ASTM E1682-96
(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: E 1682 – 96
AMERICAN SOCIETY FOR TESTING AND MATERIALS
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Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Guide for
Modeling the Colorimetric Properties of a Visual Display
Unit
This standard is issued under the fixed designation E 1682; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number 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
colorimetric output of the display unit. The accurate determination of this relationship is critical to the
goal of accurate, device-independent color simulation on a visual display unit.
1. Scope phosphor-coated screen in such a way as to display characters
and graphics.
1.1 This guide is intended for use in establishing the
3.2.2 DAC, n—an abbreviation for the term digital to analog
operating characteristics of a visual display unit (VDU), such
converter, a device for accepting a digital computer bit pattern
as a cathode ray tube (CRT). Those characteristics define the
and translating it into an analog voltage of a prescribed value.
relationship between the digital information supplied by a
3.2.3 LUT, n—an abbreviation for the term look up table, a
computer, which defines an image, and the resulting spectral
process in which input and output values are mapped in an
radiant exitance and CIE tristimulus values. The mathematical
n-dimensional table such that, for a given input value, the
description of this relationship can be used to provide a nearby
appropriate output value is “looked-up” from the table.
device-independent model for the accurate display of color and
3.2.4 VDU, n—an abbreviation for the term visual display
colored images on the VDU. The CIE tristimulus values
unit, a device interfaced to a computer for displaying text and
referred to here are those calculated from the CIE 1931 2°
graphics.
standard colorimetric (photopic) observer.
3.2.4.1 Discussion—A CRT is one type of VDU.
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Summary of Guide
responsibility of the user of this standard to establish appro-
4.1 Every color stimulus generated on a VDU is realized by
priate safety and health practices and determine the applica-
the linear (additive) superposition of the spectral power distri-
bility of regulatory limitations prior to use.
bution of three primaries. Test Method E 1336 describes how
2. Referenced Documents to measure the spectral power distributions and reduce them to
CIE tristimulus values. Practice E 1455 describes how to
2.1 ASTM Standards:
measure the CIE tristimulus values of the primaries directly.
E 284 Terminology of Appearance
An exact characterization of the VDU would require measure-
E 1336 Test Method for Obtaining Colorimetric Data from
ment of the spectral power distribution at all possible combi-
a Visual Display Unit by Spectroradiometry
nations of primary settings. Modern, computer-controlled
E 1455 Practice for Obtaining Colorimetric Data from a
VDUs will provide 256 or more levels of each of the three
Visual Display Unit Using Tristimulus Colorimeters
primaries. This results in more than 16 777 000 unique set-
3. Terminology
tings, which is far too many combinations to be measured
practically (see Note 1). Instead, a characteristic function
3.1 Definitions of appearance terms in Terminology E 284
relating the radiant output of the screen to the digital inputs
are applicable to this guide.
from the computer must be derived. Procedures are outlined for
3.2 Acronyms:
deriving a characteristic function for a computer-controlled
3.2.1 CRT, n—an abbreviation for the term cathode ray
VDU, using a minimum number of spectral radiometric mea-
tube, a device for projecting a stream of electrons onto a
surements while maintaining near optimum accuracy. Ex-
amples of deriving and testing such models are given in
This guide is under the jurisdiction of ASTM Committee E-12 on Appearance-
Appendix X1.
and is the direct responsibility of Subcommittee E12.06 on Appearance of Displays.
Current edition approved May 10, 1996. Published July 1996. Originally
NOTE 1—Different primary settings do not necessarily produce percep-
published as E 1682 – 95. Last previous edition E 1682 – 95b.
tibly different colors. For VDUs with a large number (for example,
Annual Book of ASTM Standards, Vol 06.01.
E 1682
16 777 000) of different primary settings, the number of perceptibly
should be noted that care must be taken to maintain the signal
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
colorimeter approaches the optical/electrical zero, the apparent
5.1 The color displayed on a VDU is an important aspect of
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) antirelection coatings on the face plate gloss.
or scanner, modified by the computer operator, and sent on to
This is detected in the same manner as spatial invariance.
a printer or color separation generator, or even to a paint
Again, models for this can be derived, but the complexity may
dispenser or textile dyer. The color of the final product is to
not be worth the effort.
have some well-defined relationship to the original. The most
6.1.5 Assumption 5, linearity of the DAC, can be tested with
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
spatial locations. Third, the relative spectral radiant exitance of
telephometer, measuring the luminance and chroma of each
a primary is invariant with excitation level. Fourth, there is no
primary in a dark and ambient environment. If the two readings
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
enough to produce temporal fusion (no noticeable flicker) for 6.1.7 Assumption 7, flicker rate, is a function of the display
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 Assumption 1, independence of the primaries, is tested
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 Cowen and Rowell. If the departures are small,
in an unacceptable appearance.
they may be neglected or a LUT correction applied. If the
departures are significant and maximum reproduction accuracy 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
display and a function of the application. A low-density display
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
for critical applications if this assumption is not met. The
counts and the VDU tristimulus values. The first requires the
amount of spectral variance will be a function of both position
user to adjust the video gain and offset manually such that the
and intensity of both the area of interest and the integrated area
black level and the offset cancel each other. The second method
of pollution. While such models can be derived, they may be
tries to approximate the gain and offset by trial and error. The
too complex to justify their use.
third method, the one used most commonly commercially,
6.1.3 Assumption 3, level invariance, is tested by measuring
ignores the physical origins of the signals and collects mea-
the chromaticity of a primary at several different levels. It
surements of the VDU output at a large number of points,
sampling each primary channel between the minimum and
maximum counts. The unmeasured data values are determined
Cowan, W. B., and Rowell, N., “On the Gun Independence and Phosphor
by interpolation, and a LUT is formed such that all possible
Constancy of Colour Video Monitors,” Color Research and Application, Vol 11,
1986, pp. S35–S38. combinations of primary settings can be found in the table. The
E 1682
recommended procedure in this guide conforms most closely to 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#
r,max g,max b,max
tion patches. This, then, linearizes the output of the system, and
@Z#@Z Z Z #@ B#
a linear transformation is applied to convert the linear RGB 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
g,r o,r g,r o,r
F S n D G S S n D D
2 21 2 21
g
d
r
$ 0
M 5 M k 1 k (1)
l,r l,r,max g,r o,r
F S N D G
2 2 1
5 0 for , 0 (6)
where:
g
d d
g g
M 5 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 5 the maximum spectral exitance of the (r)ed
l,r,max
$ 0
primary,
d 5 the digital setting of the (r)ed primary,
r
5 0 for , 0
N
2 −1 5 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 o,b g,b o,b
F S n D G S S n D D
2 21 2 21
k 5 the system (g)ain coefficient for the (r)ed
g,r
$ 0
primary,
k 5 the system (o)ffset coefficient for the (r)ed
o,r
5 0 for , 0
primary, and
g5 the system gamma coefficient.
Being linear, these equations are invertable. Thus the inverse
6.3.1 Similar expressions can be derived for the green and is given, in matrix notation, as follows:
blue primaries. Following the procedures given in Test Method
R 5 X X X X (7)
r,max g,max b,max
E 1336, 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)
l
p
n 1
2 2 1
g
r
d 5 ~R 22 k ! for 0 # R # 1 (8)
S D
r o,r
6.3.2 The radiance for each primary can be described as k
g,r
follows:
n 1
2 2 1
g
g
d 5S D~R 2 k ! for 0 # G # 1
g o,g
L 5 RL , L 5 GL , L 5 BL
k
l,r l,r,max l,g l,g,max l,b l,b,max
g,g
n
The scalars R, G, and B can be thought of as the display 2 2 1
g
b
d 5S D~B 2 k ! for 0 # B # 1
b o,b
tristimulus values. From Test Method E 1336, we obtain the k
g,b
relationship between the measured spectral radiance and the
CIE tristimulus values, in luminance units as follows: 7. Procedure
830 830
7.1 Analytical Method:
X 5 683 L ¯x dl5 683 L x¯ dl (3)
r * l,r l * l,r,max l
360 360
7.1.1 Once the display unit is warmed up and stabilized, it is
830 830
Y 5 683 L y¯ dl5 683R L y¯ dl necessary to display the test patches over a constant neutral
* *
r l,r l l,r,max l
360 360
background of approximately 18 % of the maximum lumi-
830 830
Z 5 683 L z¯ dl5 683R L z¯ dl nance. Measure the color of the patches following the proce-
r * l,r l * l,r,max l
360 360
dures contained in Test Method E 1336 or Practice E 1455. The
6.3.3 The linear superposition of the red, green, and blue
calculated or measured tristimulus values are used to estimate
tristimulus values yield the following:
the optimum set of
...
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