ASTM E1682-08(2013)
(Guide)Standard Guide for Modeling the Colorimetric Properties of a CRT-Type Visual Display Unit
Standard Guide for Modeling the Colorimetric Properties of a CRT-Type Visual Display Unit
SIGNIFICANCE AND USE
5.1 The color displayed on a VDU is an important aspect of the reproduction of colored images. The VDU is often used as the design, edit, and approval medium. Images are placed into the computer by some sort of capture device, such as a camera or scanner, modified by the computer operator, and sent on to a printer or color separation generator, or even to a paint dispenser or textile dyer. The color of the final product is to have some well-defined relationship to the original. The most common medium for establishing the relationship between input, edit, and output color (device-independent color space) is the CIE tristimulus space. This guide identifies the procedures for deriving a model that relates the digital computer settings of a VDU to the CIE tristimulus values of the colored light emitted by the primaries.
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 − 08 (Reapproved 2013)
Standard Guide for
Modeling the Colorimetric Properties of a CRT-Type 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 (´) 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. Terminology
1.1 This guide is intended for use in establishing the 3.1 Definitions of appearance terms in Terminology E284
operating characteristics of a visual display unit (VDU), such are applicable to this guide.
as a cathode ray tube (CRT). Those characteristics define the
3.2 Acronyms:
relationship between the digital information supplied by a
3.2.1 CRT, n—anabbreviationforthetermcathoderaytube,
computer, which defines an image, and the resulting spectral
a device for projecting a stream of electrons onto a phosphor-
radiant exitance and CIE tristimulus values. The mathematical
coated screen in such a way as to display characters and
descriptionofthisrelationshipcanbeusedtoprovideanearby
graphics.
device-independentmodelfortheaccuratedisplayofcolorand
3.2.2 DAC, n—anabbreviationforthetermdigitaltoanalog
colored images on the VDU. The CIE tristimulus values
converter, a device for accepting a digital computer bit pattern
referred to here are those calculated from the CIE 1931 2°
and translating it into an analog voltage of a prescribed value.
standard colorimetric (photopic) observer.
3.2.3 LUT, n—an abbreviation for the term look up table, a
1.2 This standard does not purport to address all of the
process in which input and output values are mapped in an
safety concerns, if any, associated with its use. It is the
n-dimensional table such that, for a given input value, the
responsibility of the user of this standard to establish appro-
appropriate output value is “looked-up” from the table.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. 3.2.4 VDU, n—an abbreviation for the term visual display
unit, a device interfaced to a computer for displaying text and
2. Referenced Documents
graphics.
2 3.2.4.1 Discussion—A CRT is one type of VDU.
2.1 ASTM Standards:
E284Terminology of Appearance
4. Summary of Guide
E1336Test Method for Obtaining Colorimetric Data From a
Visual Display Unit by Spectroradiometry
4.1 EverycolorstimulusgeneratedonaVDUisrealizedby
E1455Practice for Obtaining Colorimetric Data from a
the linear (additive) superposition of the spectral power distri-
Visual Display Unit Using Tristimulus Colorimeters
butionofthreeprimaries.TestMethodE1336describeshowto
measure the spectral power distributions and reduce them to
CIE tristimulus values. Practice E1455 describes how to
This guide is under the jurisdiction of ASTM Committee E12 on Color and
measure the CIE tristimulus values of the primaries directly.
Appearance and is the direct responsibility of Subcommittee E12.06 on Display,
An exact characterization of the VDU would require measure-
Imaging and Imaging Colorimetry.
ment of the spectral power distribution at all possible combi-
Current edition approved Oct. 1, 2013. Published October 2013. Originally
nations of primary settings. Modern, computer-controlled
approved in 1995. Last previous edition approved in 2008 as E1682–08. DOI:
10.1520/E1682-08R13.
VDUs will provide 256 or more levels of each of the three
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
primaries. This results in more than 16777000 unique
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
settings, which is far too many combinations to be measured
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. practically (see Note 1). Instead, a characteristic function
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1682 − 08 (2013)
relating the radiant output of the screen to the digital inputs measurements with pixels near the edge of the display illumi-
fromthecomputermustbederived.Proceduresareoutlinedfor nated fully. The display may have to be considered unusable
deriving a characteristic function for a computer-controlled for critical applications if this assumption is not met. The
VDU, using a minimum number of spectral radiometric mea- amount of spectral variance will be a function of both position
surements while maintaining near optimum accuracy. Ex- andintensityofboththeareaofinterestandtheintegratedarea
amples of deriving and testing such models are given in of pollution. While such models can be derived, they may be
Appendix X1. too complex to justify their use.
6.1.3 Assumption3,levelinvariance,istestedbymeasuring
NOTE 1—Different primary settings do not necessarily produce percep-
the chromaticity of a primary at several different levels. It
tibly different colors. For VDUs with a large number (for example,
should be noted that care must be taken to maintain the signal
16777000) of different primary settings, the number of perceptibly
different colors will be less than the number of primary settings.
to noise value of the color measuring instrument as the
luminance of the primary is reduced. As the signal level of a
5. Significance and Use
colorimeterapproachestheoptical/electricalzero,theapparent
5.1 ThecolordisplayedonaVDUisanimportantaspectof
chromaticity approaches that of neutral black.
the reproduction of colored images. The VDU is often used as
6.1.4 Assumption 4, absence of inter-reflections, is often
the design, edit, and approval medium. Images are placed into
violated on CRT-type displays without high efficiency antire-
the computer by some sort of capture device, such as a camera
flection(HEA)coatingsonthefaceplategloss.Thisisdetected
or scanner, modified by the computer operator, and sent on to
in the same manner as spatial invariance. Again, models for
a printer or color separation generator, or even to a paint
this can be derived, but the complexity may not be worth the
dispenser or textile dyer. The color of the final product is to
effort.
have some well-defined relationship to the original. The most
6.1.5 Assumption5,linearityoftheDAC,canbetestedwith
common medium for establishing the relationship between
a calibrated, high-precision oscilloscope. A doubling of the
input, edit, and output color (device-independent color space)
digital counts should produce a doubling of the output signal.
is the CIE tristimulus space. This guide identifies the proce-
It should be noted that RS-170 voltage levels are from −0.286
dures for deriving a model that relates the digital computer
V to +0.714 V with the range from 0 V to 0.714 V being used
settings of a VDU to the CIE tristimulus values of the colored
for signal level and 0 V to −0.286 V being used for synchro-
light emitted by the primaries.
nization during the blanking interval on a CRT-type display.
Other types of visual display units may have their own unique
6. Models
voltage ranges as well. In general, the setting of the drive
6.1 The models are based on eight basic assumptions. First,
voltage requires the simultaneous alignment of many opera-
at each pixel location on the VDU, the radiant exitance
tional parameters, the specification of which are beyond the
(emitted light per unit area) attributable to one primary type
scope of this guide. It is assumed that the signal generator and
(red, green, or blue) is invariant with the radiant exitances of
the receiver are adjusted to be within their unique operational
the other primary types. Second, the radiance exitance at one
specifications before the linearity test is performed.
spatial location is invariant with the radiant exitance at other
6.1.6 Assumption 6, ambient glare, can be tested with a
spatiallocations.Third,therelativespectralradiantexitanceof
telephotometer, measuring the luminance and chroma of each
a primary is invariant with excitation level. Fourth, there is no
primaryinadarkandambientenvironment.Ifthetworeadings
inter-reflection of light between pixel locations. Fifth, the
differ by an unacceptable amount, either the display must be
output of the digital-to-analog conversion process is linear.
outfitted with light shields or its operation restricted to a dark
Sixth, there is no ambient glare (flare) from the screen into the
environment.
observer’s eyes. Seventh, the refresh rate of the image is rapid
6.1.7 Assumption 7, flicker rate, is a function of the display
enough to produce temporal fusion (no noticeable flicker) for
electronics and display type. Chromatic flicker ceases at
the normal observer. Eighth, the pixel pitch is fine enough to
frequencies above 30 Hz. Brightness flicker ceases for most
produce spatial fusion for the normal observer. Each of the
people above 60 Hz, although some people continue to
eight basic assumptions should be tested and either verified,
experience the sensation of flicker up to 70 Hz. Most modern
noted, or corrected before deriving a characteristic model.
graphics displays operate at refresh rates above 60 Hz. Broad-
6.1.1 Assumption1,independenceoftheprimaries,istested
cast displays may operate at rates as low as 30 Hz. Low-rate
by measuring the radiometric output at several levels, as
display electronics interfaced to a high-rate display may result
described by Cowan and Rowell. If the departures are small,
in an unacceptable appearance.
they may be neglected or a LUT correction applied. If the
6.1.8 Assumption 8, pixel density, is a characteristic of the
departuresaresignificantandmaximumreproductionaccuracy
displayandafunctionoftheapplication.Alow-densitydisplay
is required, only a full table look-up method can be used to
may be adequate for displaying solid patches of color but not
create the RGB to XYZ transform.
for detailed drawings or renderings.
6.1.2 Assumption 2, spatial invariance, can be tested by
6.2 Examples of using the LUT method are also given in
measuring the center of a dark display and then repeating the
this guide for completeness. There are three possible ap-
proaches to modeling the relationship between the digital
Cowan, W. B., and Rowell, N., “On the Gun Independence and Phosphor
counts and the VDU tristimulus values. The first requires the
Constancy of Colour Video Monitors,” Color Research and Application, Vol 11,
1986, pp. S35–S38. user to adjust the video gain and offset manually such that the
E1682 − 08 (2013)
blacklevelandtheoffsetcanceleachother.Thesecondmethod 6.3.3 The linear superposition of the red, green, and blue
tries to approximate the gain and offset by trial and error. The tristimulus values yield the following:
third method, the one used most commonly commercially,
ignores the physical origins of the signals and collects mea-
X 5683 L 1L 1L x¯ dλ (4)
*
~ λ,r λ,g λ,b! λ
surements of the VDU output at a large number of points,
sampling each primary channel between the minimum and
5RX 1GX 1BX
r,max g,max b,max
maximum counts. The unmeasured data values are determined
by interpolation, and a LUT is formed such that all possible 830
combinationsofprimarysettingscanbefoundinthetable.The
Y 5683 L 1L 1L y¯ dλ
* ~ !
λ,r λ,g λ,b λ
recommendedprocedureinthisguideconformsmostcloselyto
the second method, using statistical methods to determine the
5RY 1GY 1BY
r,max g,max b,max
optimum parametric values for the gain, offset, and gamma of
each primary while requiring the smallest number of calibra- 830
tionpatches.This,then,linearizestheoutputofthesystem,and Z 5683 L 1L 1L z¯ dλ
*
~ λ,r λ,g λ,b! λ
a linear transformation is applied to convert the linear RGB
primary values to CIE tristimulus values.
5RZ 1GZ 1BZ
r,max g,max b,max
6.3 The model parameters for the red primary are related to
In matrix notation, these equations can be reduced to the
the operational variables as follows:
following:
γ
d
r
X X X X R
M 5 M k 1k (1) r,max g,max b,max
F S D G
λ,r λ,r,max g,r N o,r
2 21
Y Y Y Y G
5 (5)
r,max g,max b,max
F G F GF G
where: Z Z Z Z B
r,max g,max b,max
M = the spectral exitance of the (r)ed primary,
λ,r
where R, G, and B are defined as follows:
M = the maximum spectral exitance of the (r)ed
λ,r,max
γ
d
r
primary,
R 5 max k 1k ,0 (6)
F H S D JG
g,r n o,r
2 21
d = the digital setting of the (r)ed primary,
r
N
2 −1 = the number of digital states generated by the
γ
d
g
display driver,
G 5 max k 1k ,0
F H S D JG
g,g n o,g
2 21
k = thesystem(g)aincoefficientforthe(r)edprimary,
g,r
k = the system (o)ffset coefficient for the (r)ed
o,r γ
d
b
primary, and
B 5 max k 1k ,0
F H S n D JG
g,b o,b
2 21
γ = the system gamma coefficient.
Being linear, Eq 5 can be solved for R, G, B. Thus the
Similar expressions can be derived for the green and blue
inverse is given, in matrix notation, as follows:
primaries.
R X X X X
r,max g,max b,max
6.3.1 Following the procedures given in Test Method
G Y Y Y Y
5 (7)
r,max g,max b,max
E1336, the spectroradiometer will measure the spectral radi-
F G F G F G
B Z Z Z Z
ance (L ) of an extended diffuse source, such as a VDU. The
r,max g,max b,max
λ
spectral radiance is related to the spectral exitance as follows:
and in like manner,
M n
λ 1
2 21
L 5 (2)
γ
λ
d 5 ~R 2 k ! for 0# R#1 (8)
π S D
r o,r
k
g,r
6.3.2 The radiance for each primary can be described as
n
2 21
γ
~ !
follows: d 5 G 2 k for 0# G#1
S D
g o,g
k
g,g
L 5 RL , L 5 GL , L 5 BL
λ,r λ,r,max λ,g λ,g,max λ,b λ,b,max
n
2 21
γ
~ !
d 5 B 2 k for 0# B#1
S D
b o,b
The scalars R, G, and B can be thought of as the display
k
g,b
tristimulus values. From Test Method E1336, we obtain the
7. Procedure
relationship between the measured spectral radiance and the
CIE tristimulus values, in luminance units as follows:
7.1 Analytical Method:
830 830
7.1.1 Oncethedisplayunitiswarmedupandstabilized,itis
X 5683 L x¯ dλ 5683R L x¯ dλ (3)
* * necessary to display the test patches over a constant neutral
r λ,r λ λ,r,max λ
360 360
background of approximately 18% of the maximum lumi-
830 830 nance. Measure the color of the patches following the proce-
dures contained in Test Method E1336 or Practice E1455. The
Y 5683 * L y¯ dλ 5683R * L y¯ dλ
r λ,r λ λ,r,max λ
360 360
calculated or measured tristimulus values are us
...
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