BBC Research and Development Report Number 1972-37

UDC 621.397.132

621.391.837.32

mmm 4

RESEARCH DEPARTMENT REPORT

The contrast handling requirements
of a colour television display

No.

112/31

Research Department, Engineering Division

THE BRITISH BROADCASTING CORPORATION

RESEARCH DEPARTMENT
THE CONTRAST HANDLING REQUIREMENTS OF A COLOUR TELEVISION DISPLAY

Research Department Report No. 1972/37
UDC 621.397.132
621.391.837.32

This Report may not be reproduced in any
form without the written permission of the
British Broadcasting Corporation.

It uses SI units in accordance with B.S.
document PD 5686.

R.N. Robinson, B.Sc. Head of Research Department

(PH-96)

Research Department Report No. 1972/37

THE CONTRAST HANDLING REQUIREMENTS OF A COLOUR TELEVISION DISPLAY

Section Title Page

Summary 1

1. Introduction 1

2. Viewing conditions 1

2.1. Domestic viewing conditions 1

2.2. The viewing room used for experimental measurements 2

3. The visible contrast range under television viewing conditions 2

3.1. The luminance of subjective black as a function of ambient lighting 2

3.2. The variation of the luminance of subjective black with object size 3

4. Limitations of the display tube 4

4.1. Flare 4

4.2. Reflection of ambient light 5

5. Discussion 5

6. Conclusions 7

7. References 7

(PH-96)

October 1972

Research Department Report No. 1972/37
UDC 621.397.132
621.391.837.32

THE CONTRAST HANDLING REQUIREMENTS OF A COLOUR TELEVISION DISPLAY

Summary

A survey of domestic television viewing conditions is described. These con-
ditions were reproduced in a laboratory in order to determine the contrast range of the
eye. The dependence of the contrast range visible to the eye upon ambient illumination
and upon the form and brightness of the displayed picture have been investigated; these
results are then interpreted in terms of the contrast range available from a practical
display tube under similar conditions.

It is found that under normal viewing conditions the visible contrast range of a
television picture is severely limited by flare within the display tube and ambient light
reflected from the tube face. A reduction of the reflectance of the tube face to approxi-
mately 28% of its present value is required for good reproduction of the luminance range
visible under typical viewing conditions.

1. Introduction

Much work has been carried out to determine the opti-
mum reproduction characteristics of cinema film, trans-
parencies and reflection prints. 1 ,2,3 The problems
associated with the reproduction of scenes by colour tele-
vision have received much less attention.

The experiments described in this report were carried
out to measure the luminance range which is visible to the
eye under typical television viewing conditions; the
measurements were made using a simulated television dis-
play in order that the limitations of the eye could be
isolated from any shortcomings of the cathode-ray tube as
a display device.

The peak luminance of a practical television display
tube is a function of the properties of the phosphors and
the maximum permissible beam current. The minimum
luminance of any point on a display tube is dependent on
two main factors; these are the reflection of ambient light
incident on the tube face and light from the remainder of
the display, the so-called 'flare' of the display tube.

Measurements were made to determine by how much
these factors limit the luminance range which is visible to
the eye.

2. Viewing conditions

2.1. Domestic viewing conditions

A survey of domestic viewing conditions was carried
out in order to determine suitable test conditions. Forty
viewers were asked to measure the incident light falling on
the screens of their home receivers, after sunset, under the

lighting conditions they normally used for television view-
ing. Measurements were made using a selenium photocell
incident-light photometer held immediately in front of the
receiver screen.

Viewers were also asked to measure the distance from
the screen at which they normally sat when viewing tele-
vision. Most of the viewers who took part in this survey
owned colour television receivers. It was, however, found
that the values recorded by viewers with monochrome
receivers did not differ significantly from those with colour
receivers. No distinction is therefore made for the type of
receiver in the results of the survey, which are shown in
Figs. 1 and 2.

30

520

£10

32

64

4 8 16

incident illumination, Im/m^

Fig. 1 - Domestic viewing conditions — distribution of
inciden t ilium ina tion

(PH-96)

.30

20

|M0

s.

7 8 9

viewing distance
picture height

-10

-12

Fig. 2 - Domestic viewing conditions
viewing distance

Fig.

distribution of

1 shows the distribution of the incident illumi-
nation. The maximum and minimum values recorded in
this survey were 55 lm/m 2 and 1-6 lm/m 2 respectively. The
mean of the distribution is approximately 16-0 lm/m 2 .

The distribution of viewing distances, in terms of
picture height, is shown in Fig. 2. The total range of these
results was from 4-2 to 11-5 times picture height, the mean
value being approximately seven times picture height.

3.1. The luminance of subjective black as a function of
ambient lighting

This test was designed to measure the luminance
which the eye perceives as black when bright and dim areas
are viewed simultaneously. It was found that tests carried
out using separate illuminated test charts of high and low
brightness attached to the illuminated screen produced
results which varied over a wide range and were usually very
inconsistent. The cause of this inconsistency lay in the fact
that it was found difficult to prevent the eye from tem-
porarily resting on either the bright or dim area, and
accommodating to suit that particular luminance. A test
chart which included bright and dim areas was designed so
that it required the eye to look simultaneously at the two
areas. This chart (see Fig. 3) consisted of a grey scale
(105 mm x 50 mm) with five steps of reflectance ranging
from 4-5% to 13%, together with a strip of white card
(105 mm x 20 mm) of reflectance 95%, which was attached
centrally along the length of the grey scale. The chart was
mounted vertically on the rear-illuminated screen. One
slide projector, containing a suitable mask and equipped
with an iris diaphragm, was used to illuminate the whole
chart at a low brightness level. A second projector pro-
duced a bright illuminated area (20 mm x 20 mm), with a
fine horizontal indicating line across its centre, on the white
strip of the test chart; this latter projector was mounted on
an adjustable stand which enabled the illuminated area on
the test chart to be moved up and down the white strip of
the test chart.

2.2. The viewing room used for experimental measure-
ments

The experiments described in the following sections
were carried out in a viewing room in which domestic
viewing conditions could be simulated. The room was
illuminated by overhead fluorescent lighting controlled by
a thryistor dimmer; a range of illumination from 0-3 to
30 lm/m 2 could be provided with negligible change in colour
temperature of the illuminant (D 65 ). The walls of the
viewing room had a mean reflectance of 60%, a value typical
for domestic wall covering.

3. The visible contrast range under television view-
ing conditions

The experiments carried out in order to make direct
measurements of the properties of the eye, under conditions
simulating television viewing, were carried out using a trans-
lucent perspex screen 200 mm x 320 mm which was illumi-
nated from the rear to a level of 13 cd/m 2 , corresponding to
the mean luminance of typical scenes portrayed on a tele-
vision screen.* Reflective test charts were then attached to
the screen and illuminated from a light source in front of the
screen. The screen was positioned a distance of 2 metres
from the observer, so that the angle it subtended at the eye
of the observer was approximately the same as that sub-
tended by a television screen under typical viewing con-
ditions.

* The value of 13 cd/m assumes a peak display luminance of 100
cd/m 2 .

illuminated
area -lOOcd/m 2

* the illuminated area
is capable of being moved
up and down the whole
length of the chart.

4'5%

5'9%

77%

-10%

13%

white card
reflectance 95%

Fig. 3 - Test chart for measurement of visible contrast range

Measurements of the perceived black level of the eye
were made by asking the subject to move the spot along the

chart until the indicating line across the bright spot was
opposite the first step in the grey scale which was distin-
guishable from black.

ambient lighting indicate that the characteristic shown in
Fig. 4 asymptotes to a value of luminance of approximately
0-3 cd/m 2 .

The incident light falling on the whole test chart from
the first projector was set initially to a predetermined level.
The second projector was adjusted so that the luminance of
the bright square was 100 cd/m 2 , corresponding to a picture
highlight.

If the subject set the bright spot accurately at a
division in the grey scale other than that between the two
darkest steps, it was assumed that the perceived black level
lay between the luminances of the grey-scale steps on either
side of the division. A more refined measurement of the
black level was made by reducing the level of grey-scale
illumination until the subject found great difficulty in
accurately positioning the bright spot. When this condition
occurred, the threshold of luminance was defined as the
luminance of that grey scale step, of higher luminance,
adjacent to the division at which the bright spot had been
set. It was assumed that below this luminance all objects
appeared black and that detail was invisible.

This test was carried out on each subject at six levels
of room lighting adjusted to produce a range from 1-0 to
30 lm/m 2 incident upon the test chart. The mean values
of the luminance threshold obtained from six subjects are
shown in Fig. 4, plotted against ambient illumination. The
standard deviation of the results at any level of ambient
illumination was approximately 20%.

2 34567910 2

ambient light incident on display, Im/m^

Fig 4 - Luminance of subjective black as a function of

ambien t lighting for a low luminance area 2,0 mm square at

a viewing distance of 2 m

Fig. 4 shows that, for a typical value of ambient
lighting level of 16 lm/m 2 , areas of a television picture at a
luminance of less than 0-8 cd/m 2 will appear black. If the
peak luminance of the display is 100 cd/m 2 the contrast
range visible to the eye will be about 125 : 1. At higher
levels of room lighting the adaption of the eye is controlled
principally by the luminance of objects surrounding the
screen, and the luminance of subjective black rises rapidly
with the increase in lighting level. At low levels of room
lighting the luminance of the screen itself becomes dominant
and the room lighting has little effect on the visible lumi-
nance range. Experiments carried out at very low levels of

3.2. The variation of the luminance of subjective black
with object size

It has been found 4,5 that the measured low luminance
threshold is greatly dependent on the area of the test field
which is used. Tests were therefore carried out to deter-
mine the variation of the threshold with the dimensions of a
low-luminance area, under television viewing conditions.

Square test-areas with dimensions from 10 mm to
80 mm were used; each consisted of two strips of photo-
graphic paper with reflectances of approximately 6% and
8%. These test areas were attached in turn to the illumi-
nated screen used in the previous experiments and a pro-
jector, containing a suitable mask, was used to illuminate
the test area without affecting the illumination of the rest
of the screen. At a fixed level of room lighting, subjects
were asked to reduce the illumination of the test area, by
means of an iris diaphragm on the projector, until the dif-
ference in brightness of the two sections of the test area
could only just be distinguished. The luminance of the
strip of higher reflectance was then measured and regarded
as that of subjective black. The mean results of this test
are shown in Fig. 5.

10

2 3 4 5 6 7

dimensions of square test area, mm

9 10'

Fig. 5 ■

Luminance of subjective black as a function of area

Ambient light incident on display = 16 lm/m
Viewing distance = 2 m

This curve shows that at a viewing distance of 2 m the
eye regards, as black, a value of luminance which, for
images of less than 50 mm square, increases rapidly as the
size of the image is reduced.

An object 50 mm square, viewed at a distance of 2
metres, subtends an angle of approximately 1 -4° at the eye,
and the above result agrees well with experiments carried
out by Lowry.

4. Limitations of the display tube

Measurements were made of the contrast range which
can be displayed on a typical shadow-mask display tube.
These measurements were made in such a way that the con-
trast range of the tube could be compared directly with the
limitations of the eye described in Section 3.

It was found that the minimum luminance which can
be displayed on a shadow-mask tube is limited by two
factors; these are the flare in the display tube, and the
reflectance of ambient light incident on the tube face.
These are considered separately using typical viewing con-
ditions where applicable.

4.1. Flare

The flare was measured on a typical 630 mm shadow
mask colour display tube mounted in a television monitor.
A switching signal was fed into the monitor such that the
left-hand half of the screen only was energised to approxi-
mately peak white (100 cd/m 2 ). A mask made from low
reflectance paper was placed in front of the monitor such
that only a vertical strip of the screen, 3 mm by 50 mm, was
visible through a slit in the mask. The light output from
the tube at various distances to the right of the illuminated
area was measured with a photometer as the mask was
moved across the tube face. The results of these measure-
ments, which are shown in Fig. 6, indicate an approximately
logarithmic relationship between screen luminance and
distance from the illuminated area.

3 45679 10 23 45679 -I0 2

distance from illuminated area, mm

Fig. 6 - Display tube flare
Luminance of energised portion of screen =100 cd/m

Flare in a display tube may be introduced by two
different effects. So-called optical flare is caused by
internal reflection of light within the faceplate of the tube.
A second type of flare, which will be referred to as electron
flare, is caused by electrons landing on parts of the screen
other than where intended.

The effects of these two forms of flare in a colour
display tube are different. If a coloured area is displayed
on the tube, optical flare will cause a cast of the same hue
to be seen on surrounding areas of the screen. In the case
of electron flare there is an equal probability of stray
electrons landing on a phosphor dot of any of the three
colours. The colour of light emitted by electron flare will
therefore be independent of the colour of an illuminated
area displayed on the tube, and will tend to be neutral
(depending upon the relative phosphor efficiencies).

The relative significance of the two types of flare was
estimated by switching off two of the guns of the display
tube so that the left-hand half of the screen displayed one
primary colour. The flare which could be seen on the right
hand half of the screen appeared highly saturated close to
the illuminated area, but gradually became desaturated at
greater distances. At about 50 mm from the illuminated
area the flare appeared almost neutral and was virtually
independent of the colour displayed on the left-hand half of
the screen.

From these observations it can be concluded that
optical flare in the faceplate of the display tube may cause
hue and saturation errors in areas of the picture in close
proximity to regions of high luminance. Electron flare
causes desaturation of the picture over a wider area,
although of a less pronounced form. Both types of flare
decrease the available contrast range which can be displayed
in a picture, by increasing the luminance of the dark areas.

Further experiments were carried out to estimate the
extent to which flare reduces the contrast ratio which can
be displayed in a typical picture. A waveform was applied
to the monitor such that the whole of the screen was
illuminated to a level of 13 cd/m 2 (assumed to be the mean
picture luminance) except for a small black square in the
centre of the screen. The mean luminance in the central
square due to flare was measured whilst masking the
remainder of the picture with low-reflectance paper.
Measurements were made with different sizes of square
from 10 mm to 80 mm.

0-5

:04

;0 3

,0-2-

2 3 4 5 6

dimensions of square test area, mm

10 £

Fig. 7 - Mean luminance of flare vs dimensions of test area

Luminance of energised portion of screen = 13 cd/m

The results of these measurements, shown in Fig. 7,
indicate that the mean luminance due to flare decreases
logarithmically with increase in size of the low luminance
area.

4.2. Reflection of ambient light

Ambient light is reflected from both the front surface
of the display tube faceplate, and the phosphor coating at
the rear of the faceplate. The polished front surface causes
specular reflections to be seen by the viewer, while reflec-
tions from the phosphor are diffusely reflected with a lumi-
nance proportional to the mean incident light.

The reflectance of a display tube was measured in the
viewing room in which the previous subjective tests were
carried out; overhead room lighting was used to provide the
illumination. A spot photometer was placed in front of
the receiver in a position from which direct specular reflec-
tions of the room lighting in the screen could not be seen.
The mean luminance of the screen was compared with that
of a magnesium carbonate block placed in front of the
screen.

The reflectance of the tube measured in this way was
found to be 21%.

5. Discussion

The separate effects of flare and reflection of ambient
light on the performance of a colour display tube have been
described in Section 4. These effects combine to produce
an effect which will be termed 'background luminance':
this is inevitably added to the displayed picture. The
greatest subjective impairment is the increased luminance
and desaturation of colours in low luminance areas of the
picture.

In order to estimate the subjective impairment which
it causes, the background luminance level due to flare^and
reflected ambient light must be compared with the lumi-
nance of subjective black under similar viewing conditions.
Ideally, any background luminance in the displayed picture
should be well below the luminance of subjective black for
the particular viewing conditions considered so that it
causes a negligible change in perceived brightness within the
visible luminance range.

Although no tests have yet been carried out to grade
the subjective impairment of picture reproduction due to
background luminance, it will be assumed that significant
impairments are caused when the background luminance
exceeds the luminance of subjective black since the dis-
played contrast will be reduced. These effects will be dis-
cussed in terms of a display tube with a diagonal of 630 mm
viewed at a distance of seven times picture height (approxi-
mately 2-8 metres).

It should be noted that the measurements of lumi-
nance of subjective black described in Section 3 were
carried out on a smaller screen at a viewing distance of 2
metres. Since it has been found that these measurements

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are a function of the visual angle subtended by the image
they must apply to an increased size of aperture when the
viewing distance is increased. In order that the visual
angles be comparable under different viewing conditions,
the dimensions of test areas must be changed in the ratio of
the viewing distances. Fig. 4 thus shows the luminance
that would be seen as black in an area 28 mm square at a
viewing distance of 2-8 metres. This can then be compared
with the characteristic of display-tube background-lumi-
nance for a similar aperture.

20

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1 2 3 4 5 6 7 9 10 ,2 3

ambient light incident on display tube, lm/m 2

Fig 8 - Background luminance of a 630 mm display tube as
a function of ambient lighting for a 28 mm square test area

(a) Reflection of ambient light from tube face

(b) Display tube flare (optical + electron) measured with surround
luminance of 13cd/m 2

(c) Flare + reflection of ambient light

The luminance caused by reflection of ambient light
is shown in Fig. 8(a), as a function of ambient light for a
display-tube screen-reflectance of 21%. The mean lumi-
nance due to flare in a 28 mm square area, derived from
Fig. 7 is shown in Fig. 8(b). The total background lumi-
nance for a 28 mm square area can be obtained by addition
of these two characteristics (Fig. 8(c)).

The characteristic showing the variations of back-
ground luminance, with area, at a fixed ambient illumi-
nation can be derived by a similar method.

Fig. 9(a) shows the luminance of the screen due to
reflection of ambient illumination at a level of 16 lm/m 2 .
The variation of flare luminance with the area is shown in
Fig. 9(b). The total luminance of the area, Fig. 9(c), is
obtained by addition of the ordinates of 9(a) and 9(b).

The variation of screen background luminance with
ambient illumination and size of test area have been re-
plotted in Figs. 10 and 11 together with the characteristics
of the luminance of subjective black under similar viewing
conditions.

From Fig. 10 it can be seen that for an area of 28 mm
square the background luminance of the display tube
exceeds the luminance of subjective black for levels of
ambient light above T5 lm/m 2 . At high levels of illumi-
nation, above 15 lm/m 2 , the luminance of the area is
nearly twice that of subjective black.

SO-4

2 3 4 5 6 7

dimensions of square test area, mm

9 -K) £

Fig. 9 - Background luminance of display tube as a function

of dimensions of test area at an ambient lighting level

of 16 lm/m 2

(a) Reflection of ambient light from tube face

(b) Display tube flare (optical + electron)

(c) Flare + reflection of ambient light

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2 345679 10 2

ambient light incident on display, lm/m 2

4 5

Fig. 10 - A comparison of the background luminance of the

display tube and the luminance of subjective black as

functions of ambient light

(a) Background luminance of display tube

(b) Luminance of subjective black

Fig. 1 1 shows the variation of background luminance,
and subjective black, with the size of the test area at the
mean domestic lighting level of 16 lm/m 2 . It is apparent
from these characteristics that, despite the reduction of the
influence of flare, the difference between the background
luminance of the display tube and the luminance of sub-
jective black increases rapidly with the size of the test area.
This implies that a greater impairment of luminance repro-
duction will occur if the picture contains large areas of low
luminance. If areas 100 mm square are to be displayed as

10

2 3 4 5 6 7 9 -KT

dimensions of square test area, mm

Fig. 11 - A comparison of the background luminance of the

display tube and the luminance of subjective black as

functions of the dimensions of the test area

(a) Background luminance of the display tube

{b) Luminance of subjective black

black the background luminance must be reduced to 27% of
its present value. The curvature of the characteristic of the
luminance of subjective black for large areas, shown in Fig.
11 (ft), suggests that no further reduction of background
luminance is necessary for portrayal of areas larger than
100 mm square.

It can be seen from Fig. 9 that, at an ambient lighting
level of 16 lm/m 2 , it is the reflection of ambient light from
the tube face which gives rise to the major component of
the background luminance. It is therefore necessary to
reduce the reflectance of the tube face in order to extend
the contrast range of the tube.

In the type of tube used in these experiments, the
reflectance of the phosphors is approximately 70%. The
light reflected from the phosphor coating is reduced to
approximately 21% of the incident light by a neutral face-
plate with a transmission of 54%. In order to reduce the
reflectance of the tube face to 27% of its present value, the
transmission of the faceplate could be reduced from 54% to
28%, This would require the light output of the phosphors
to be increased by 93% to maintain the peak luminance at
its present value.

Alternatively, the reflectance of the phosphor coating
could be reduced. One method which may be used to
reduce the effective reflectance of the phosphor coating is
to cover the non-emitting areas of the screen between the
phosphor dots with a matrix of low-reflectance black
material. 6 This technique enables the reflectance of the
phosphor coating to be reduced by up to 50% without
reduction of the peak luminance of the tube. A tube in-
corporating a black-matrix phosphor coating would require
a neutral faceplate with a transmission of 40% to reduce the
background luminance to 27% of that of the measured
tube. In this case the light output of the phosphors must
be increased by 36% in order to maintain the same peak
luminance.

6. Conclusions

When a television receiver is viewed under domestic
conditions, background luminance is added to the picture
due to flare and the reflectance of ambient light from the
tube face. This causes an increase in the luminance of black
areas of the picture, and a reduction in saturation of colours
displayed at low luminance. These impairments are most
noticeable when the picture contains large areas of low
luminance.

In order to minimise these effects the reflectance of
the tube face must be reduced, so that the background
luminance is less than the luminance of subjective black.
Under typical viewing conditions this requires the trans-
mission of the tube faceplate to be reduced from its
present value of 54% to approximately 28%. The light
output from the phosphors must then be increased by
about 90% in order to maintain the present value of peak
luminance.

If a tube incorporating a black-matrix phosphor
coating is used, a satisfactory value of background lumi-
nance will be obtained if the transmission of the faceplate is
40%. In this case the light output of the phosphors must
be increased by 36% in order to maintain the present value
of peak luminance.

If phosphors can be developed to enable these modi-
fications to be carried out, it should be possible to display a
contrast of 250 : 1 under typical viewing conditions. In
order to make full use of this improvement, it would be
necessary for the contrast characteristic of the transmitted
signal to be modified accordingly^

Despite the limitations of currently available display
tubes, a contrast of 200 : 1 can be portrayed if the level of
ambient light incident on the face of the tube is sufficiently
low. Such low lighting levels do prevail in some viewers'
homes; it is therefore expected that a considerable improve-
ment in picture quality would be apparent for these viewers
if the transmitted contrast range could be extended.

7. References

1. HUNT, R.W.G., PITT, I.T. and WARD, P.C. 1969.
The tone reproduction of colour photographic materials.
/. photogr. Set, 1969, 17, 6, pp. 198 - 204.

2. HUNT, R.W.G. 1969. The effect of viewing con-
ditions of required tone characteristics in colour photo-
graphy. Br. Kinematogr. Sound & Telev. Soc. Journal,
51, 7, pp. 268 - 275.

3. BARTLESON, C.J. and BRENEMAN, E.J. 1967.
Brightness reproduction in the photographic process.
Photogr. Set Engng, 1967, 11, 4, pp. 254- 262.

4. LOWRY, E.M. 1951. The luminance discrimination
of the human eye. /. Soc. Motion Pict. Telev. Engrs,
1951, 57, 3, pp. 187- 197.

5. LOWRY, E.M. and JARVIS, J.G. 1956. The lumi-
nance of subjective black. /. Soc. Motion Pict. Telev.
Engrs, 1956, 65, 8, pp. 41 1 - 414.

6. FIORE, J.P. and KAPLAN, S.H. 1969. A second
generation color tube providing more than twice the
brightness and improved contrast. IEEE Trans. Beast
& Telev. Receiv., 1969, BTR-15, 3, pp. 267 - 275.

SMW/AM

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