Chiếu sáng trong truyền hình
(English)

Television Lighting

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.
Level
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 camera.
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

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.
Color Temperature
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: Color Temperature
Temperature Source Color
1950 Candlelight Orange
2870 Normal Incandescent Orange