ASTM E1682-96(2001)
(Guide)Standard Guide for Modeling the Colorimetric Properties of a Visual Display Unit
Standard Guide for Modeling the Colorimetric Properties of a Visual Display Unit
SCOPE
1.1 This guide is intended for use in establishing the operating characteristics of a visual display unit (VDU), such as a cathode ray tube (CRT). Those characteristics define the relationship between the digital information supplied by a computer, which defines an image, and the resulting spectral radiant exitance and CIE tristimulus values. The mathematical description of this relationship can be used to provide a nearby device-independent model for the accurate display of color and colored images on the VDU. The CIE tristimulus values referred to here are those calculated from the CIE 1931 2° standard colorimetric (photopic) observer.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E1682–96(Reapproved2001)
Standard Guide for
Modeling the Colorimetric Properties of a Visual Display
Unit
This standard is issued under the fixed designation E1682; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This guide provides directions and mathematical models for deriving the relationship between
digital settings in a computer-controlled visual display unit and the resulting photometric and
colorimetricoutputofthedisplayunit.Theaccuratedeterminationofthisrelationshipiscriticaltothe
goal of accurate, device-independent color simulation on a visual display unit.
1. Scope 3.2 Acronyms:
3.2.1 CRT, n—an abbreviation for the term cathode ray
1.1 This guide is intended for use in establishing the
tube, a device for projecting a stream of electrons onto a
operating characteristics of a visual display unit (VDU), such
phosphor-coated screen in such a way as to display characters
as a cathode ray tube (CRT). Those characteristics define the
and graphics.
relationship between the digital information supplied by a
3.2.2 DAC, n—anabbreviationforthetermdigitaltoanalog
computer, which defines an image, and the resulting spectral
converter, a device for accepting a digital computer bit pattern
radiant exitance and CIE tristimulus values. The mathematical
and translating it into an analog voltage of a prescribed value.
descriptionofthisrelationshipcanbeusedtoprovideanearby
3.2.3 LUT, n—an abbreviation for the term look up table, a
device-independentmodelfortheaccuratedisplayofcolorand
process in which input and output values are mapped in an
colored images on the VDU. The CIE tristimulus values
n-dimensional table such that, for a given input value, the
referred to here are those calculated from the CIE 1931 2°
appropriate output value is “looked-up” from the table.
standard colorimetric (photopic) observer.
3.2.4 VDU, n—an abbreviation for the term visual display
1.2 This standard does not purport to address all of the
unit, a device interfaced to a computer for displaying text and
safety concerns, if any, associated with its use. It is the
graphics.
responsibility of the user of this standard to establish appro-
3.2.4.1 Discussion—A CRT is one type of VDU.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
4. Summary of Guide
2. Referenced Documents 4.1 Every color stimulus generated on aVDU is realized by
the linear (additive) superposition of the spectral power distri-
2.1 ASTM Standards:
bution of three primaries. Test Method E1336 describes how
E284 Terminology of Appearance
to measure the spectral power distributions and reduce them to
E1336 Test Method for Obtaining Colorimetric Data from
CIE tristimulus values. Practice E1455 describes how to
a Visual Display Unit by Spectroradiometry
measure the CIE tristimulus values of the primaries directly.
E1455 Practice for Obtaining Colorimetric Data from a
2 An exact characterization of the VDU would require measure-
Visual Display Unit Using Tristimulus Colorimeters
ment of the spectral power distribution at all possible combi-
3. Terminology nations of primary settings. Modern, computer-controlled
VDUs will provide 256 or more levels of each of the three
3.1 Definitions of appearance terms in Terminology E284
primaries. This results in more than 16777000 unique set-
are applicable to this guide.
tings, which is far too many combinations to be measured
practically (see Note 1). Instead, a characteristic function
This guide is under the jurisdiction of ASTM Committee E12 on Color and
relating the radiant output of the screen to the digital inputs
AppearanceandisthedirectresponsibilityofSubcommitteeE12.06onAppearance
fromthecomputermustbederived.Proceduresareoutlinedfor
of Displays .
deriving a characteristic function for a computer-controlled
Current edition approved May 10, 1996. Published July 1996. Originally
published as E1682–95. Last previous edition E1682–95b.
Annual Book of ASTM Standards, Vol 06.01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1682–96 (2001)
VDU, using a minimum number of spectral radiometric mea- for critical applications if this assumption is not met. The
surements while maintaining near optimum accuracy. Ex- amount of spectral variance will be a function of both position
amples of deriving and testing such models are given in andintensityofboththeareaofinterestandtheintegratedarea
Appendix X1. of pollution. While such models can be derived, they may be
too complex to justify their use.
NOTE 1—Different primary settings do not necessarily produce percep-
6.1.3 Assumption3,levelinvariance,istestedbymeasuring
tibly different colors. For VDUs with a large number (for example,
16777000) of different primary settings, the number of perceptibly the chromaticity of a primary at several different levels. It
different colors will be less than the number of primary settings.
should be noted that care must be taken to maintain the signal
to noise value of the color measuring instrument as the
5. Significance and Use
luminance of the primary is reduced. As the signal level of a
5.1 The color displayed on aVDU is an important aspect of
colorimeter approaches the optical/electrical zero, the apparent
the reproduction of colored images. The VDU is often used as
chromaticity approaches that of neutral black.
the design, edit, and approval medium. Images are placed into
6.1.4 Assumption 4, absence of inter-reflections, is often
the computer by some sort of capture device, such as a camera
violated on CRT-type displays without high efficiency antire-
or scanner, modified by the computer operator, and sent on to
flection (HEA) antirelection coatings on the face plate gloss.
a printer or color separation generator, or even to a paint
This is detected in the same manner as spatial invariance.
dispenser or textile dyer. The color of the final product is to
Again, models for this can be derived, but the complexity may
have some well-defined relationship to the original. The most
not be worth the effort.
common medium for establishing the relationship between
6.1.5 Assumption5,linearityoftheDAC,canbetestedwith
input, edit, and output color (device-independent color space)
a calibrated, high-precision oscilloscope. A doubling of the
is the CIE tristimulus space. This guide identifies the proce-
digital counts should produce a doubling of the output signal.
dures for deriving a model that relates the digital computer
It should be noted that RS-170 voltage levels are from −0.286
settings of a VDU to the CIE tristimulus values of the colored
V to +0.714 V with the range from 0 V to 0.714 V being used
light emitted by the primaries.
for signal level and 0 V to −0.286 V being used for synchro-
nization during the blanking interval on a CRT-type display.
6. Models
Other types of visual display units may have their own unique
6.1 The models are based on eight basic assumptions. First,
voltage ranges as well. In general, the setting of the drive
at each pixel location on the VDU, the radiant exitance
voltage requires the simultaneous alignment of many opera-
(emitted light per unit area) attributable to one primary type
tional parameters, the specification of which are beyond the
(red, green, or blue) is invariant with the radiant exitances of
scope of this guide. It is assumed that the signal generator and
the other primary types. Second, the radiance exitance at one
the receiver are adjusted to be within their unique operational
spatial location is invariant with the radiant exitance at other
specifications before the linearity test is performed.
spatiallocations.Third,therelativespectralradiantexitanceof
6.1.6 Assumption 6, ambient glare, can be tested with a
a primary is invariant with excitation level. Fourth, there is no
telephometer, measuring the luminance and chroma of each
inter-reflection of light between pixel locations. Fifth, the
primaryinadarkandambientenvironment.Ifthetworeadings
output of the digital-to-analog conversion process is linear.
differ by an unacceptable amount, either the display must be
Sixth, there is no ambient glare (flare) from the screen into the
outfitted with light shields or its operation restricted to a dark
observer’s eyes. Seventh, the refresh rate of the image is rapid
environment.
enough to produce temporal fusion (no noticeable flicker) for
6.1.7 Assumption 7, flicker rate, is a function of the display
the normal observer. Eighth, the pixel pitch is fine enough to
electronics and display type. Chromatic flicker ceases at
produce spatial fusion for the normal observer. Each of the
frequencies above 30 Hz. Brightness flicker ceases for most
eight basic assumptions should be tested and either verified,
people above 60 Hz, although some people continue to
noted, or corrected before deriving a characteristic model.
experience the sensation of flicker up to 70 Hz. Most modern
6.1.1 Assumption1,independenceoftheprimaries,istested
graphics displays operate at refresh rates above 60 Hz. Broad-
by measuring the radiometric output at several levels, as
cast displays may operate at rates as low as 30 Hz. Low-rate
described by Cowen and Rowell. If the departures are small,
display electronics interfaced to a high-rate display may result
they may be neglected or a LUT correction applied. If the
in an unacceptable appearance.
departuresaresignificantandmaximumreproductionaccuracy
6.1.8 Assumption 8, pixel density, is a characteristic of the
is required, only a full table look-up method can be used to
displayandafunctionoftheapplication.Alow-densitydisplay
create the RGB to XYZ transform.
may be adequate for displaying solid patches of color but not
6.1.2 Assumption 2, spatial invariance, can be tested by
for detailed drawings or renderings.
measuring the center of a dark display and then repeating the
6.2 Examples of using the LUT method are also given in
measurements with pixels near the edge of the display illumi-
this guide for completeness. There are three possible ap-
nated fully. The display may have to be considered unusable
proaches to modeling the relationship between the digital
counts and the VDU tristimulus values. The first requires the
user to adjust the video gain and offset manually such that the
Cowan, W. B., and Rowell, N., “On the Gun Independence and Phosphor
blacklevelandtheoffsetcanceleachother.Thesecondmethod
Constancy of Colour Video Monitors,” Color Research and Application, Vol 11,
1986, pp. S35–S38. tries to approximate the gain and offset by trial and error. The
E1682–96 (2001)
third method, the one used most commonly commercially,
Y 5683 ~L 1 L 1 L ! y¯ dl
* l,r l,g l,b l
ignores the physical origins of the signals and collects mea-
surements of the VDU output at a large number of points,
5 RY 1 GY 1 BY
r,max g,max b,max
sampling each primary channel between the minimum and
maximum counts. The unmeasured data values are determined Z 5683 L 1 L 1 L z¯ dl
~ !
* l,r l,g l,b l
by interpolation, and a LUT is formed such that all possible
combinationsofprimarysettingscanbefoundinthetable.The
5 RZ 1 GZ 1 BZ
r,max g,max b,max
recommendedprocedureinthisguideconformsmostcloselyto
In matrix notation, these equations can be reduced to the
the second method, using statistical methods to determine the
following:
optimum parametric values for the gain, offset, and gamma of
@X#@X X X #@ R# (5)
r,max g,max b,max
each primary while requiring the smallest number of calibra-
Y 5 Y Y Y · G
tionpatches.This,then,linearizestheoutputofthesystem,and @ # @ # @ #
r,max g,max b,max
a linear transformation is applied to convert the linear RGB
@Z#@Z Z Z #@ B#
r,max g,max b,max
primary values to CIE tristimulus values.
where R, G, and B are defined as follows:
6.3 The model parameters for the red primary are related to
g
d d
r r
the operational variables as follows:
R 5 k 1k for k 1 k
F g,rS n D o,rG S g,rS n D o,rD
2 21 2 21
g
d
r
$0
M 5 M k 1 k (1)
l,r l,r,maxF g,rS N D o,rG
2 21
50 for ,0 (6)
where:
g
d d
g g
M = the spectral exitance of the (r)ed primary,
G 5 k 1k for k 1 k
l,r
g,g o,g g,g o,g
F S n D G S S n D D
2 21 2 21
M = the maximum spectral exitance of the (r)ed
l,r,max
$0
primary,
d = the digital setting of the (r)ed primary,
r
50 for ,0
N
2 −1 = the number of digital states generated by the
g
d d
b b
display driver,
B 5 k 1k for k 1 k
g,b n o,b g,b n o,b
F S D G S S D D
2 21 2 21
k = the system (g)ain coefficient for the (r)ed
g,r
$0
primary,
k = the system (o)ffset coefficient for the (r)ed
o,r
50 for ,0
primary, and
Beinglinear,theseequationsareinvertable.Thustheinverse
g = the system gamma coefficient.
is given, in matrix notation, as follows:
6.3.1 Similar expressions can be derived for the green and
blueprimaries.FollowingtheproceduresgiveninTestMethod R 5 X X X X (7)
r,max g,max b,max
E1336, the spectroradiometer will measure the spectral radi-
G 5 Y Y Y Y
r,max g,max b,max
ance (L ) of an extended diffuse source, such as a VDU. The
l
B 5 Z Z Z Z
r,max g,max b,max
spectral radiance is related to the spectral exitance as follows:
and in like manner,
M
l
L 5 (2) n 1
l
2 21
p
g
r
d 5S D~R 22 k ! for 0# R#1 (8)
r o,r
k
g,r
6.3.2 The radiance for each primary can be described as
n
2 21
follows:
g
g
d 5S D~R 2 k ! for 0# G#1
g o,g
k
g,g
L 5 RL , L 5 GL , L 5 BL
l,r l,r,max l,g l,g,max l,b l,b,max
n 1
2 21
g
b
d 5 ~B 2 k ! for 0# B#1
The scalars R, G, and B can be thought of as the display S D
b o,b
k
g,b
tristimulus values. From Test Method E1336, we obtain the
relationship between the measured spectral radiance and the 7. Procedure
CIE tristimulus values, in luminance units as follows:
7.1 Analytical Method:
830 830
7.1.1 Oncethedisplayunitiswarmedupandstabilized,itis
X 5683 L ¯x dl5683 L x¯ dl (3)
* *
r l,r l l,r,max l
360 360
necessary to display the test patches over a constant neutral
830 830
background of approximately 18% of the maximum lumi-
Y 5683 L y¯ dl5683R L y¯ dl
r * l,r l * l,r,max l
360 360
nance. Measure the color of the patches following the proce-
830 830
durescontainedinTestMethodE1336orPracticeE1455.The
Z 5683 L z¯ dl5683R L z¯ dl
r * l,r l * l,r,max l
360 360
calculated or measured tristimulus values are used to estimate
the optimum set of values for the model parameters and the
6.3.3 The linear superposition of the red, green, and blue
coefficients of the XYZ to RGB conversion matrix.The patches
tristimulus values yield the foll
...
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