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
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio-Electronics articles I have already
posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage Radio-Electronics articles.
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
"color-blind" 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
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.
Physicists 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
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.
It was 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 cyan.
Fig. 4 - Red, green and blue combine to form white.
Fig. 5. - Each color signal voltage has a phase as well as an
Fig. 6 - Black and white patterns which generate
the sensation of colors when rotated.
Note: 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 color.
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
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°.
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.
It 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
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. 6-b.
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
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.
known facts of "color blindness" also support the theory of three color
receptors in the retina, responding in the regions of red, green and
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
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 colors.
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
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.
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 the filter.
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
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%.
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 %.
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
This particular (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
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.
Posted January 7, 2014