
coated with two coats of a suitable integrating
7.2.12 A Lummer-Brodhun type photometric
sphere paint * applied over flat white undercoat
cube * is recommended for visual work in
of oil paint.
conjunction with the inverse square law.
7.2.6 The diffusion coefficient ** of the
7.2.13 When the Martens type polarization
sphere shall be 0.98 to 1.02.
photometer is used with the integrating sphere,
the general arrangement of parts shown in
7.2.7 Angular deviation from normal of the
Fig. 2 shall be followed. The lens system
light collected by the receiver shall not exceed
between the source and the sphere is not
10 degrees.
essential but it is recommended since it gives
7.2.8 Suitable lamp houses, diaphragms, and
an increase in the brightness of the photometric
shields shall be used to reduce stray radiation
field without an increase in the size of the area
to such an extent that its effect is negligible.
covered by the beam on the sphere wall. Any
7.2.9 The source of radiation shall be
clear bulb tungsten lamp which is operated
sufficiently intense for the flux reaching the
under such conditions as to give a color tempe-
receiver to be adequate for efficient operation.
rature of 3 O00 OK may be used**.
For visual work the luminance of the photo-
7.3 Procedure when Inverse Square Law is Used
metric field shall be not less than 1 foot-Lambert
(3.4 nits). The spectral energy distribution
7.3.1 Precautions shall be taken to prevent
of the radiation from the source shall be ap-
stray radiation from entering the sphere, from
propriate to the spectral type of density desired.
falling on the sample, or from entering the
See paragraphs 4.2.2, 5.1.2, and 6.1.2.
photometric cube.
7.2.10 The receiver of the flux shall be a
7.3.2 With no sample in the beam the
suitable radiation-sensitive device having a
movable source (see Fig. 1) shall be set at a
spectral sensitivity appropriate to the spectral
distance, lo, such that the two halves of the
type of density desired. See paragraphs 4.2.2, photometric field are balanced.
The intensity
5.1.2, and 6.1.2.
of the comparison field shall be adjusted so that
Io is not less than 2 meters.
7.2.11 Auxiliary apparatus for work in con-
junction with the photometric inverse square
7.3.3 The sample shall be placed over the
exit aperture of the sphere, and in the case of
* A suitable integrating sphere paint produces a high
photographic films or plates, the emulsion
reflective and diffusing surface which is spectrally nonselec-
surface shall be in contact with the sphere. The
tive. A paint composed of titanium dioxide pigment con-
centrated in a clear vehicle is considered suitable for this movable source shall then be moved toward the
purpose.
sphere, to a distance, Is, at which the two
** The diffusion coefficient is the ratio A/B, where A
halves of the photometric field are equal.
is the area under the curve obtained by plotting the relative
candlepower of the exit aperture of the sphere as a func-
tion of the angle of view over the
...