Chiếu sáng trong truyền hình - Television Lighting Television is a means of changing

Chia sẻ: Nguyenhoang Phuonguyen | Ngày: | Loại File: PDF | Số trang:6

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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....

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Nội dung Text: Chiếu sáng trong truyền hình - Television Lighting Television is a means of changing

Chi u sáng trong truy n 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 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 consumer camera. 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 either setting. Contrast
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. 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 3200 --------------Most Photo or TV Lights -------Orange
3400 ----------------Some Photo Lamps ----------Orange 3500-4000 -----------Fluorescent Lamps ----------Green 5500 ------------------Midday Sunlight ------------Blue 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.