so many things in life that we take for granted - aspirin, automatic
clothes washers, drill motors and bits, eyeglasses, rifles, bicycles,
transistors, to name a few - we rarely think about the effort that
went behind the end product that is now enjoyed. Even relatively
simple devices like scissors are the result of someone saying to
himself or herself, "Self, I need something to make cutting fabric
and paper and hair simpler and neater, so what might that thing
look like?" Then, after making a working prototype, improvements
are made based on empirical testing from usage, improvements are
made in the form factor, materials, size, etc., until evolution
results in what can be purchased today. If you have ever been in
a product design cycle, either privately or corporately, then you
know the process well. A concept as sophisticated as developing
television is much more involved than developing a pair of scissors,
but the fundamentals are the same. Implementing color TV for commercial
broadcast had the dual difficulty of not just producing affordable
and reliable color images, but also being backwards compatible with
black and white (B&W) broadcasts and TV sets already in place.
Adding stereo sound to the original monaural FM radio broadcasts
had the same requirement. This article from a 1958 edition of Radio-Electronics
does a superb job of describing the work that went into implementing
color television. You might be as surprised as I was to learn how
much new science was required in the realm of human vision and color
perception in order to design a high quality transmission and presentation.
Prior to color TV, most color perception research went into reflected
light (paint, ink, crayons, etc.) whereas for the first time in
human history it was necessary to determine the peculiarities of
emitted light from a light source (cathode ray tube).
January 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. All copyrights hereby acknowledged.
The Strange World of Color Vision
By Robert G. Middleton
Our cover this month is an abstraction
based on a suggestion by Mr. Middleton to illustrate this
article. The two odd-looking discs are used to produce sensations
of color from black and white elements only. (See Fig. 6.)
The bar slanting across the bottom is, of course, the color
spectrum as most of us know it, and the upright panel is
the color spectrum of a color-anomalous or
person, whose world of color is based on stimuli from two
instead of three primaries. The floating colored dots or
bubbles, chosen to produce a random display of color, are
actually a nighttime color photograph of colored lights
on a Ferris Wheel deliberately shot out of focus. The picture
was taken by Jay Maisel.
Fig. 1 - As wavelength of electromagnetic waves is raised
from 380 to 780 millimicrons, we see in succession all
the colors of the rainbow.
Fig. 2 - The standard color map or chromaticity diagram.
All colors are arranged around white as an optical
Color is by no means simple - it is often incomprehensible
and always tricky; but its apparent inconsistencies make compatible
color television possible.
Children and simple folk
suppose that the colors they see really exist in nature and scoff
at the idea that colors exist only in the mind.
explain that the colors we see correspond to waves of electromagnetic
energy from 4 to 8 X 1014 cycles per second. Electromagnetic
waves used in radio and television transmission have longer wavelengths.
Of course, light waves are not color, any more than radio
waves are color. Physicists do not attempt to explain further and
physiologists cannot. Psychologists are baffled and philosophers
offer various theories which cannot be proved or disproved.
In spite of the unsatisfactory state of our knowledge concerning
color vision, many interesting laws have been discovered, upon which
the technology of color television rests. Some of these laws are
well known while others are familiar only to specialists.
Of all our body's organs, the eye
is most remarkable. Loss of hearing is a personal tragedy but loss
of sight is a calamity - our eyes provide us with more information
concerning the external world than any other organ.
once supposed that the eye is a frequency-sensitive organ because
we see various colors when electromagnetic waves of various frequencies
enter the eye. As shown in Fig. 1, a wavelength of 475 mμ (millimicrons,
mμ) causes us to see blue, 520 mμ green, 578 mμ yellow and
700 mμ to see red.
Fig. 1 shows in a limited manner the information given in Fig. 2.
The chromaticity diagram (Fig. 2) shows around its border the wavelengths
of light corresponding to the common colors. Note that there are
colors along the base of the diagram to which no single wavelength
of light corresponds. This is a rather unexpected fact which is
discussed later in the article.
Fig. 3 - Red and green combine to form yellow; red and blue
combine to form magenta; green and blue combine to form
Fig. 4 - Red, green and blue combine to form white.
Fig. 5. - Each color signal voltage has a phase as well
as an amplitude.
Fig. 6 - Black and white patterns which
generate the sensation of colors when rotated.
I printed the patterns of Fig. 6 and glued a round toothpick
through the center of each. (a) sort of looked bluish and
(b) looked sort of reddish, but there was no clearly discernable
In view of such experimental
data, it is reasonable to conclude that each color we see has a
corresponding frequency or wavelength. However, there are difficulties
which make this conclusion unacceptable.
When we mix red
light with green light, we do not see either of these two colors.
Instead we see a new color: yellow. On this basis, we must abandon
the definition that yellow corresponds to a wavelength of 578 mμ,
since yellow is also produced by a combination of two other wavelengths.
In fact, investigation has shown that the many thousands
of colors which we see can be obtained by mixing only three colored
lights - red, green and blue - in various proportions. Color television
operates upon this law, the law of trichromatic vision. Fig. 3 illustrates
how the primary colors of red, green and blue combine by pairs to
form the complementary colors of yellow, cyan and magenta. Fig.
4 shows how the three primary colors combine to form white.
To obtain compatible operation of black-and-white and color
TV receivers, wavelengths of light are transmitted as various phase
angles of a color subcarrier. This is shown in Fig. 5. Burst is
taken as the reference frequency. Red, which has a wavelength of
700 mμ, is transmitted as a phase angle of 76.5°. Magenta, which
is a combination of 700 mμ and 475 mμ, is transmitted as a
phase angle of 119°.
The colors seen in the spectrum of
a prism are 100% saturated. They are pure colors. Saturated colors
are vivid. Desaturated colors are pale - they have a pastel shade.
The wavelength of a desaturated color is the same as the wavelength
of the same saturated color. However, white light is mixed with
a saturated color to make a desaturated color. In Fig. 5, the relative
voltages of the saturated colors are shown by the lengths of the
vectors. Now, if we shorten the length of the red vector to half
that shown in Fig. 5, we transmit pink - a desaturated red.
is no mystery that hues are transmitted in terms of phase, and that
saturations are transmitted in terms of voltage. These values are
easily calculated throughout. We encounter the unknown only when
we attempt to understand how a color such as yellow is seen when
the eye is viewing a mixture of red and green lights. Perhaps we
shall never know.
White is a mixture
of red, green and blue. Black is the absence of visible electromagnetic
wave energy. As white can be produced from colors, conversely, colors
can be produced by suitable arrangements of black and white. For
example, when we mount the disk shown in Fig. 6-a on the shaft of
a variable-speed motor, we see an arc of color in the rotating pattern.
As the speed of the motor is varied, the hue of the color changes
accordingly. A disk which produces red and yellow is shown in Fig.
It is thought that there may be three types of color receptors in
the retina of the eye, with peak responses to wavelengths in the
regions of red, green and blue. These color receptors are not sharply
tuned but have overlapping responses or considerable bandwidth.
Furthermore, when these color receptors are simultaneously energized
by white light which is then suddenly stopped, the response of the
color receptors does not fall to zero at once, but requires a small
time interval to decay to zero.
Fig. 7-a - Splitting white light into a color spectrum;
b - recombination of color spectrum produces white light.
Fig. 8 - 0ptical filters use resonant electrical circuits
which are provided by nature in the electronic orbits of
the atoms comprising the filter glass.
Fig. 9 - The brightness of a white bar, like that of a color
bar, is equal to the sum of the brightness of its components.
Fig. 10 - The brightness of a color is equal to the sum
of the brightness of its components.
Fig. 11 - Color is characterized by three quantities: brightness,
hue and saturation. Color brightness is determined only
by Y-voltage value and is not affected by chrominance voltage.
Fig. 12 - The chrominance voltages produced by scanning
25%, 50%, 75% and 100% saturated red. Length of chrominance
vector is directly proportional to color saturation. Its
phase does not change with change in saturation.
Fig. 13 - Compatibility of color-TV and black-and-white
TV requires that the Y signal as used in black-and-white
transmission be unaltered.
Because the color receptors
have differing decay times, a residual unbalance of response occurs
from the receptors during the decay time so that we see color in
the black-and-white whirling pattern, which attacks the eye with
sudden changes from black to white and vice versa. The disk should
be rotated counterclockwise at a speed on the borderline of persistence
of vision, and the level of daylight or artificial light adjusted
to produce the maximum intensity of color in the whirling pattern.
Thin, fairly intense red rings, blue tails and yellow fields will
appear under suitable conditions.
The known facts of "color
blindness" also support the theory of three color receptors in the
retina, responding in the regions of red, green and blue.
Test charts are available, which reveal the presence of various
types of color blindness in afflicted persons. Some persons are
completely color-blind, but still get about quite handily, since
black-and-white vision remains (provided by the rods in the retina)
although the color receptors (cones) are incapable of response.
All of us are color-blind in dim light. If you walk out
of a lighted room into a dimly lit hall and look at a color chart,
you will find that all the colors appear to be in shades of gray.
We are also color-blind to very small patches of color and
semi-color-blind to somewhat larger patches. All colors seem to
drift into orange or cyan hues as a patch of the color is reduced
in size. This is the I axis of color television and is the basis
of wide-band color transmission.
Development of the eye
In an embryonic infant, the retina develops as an outgrowth
of the fore-brain. However, the function of vision is located in
the hind-brain and persons who suffer injury to the hind-brain are
as blind as if their eyes had been put out.
If a person
has the hind-brain intact, but his eyes have been injured, he may
still see flashes of light and colors as a result of mechanical
stimulation of the exposed ends of the optic nerves. A somewhat
similar response is observed by normal persons. If the eyelids are
closed and pressure is applied to the edge of the eyeball, rings
of yellowish light are seen.
How the sensation of light
is produced in the mind when the optic nerves are energized is not
known. As far as color television is concerned, it is essential
only that we know the laws of color mixture whereby any desired
color can be synthesized by a suitable mixture of the three primary
Fig. 7-a shows how a prism can decompose white light
into a spectrum of its color components. Fig. 7-b shows how the
color spectrum can be recombined into white light again. This is
not a very striking demonstration. However, we find that we can
remove large regions of the spectrum, leaving only the colors at
the extreme ends and a color at the middle, and still obtain white
on re-composition. This robbed white is quite undistinguishable
by the human eye from the first white.
If we were unable
to rob the spectrum in this manner, color television as we know
it today would be impossible. It is hard enough to transmit three
primary-color signals in a 6-mc channel containing a black-and-white
signal and sound signal without contending with individual signals
for tens of thousands of particular hues.
Robbing the color
A rainbow spectrum of light is conveniently robbed
by use of filters, as depicted in Fig. 8. Optical filters operate
like electric or electronic filters, except that the resonant circuits
utilized are contained in the electronic orbits of the atoms in
All matter is an arrangement of electrical forces
or fields. The electrons comprising a substance have a certain frequency
of rotation about the nucleus of the atom and are coupled to external
space just as an antenna is coupled to space. Incident electromagnetic
energy can be absorbed by electrons in certain orbits, just as a
tuned circuit absorbs electromagnetic energy from an antenna.
Thus, an optical filter is an electronic wavetrap, of atomic
dimensions, which is tuned to the frequency of the electromagnetic
energy recognized by us as a given color. There is no basic difference
between an optical filter and a wave trap except that the wavelengths
used in the optical filter are shorter.
Color can be specified
on a technical basis in terms of brightness, hue and saturation.
Fig. 9 shows that the brightness of a white bar is equal to the
sum of the brightnesses of its components (red, green and blue).
The brightness of red, as seen on a black-and-white picture tube,
is 30%; the brightness of green is 59%; the brightness of blue is
11 % - hence, the brightness of white is 100%.
Fig. 10 shows
that the brightness of a color is equal to the sum of the brightnesses
of its components. Yellow has a brightness, as seen on a black-and-white
picture tube, of 89%. Yellow is comprised of green which has a brightness
of 59% and of red which has a brightness of 30% - hence, the brightness
of yellow is the sum of these brightnesses, or 89 %.
brightness of a color, as transmitted from a color TV station, is
given by the level of the Y (black-and-white) signal component,
as shown in Fig. 11. To this brightness signal level is added a
3.58-mc chroma signal. The chroma signal specifies hue and saturation.
Fig. 12 shows how the voltage of the chroma signal specifies the
saturation (vividness) of the color. We have already seen how the
phase of the chroma signal specifies color (hue).
(NTSC) signal arrangement has been established to provide compatibility
in black-and-white and color TV reception. Fig. 13 shows how a succession
of red, green and blue bars is transmitted as a series of video
levels at 30%, 59% and 11%. This is the only part of the complete
color signal which is seen by a black-and-white TV receiver.
The chroma signal has a relatively high frequency (3.579545
mc) which is largely filtered out in the if and video amplifier
of a black-and-white TV receiver. In a color TV receiver, however,
the chroma signal component is not rejected, but is processed through
the chroma circuits of the color receiver.
The color receiver
responds to the phase of the chroma signal by displaying a corresponding
hue; it responds to the voltage of the chroma signal by displaying
a corresponding color saturation. This is how we get compatibility.