Television is a means of changing patterns of light into electrical
signals for storage or transmission and then recreating those patterns
on a screen. In order to do this well, the television camera must be
presented with properly illuminated scenes. The three important
considerations are overall level, contrast range, and color temperature.
Lighting levels for television are generally set by adjusting the
incident light, or the light striking the subject. The unit of measure
for incident light is the foot candle, which is the amount of light
produced by a standard candle at a distance of one foot. Lighting
measurements are made using an incident light meter, which has a white
plastic cover over the sensing element and a logarithmic scale
calibrated in foot candles. To measure the useful incident light for
television, the meter is held near the subject and pointed toward the
The minimum acceptable level for color television depends on the
ability of the lens to transmit light to the camera, the sensitivity of
the pickup tube or chip, and the amount of depth of field you need. For
high-quality pictures you need something between fifty and two hundred
foot candles. Most cameras can be operated in light ranging from the
minimum up to ten thousand foot candles, or fairly bright sunlight.
Where lighting conditions fall outside this range, steps must be taken
to bring the lighting level into line with the capabilities of the
camera. With too little light, additional lighting must be added. With
too much, special neutral density filters must be used on the camera.
Absolute rock bottom
You'll see cameras advertised as 2 LUX or 4 LUX cameras. 2 LUX is equal
to .19 foot candles. 4 LUX is about .37 foot candles. I was suspicious,
so a number of years ago I set up an ordinary candle one foot away from
a white square on a black background. I tested two cameras. The first
was a popular CCD camera requiring four LUX for minimum illumination.
The second was a broadcast camera using Saticon pickup tubes. At a
nominal one foot candle the CCD camera produced 40 IRE units of video,
but the amount of noise in the picture was very objectionable. At four
foot candles the CCD camera produced 100 IRE units with an acceptable
noise level. The broadcast camera produced 20 IRE units at one foot
candle with the "boost" set at 18 dB. At four foot candles, it produced
46 dB at 0 boost and 95 dB at 9dB boost. At four foot candles the
broadcast picture was obviously superior to the picture from the
The difference at one foot candle is essentially the willingness to
tolerate more noise in the CCD camera, giving it more apparent
sensitivity under extremely low light situations. To mask some of
the noise at low light levels, consumer cameras often use a setup, or
black level, of zero IRE, rather than the 7.5 IRE broadcast
standard. Some cameras that automatically boost the signal in low
light situations can also be run in manual mode where you can control
how much boost you want to use.
Lighting levels of five to fifteen foot candles are common in homes,
while office settings tend to range from fifty to sixty foot candles.
Keeping the reservations in the preceding paragraph in mind, consumer
camcorders should have plenty of light for acceptable pictures in
Contrast refers to the difference in brightness from the darkest parts of a scene to the brightest.
Useful contrast for NTSC television is determined by the amplitude of
the video signal. The NTSC standard calls for a "peak to peak"
amplitude of one volt at 75 ohms impedance. Only seven tenths of a volt
is used for the luminance, or black and white part of the signal.
Common digital video signals are 24 bit color, with eight bits each for
red, green, and blue. This scheme allows for 256 individual
shades from dark to light for each color. Since 24 bit color
allows for over sixteen million colors, the limited number of shades
available for each color isn't usually a problem, although the
luminance steps may be visible in monochromatic scenes.
If there's too little contrast many receivers will produce a
flat, grayish picture. If there's too much contrast, details in the
brightest and darkest parts of the picture will be lost and the picture
will look too harsh.
Since contrast is actually light reflected from the subject, it's
measured using a reflectance light meter. The meter is held near a
variety of very light and very dark parts of the subject and pointed
toward each part of the subject to be measured. The ideal contrast
range for NTSC television is about twenty to one. This corresponds to a
difference of about four and one half f-numbers between the darkest and
brightest parts of the picture on a reflectance light meter. In
practice, actual contrast ranges are rarely measured using a meter. A
subjective analysis based on camera output is generally sufficient.
The third consideration is color temperature. Every source of light has
a characteristic color. This color is related to its "temperature."
Lower color temperatures tend to be red or orange while higher
temperatures tend to be green or blue. Color temperatures are measured
in degrees Kelvin. Some examples:
||Most Photo or TV Lights
||Some Photo Lamps
The eye "remembers" how things are supposed to look and interprets
color accordingly, regardless of the color temperature of lighting
sources. A white sheet of paper seems white whether viewed under an
incandescent lamp or sunlight. The eye can even adjust for "correct
color" when two light sources of different colors are present in the
same scene. Sunlight streaming into a room which is also lit by
incandescent lamps doesn't make objects it strikes appear bluish.
Television cameras aren't so versatile. They must be set up to render
color in a way that's pleasing to the eye. They can do this only if all
of the important lighting sources within a scene have the same color
temperature. A combination of filters and electronic adjustments is
used to adapt color cameras to each new lighting situation. Most
cameras can adjust automatically to typical color temperatures.
They cannot resolve conflicts when major picture elements are lit at
different color temperatures.