Color temperature

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240
WHAT COLOR IS YOUR TEMPERATURE: THE ART OF RECORDING
PERFECT COLORS
Have you ever bought a suit or dress in a store, then
brought it out into the daylight only to find the color looked
yucky? Have you ever taken photographs both outside in daylight
and inside in incandescent lighting then found that the pictures
taken inside all looked reddish? Have you ever wondered why some
of your videotapes have a blue or a magenta cast to them or
people's skin looked yellow or greenish? What makes this happen?
The answers are revealed through physics, psychology, and
electronics. The physics will explain how light makes colors.
The psychology will explain how our brain interprets those
colors. Finally, we will see how the engineers have devised
circuits to accurately record those colors. Knowing a little
about these three subjects prepares you to answer the questions
in the previous paragraph.
The Science of Color
Light is made of electromagnetic waves of varying sizes.
Most of these waves are so small that 160 of them fit in the
thickness of a human hair. The light that our eyes can see has a
size (wavelength) between 380 and 780 nanometers (billionths of a
meter). Our eyes interpret the short waves as bluish, and as
they become longer they become green, yellow, and finally red at
the 780 nanometer end of the spectrum. There are even colors we
cannot see. There is infrared (redder than red) that could be
described as more heat than light. Snakes sense infrared in
order to target their prey. Using infrared sensors, soldiers can
see warm vehicles and people in the dark. Ultraviolet colors are
bluer than blue.We cannot see them but insects can and use
these colors to find necter in flowers. Where our eyes would see
a white flower, their eyes would see a bull's-eye.
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Just as an insect's eyes are different from our own, your
camcorder's "eye" is also different from ours. It emphasizes
some colors we don't see very well.We can see some colors it
doesn't see very well. As a result, your camera sees the world
slightly different from we do. These differences are quite
small, but become more noticeable under certain lighting
conditions. Cameras have a hard time reproducing browns and
saturated red colors. Cameras tend to see blue a little better
than we do.
After all these years of Tide commercials, let's ask
ourselves the dumb question, what is white? Physicists tells us
it is the sum of all the colors added together, a mixture of many
different wavelengths of electromagnetism. Just as an artist can
mix a few colors on his/her palette to make a painting of many
thousands of colors, nature can paint our world from a palette of
just three primary colors: red, green, and blue. With these
three colors we can make every other color our eyes can see.
To understand this concept, try this experiment. Buy
yourself a red floodlight, a green floodlight, and a blue
floodlight. Some evening, darken the room, and aim the red
floodlight at a white wall. You will see red. Turn it off and
aim the blue floodlight at the white wall. Naturally, you will
see blue. Aim the green floodlight alone and you will see green.
Now for the magic, turn on the green and red together and the
colors will mix to create yellow. Cover part of the green light
with your hand diminishing the green and the result will look
more orange. Diminish the red and the result will look
greenish-yellow. Thus you can get a broad array of colors just
by mixing various proportions of green and red.
Similarly, aim just the red light and the blue light at the
wall. The result will be magenta. Varying the intensity of
these two lights creates another spectrum of colors.
Aim the green and blue lights at the wall and you get cyan,
sort of an aqua. At this point the neighbors have probably
called the police thinking aliens have invaded your house.
Switch on all three lights and you'll see white. Equal
amounts of red, green, and blue make white. Vary the intensity
of any light and you create a pastel of any color. Bright colors
result from using bright lights. Dim the lights and you make
dark colors. Turn them all out and you see black, naturally.
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Red, green, and blue are not the only primary colors; there
are other trios that can make all the colors. Because red,
green, and blue are easy to work with, they have become the
primary colors used in the television industry for designing TV
cameras and TV sets.
Combining red, green, and blue colors is called additive
mixing. You add colors together to get more colors. The more
light you add of any color, the brighter the final image looks.
This is just the opposite of subtractive color, what happens when
an artist mixes paints.
If an artist mixes red, green, and blue paint together, the
result doesn't look white, it looks an icky brown. But I'm
getting ahead of myself. Time for another question.
What makes a grapefruit yellow or an apple red?
Nearly all surfaces absorb sunlight and reflect sunlight.
A perfectly black surface absorbs all the light. A white surface
(like the wall you used in the previous experiment) reflects
nearly all the light (nothing is perfect, so a little light
always gets absorbed). If you shine white light on a perfectly
yellow grapefruit, the blue, green, and most other wavelengths of
white light will be absorbed by the surface while the yellow
wavelength is reflected. Even though you started with white
light, your eye only sees yellow after the grapefruit's surface
has filtered out all but the yellow.
The artist does the same thing when painting. Blue paint
on a white canvas will absorb all the colors except the red,
which is reflected back to your eye. Dab some yellow paint
elsewhere on the canvas and only yellow light will reflect from
that area and you will see yellow. Now for more magic: mix some
of the yellow paint with the blue paint and you see green. Mix
red, green, and blue together and you'll get ... icky brown.
What happened here? Wasn't red, green, and blue supposed to
equal white?
When we aimed spotlights at the white wall, this was
additive color mixing. Each color added to the others to make
another color. When we mixed paint together, however, we are
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using subtractive color mixing. Each paint color absorbs nearly
all the colors except its own and reflects just that color. The
red paint subtracted nearly all the other colors except red. The
blue paint subtracted nearly all the other colors except blue.
The green paint subtracted nearly all the other colors except
green. Mix the three together and we have subtracted nearly all
of the colors reflecting none. Theoretically, the mixture should
look black, but in the real world nothing is perfect and instead
we see an icky brown.
All of this is building up to something: the color that
meets your eye (or your camcorder) is a combination of two
things; the color of the light you start with, and the color that
is reflected from the surface. The two react in surprising ways.
Back to our first experiment. If we aimed the blue light
at a red wall, what do you suppose we'd see? You'd see black or
almost black. Blue light strikes the wall but the wall absorbs
blue light and reflects only red. But there wasn't any red.
Therefore there is no reflection from the wall and it looks
black.
This explains why your clothes look different in the store
light and in the sunlight. Sunlight contains nearly all the
color wavelengths allowing you to see all the colors that are in
the suit. In the store, however, the fluorescent lights have
lots of green, yellow, and blue wavelengths but little red. The
store light may look white to you but it really isn't. Meanwhile
there is almost no red light to reflect off the red stripes in
the suit, making them dark. If the red stripes have a tiny bit
of blue in them and the blue light is very strong, you may see a
very bluish-red. Take the suit out into the sunlight and the red
will overpower the blue the way it is supposed to.
Incandescent lights in a store have a reddish hue. Blue
colors will look weak and red colors will look strong.
The same thing that happens to suits happens to faces.
Fluorescent lights turn faces yellow/green, while incandescent
lights turn faces reddish. Only sunlight tells the truth.
And even sunlight cannot be trusted all the time. In the
early morning and in late afternoon, dust and pollution in the
air make the sun reddish. A cloudy day filters out red colors
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making the sun bluish. A bright blue sky also adds some blue to
your picture. In the real world it is almost impossible to get
perfectly white light to shoot by. How we get around this
problem will be explained in a moment, but first another physics
lesson.
Physicists love to measure and quantify things. If you
said a light was reddish, they'd say "how red...give me a
number." The number given to the redness or blueness of white
light is called color temperature and is measured in degrees K).
A Kelvin degree is about 273° higher than the same temperature
centigrade. Color temperature is derived by heating a very black
object (called a "black body") hotter and hotter. At first the
object would glow red at 500 degrees Kelvin, then orange at 2000
degrees, and be white hot at 3500° . Applying more heat in Tim
Taylor fashion, the body would glow bluish white at 6000 degrees
through 10,000 degrees. Above 10,000 degrees, the color gets no
bluer. Probably the instrument melts at this point, setting off
smoke detectors all over the lab. Anyway, thanks to physicist
Max Planck (who first described this phenomenon) and his local
fire department, the subtle coloration of white light can be
described by its color temperature.
The Psychology of Light
The human brain is a wonderful thing. In conjunction with
the eye, it adapts to all kinds of lighting situations. To our
eyes, a friend's face will look normal color at noon time,
sunset, under the fluorescent lights of a store, or the
incandescent lights of the living room. The face is a markedly
different color in each of these situations but our brain makes
unconscious adjustments so that the color of the face "makes
sense."
Photographic and video cameras, however, don't have brains.
Take a photograph of a face outdoors and it will look normal.
Take another shot illuminated by the living room lamp and the
face will appear very red. Shoot the face again under the
fluorescent kitchen light and it will look yellow/green.
Photographers adapt to these situations using colored
filters over their camera lenses. When shooting indoors, they
use a bluish filter to counteract excessive red coming from
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incandescent lights. When shooting under fluorescent lights,
another color filter is used. It is also possible to change the
type of film used from "daylight" (film that is color balanced
for outdoor's colors) to "indoors" (color balanced for reddish
incandescent light).
Professional TV cameras use the same mechanism to adjust to
various lighting conditions. Built into these cameras are color
filter wheels which can be rotated to place the properly colored
filter behind the camera lens. Professional cameras also require
other color adjustments; this is just one of them.
The Electronics of Color Balance
Place a sheet of paper under perfectly white light and aim
a color camera at it. The optics of the camera will be breaking
the white image into the primary colors red, green, and blue.
The camera's electronics will measure the red, green, and blue
and supposedly these three colors should be equal (remember in
our physics lesson that equal amounts of red, green, and blue
equals white). If the three electronic signals are not equal,
the camera is misadjusted. The image may look pinkish or bluish
instead of white. The problem can be corrected by decreasing the
strength (gain) of the offending color or increasing the strength
of the opposite colors. You could see the results of your
adjustments on a well calibrated TV screen or by viewing various
test instruments such as vectorscopes. Some years ago industrial
cameras had little meters helping you to make equal signals from
the red, green, and blue circuits.
Less talked about, but still important to professionals, is
black balance. Here, the camera is capped (no light enters) and
the image should be pure black. If it has a slight tint, the
circuits are adjusted to remove the offending color.
Once a color TV camera is adjusted for black balance and
white balance using a white sheet under white light, all of the
rest of the colors will take care of themselves, under the same
light.
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Now what happens if the camera moves to another lighting
situation such as indoors? Faces will look red. To solve the
problem, the camera is white balanced again. A white sheet is
illuminated by the indoor light and the camera is aimed at the
sheet. The sheet will appear reddish because the light striking
it was reddish. Using various instruments, the camera engineer
would turn down the strength of the red circuit, and maybe boost
the blue until again, red = green = blue. Now all the other
colors should look proper for indoors. Move to another scene
with different lights, and the process starts all over again.
Auto White Balance
As cameras improved, automatic circuits took the place of
knob twiddlers. This made it possible to aim your camera at a
white sheet of paper illuminated by your scene's light, and push
one button. The camera's circuits adjusted themselves so that
red = green = blue. Let up on the button and the camera stays
locked to these settings until you perform the procedure again.
As noted before, professional cameras make two adjustments:
By turning a color balance wheel a glass filter will change the
colors to some degree. After this, the camera is black and white
balanced to "perfect" the color rendition.
Some industrial cameras, instead of using a white card, use
a milky colored lens cap to make a white image. Here you would
aim the camera at the illuminated scene, cover the lens with a
milky cap, and press the auto-white balance button. All the
colors of the scene would mix together to white by the
translucent cap and the camera would adjust to that color. This
system is not as accurate as using the white paper because it can
be fooled by large amounts of one color in the scene.
If you forget to carry out a white balance when you start
up your camera or you change lighting, your pictures will have
the wrong hue. Faces may look green or magenta, grass might look
blue. In most cameras there is nothing to remind you that your
color balance is off. Your monochrome viewfinder doesn't show
color. Most color viewfinders are too inaccurate to appraise you
of this problem. Color balancing is just something you have to
remember to do. After watching years of green faces your family
will probably remind you ... in unison ... whenever you lift your
camcorder to your eye.
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Some cameras automatically perform an auto-white balance
when you turn them on. This system is fooled, however, if the
camera is not aimed at the scene when turned on; maybe it
auto-balances on the color of your shoes.
Continuous White Balance
Wouldn't it be nice if your camera could adjust its white
balance automatically as you shot, even if you changed from one
scene to another. This is what the manufacturers have attempted
to give us with a feature called continuous white balance.
Here's how a couple of systems work.
Some camcorders have external sensors on them. One sensor
is for the color R-Y (pronounced R minus Y which represents the
color red with a luminance --- the sum of red, green, and blue
colors --- subtracted from it). The second sensor measures B-Y,
blue with the luminance subtracted. Each of these sensors sit
behind a milky plastic screen that averages all the colors in the
scene into some sort of white. These two signals are compared
and circuits in the camera try to make them equal.
The system works pretty well under most lighting conditions
and most scenes. If the lighting was a little reddish or if the
walls of the room reflected a tinge of green onto the scene, the
circuit would notice the overabundance of red or green and reduce
the gain of those circuits, balancing out the colors and making
them appear white.
The sensors, because they are omni-directional, are easily
tricked. Say, for instance, you are inside a room illuminated
with incandescent light shooting through a window to the
outdoors. Even though your picture is composed entirely of the
outdoor shot, your sensors are picking up some red from the
indoors. To compensate, your camera reduces the red gain, making
the picture look bluer than it should be. This continuous white
balance system works well only when the camera is in the same
light as the subject.
High end consumer and prosumer camcorders use another
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continuous white balance system where the colors are sensed
directly through the camera's lens (TTL --- Through The Lens).
Here the camera's image sensor chips make R-Y and B-Y
signals. The data is compared to data in a look-up table in the
camera's computerized memory. If calculations show the numbers
to be close to what white would be, the camera makes adjustments
to its red and blue circuits to make the R-Y and B-Y signals add
up to white. If, however, the data comes nowhere near what white
would be, the circuit figures the picture is supposed to have
colors far from white, and leaves the picture alone.
TTL color balancing is more accurate than external sensors
on the camera because the circuits do more thinking. They don't
try to fix everything; they just try to fix things that seem
fixable. Also TTL sensing, because it is done through the lens,
senses only the picture the camera sees, not the superfluous
surroundings of the camera.
Things You Can Do To Improve the Color of Your Pictures
Colors will always look best with plenty of light. Sensors
cannot sense colors that are lost in the noise of a weak signal.
Adequate light provides strong signals for the camera to process.
Illuminate your scene with one color temperature only.
Fluorescent light on one side of the face and sunlight on the
other will give Grandma a rosey cheek on the sun side and
necrotic gray/green on the other. Similarly, incandescent light
on one side of a face will make it look sunburned compared to the
sunlight on the other. Your camera can adjust for one kind of
light or the other or average both. In none of these cases is
the camera adjusted exactly right. If daylight illuminates one
side of the face, use a white or silvery reflector to brighten up
the shadowy side. If you have enough light inside an office to
make a good picture, close the blinds so that bluish daylight
doesn't unbalance the color. If using incandescent lights or
movie lights or professional studio lights, use all the same
kind. Standard home light bulbs are redder (about 2000° K) than
movie lights and TV camera lights (about 3500° K).
If your camera has manual or auto white balance, remember
to use it. Aiming a camera at a white sheet of paper illuminated
by the lights in your scene is the most accurate way to set your
white balance. Second best is to use the milky white lens cap
method.
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If your camera has continuous white balance but you can
switch this feature off, do so, perferring manual white balance.
No automatic circuit can outthink a careful human with a white
card.
If your camcorder only works in the continuous white
balance mode, avoid large amounts of any one color in the
background. The multiple colors found in nature average each
other out causing no harm, but if you stand Uncle Hobbitt in
front of a huge green chalkboard or a bright yellow wall, he'll
look ready for Halloween.
You don't have to be too particular about using a white
sheet. A white wall, a tee shirt (but not the yellowed ones in
my closet), or even a printed newspaper will work pretty good
(the black print doesn't effect the color measurements). I even
know a professional who used his cat's stomach to white balance
his camera.
Incidentally, for artsy shots, you can fool your camera's
white balance on purpose. If you white balance on the blue sky
or your blue jeans, your pictures will come out with a coral or
sepia tone. Whatever color you white balance on, the opposite
color will tint your image.
So what do you do if you have forgotten to white balance
your camera and an entire wedding looks like nuptials from
Neptune? If you don't mind a little knob twiddling while showing
your tape, you could try adjusting your TV. Unfortunately,
adjusting your TV's hue control will fix one color while messing
up several others. Perhaps by turning down the color saturation
control, you can make the mistake less obvious. Another solution
is to play your tape through a gadget called a color corrector.
This device can boost one color while reducing another, restoring
your colors to near normal, but at the cost of going down one
more tape generation. Some special effects switchers like the
AVE5 will allow you to mix a slight amount of background color
with your picture. By copying your tape through such a device,
you can add a little red back into Grandma's cheeks.
Remember, for best color balance, don't shoot until you've
seen the white of the color balance card.
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