Цветовая температура
На цветном телевидении больше внимания уделяется передаче белого, чем всех других цветов. Прежде, чем делать любые записи или передачи, важно, чтобы камеры имели соответствующий баланс белого,
в котором камера градуируется до необходимого «белого света». Для того, чтобы сделать это, нужно направить камеру на белый лист или диаграмму настройки, которые имеют правильное освещение, затем задействовать автоматический баланс белого камеры так, чтобы сигналы RGB камеры были скорректированы, что должно привести к тому, что белый лист будет выглядеть белым на телевизионном мониторе, или полутоновая шкала будет выглядеть нейтрально.
Допустимые отклонения
Colour temperature
In colour television we are more concerned with the reproduction of white than any other colour. Prior to making any recordings or transmission, it is essential that the cameras have been properly white balanced, in which the camera is calibrated to an appropriate ‘white light’. To do this, expose the camera to a white card or line-up chart which has been suitably illuminated, then
operate the camera’s auto-white balance so that the camera RGB signals are equalized, resulting in the white card looking white on a TV monitor, or the grey scale looking neutral. So, how can we define what we mean by white? One convenient way is to use the concept of colour temperature. The colour temperature of a light source is that temperature at which the colour of light from a black body radiator (e.g. a poker or furnace) is identical to that of the light source. When an iron poker is cold it will look black, i.e. no light is reflected from it; as it is heated up it will glow first red, then amber, yellow, white and then blue, where the colour of the light relates to the temperature of the poker.
Note an important psychological difference. We associate red colours with warmth and blue colours with cold, but with colour temperature, as the source becomes physically hotter, the colour temperature increases and it becomes bluer!
Colour temperature is measured in Kelvin (K) where 0 Kelvin corresponds to -273°C or 0° Absolute. A tungsten halogen light source, at full mains voltage, has a colour temperature of 3200 K. Average summer sunlight is quoted as 5500 K. Strictly speaking this term colour temperature should only be used for incandescent sources, i.e. those which are glowing because they are hot — e.g. tungsten, carbon, candle, gas mantle, the sun.
However, it is useful to quote a correlated colour temperature for discharge sources. The correlated colour temperature of a discharge source is the colour temperature which best matches the discharge source.
Tolerances
Having defined our white point by using the concept of colour temperature, what deviations, if any, can be tolerated? Typical tolerances are ±150 ␣ when white balanced to tungsten sources (3200 K), and ±400 ␣ when white balanced to daylight sources (5500 K). These tolerances apply to lighting face tones; on scenery a greater deviation can usually be tolerated since the viewer is not aware of the true colour of the
scenery but of course is very aware of face tones and very critical of the way in which they are reproduced
