In this series of articles, we will explain broadcasting for IT engineers. Television is an illusion, there are no moving pictures and todays broadcast formats are heavily dependent on decisions engineers made in the 1930’s and 1940’s. In this article we start to look at the human visual system and color temperature.
The human visual system (HVS) is extremely complex and embraces the interaction of the eyes and brain. Optical illusions are a play on this system and we can often be fooled into seeing either something that doesn’t exist or is a distortion of reality.
Color television is such a play. In broadcasting we do not truly transmit color, that is, we do not send infinitely varying signals that are representations of the colors in the electromagnetic spectrum at each pixel at a point in time. Instead, we take advantage of the fact that we can synthesize color using predominantly red, green and blue light.
In Part 3 – Video Lines, we looked at the different types of receptors in the human eye, specifically rods and cones. Simplistically, rods are used to detect monochrome light, and cones detect color.
Warm or Cold?
Color temperature, simply stated, is the system used to describe the color characteristics of light. Broadcast engineers, cinematographers and production crew will often refer to a scene as warm or cold. Warm is the lower color temperature and appears as yellow or orange. Cold is the higher color temperature and often is perceived as blue.
Although light may appear to be the same color throughout the day, it does in fact change. Our HVS is so good, that it automatically corrects our internal color representation, so all colors are perceived to be the same throughout the day, or under different lighting. Only when we compare two dissimilar sources of color temperature side-by-side do we see a difference.
During sunrise, the color temperature is in the region of 2,000 Kelvin (K), and as the day progresses the color temperature changes to 5,500K, and then back to 2,000K at sunset as the sunlight disappears over the horizon.
Strangely, although duller, a cloudy day has a color temperature of 7,000K and the shade under a blue sky is 10,000K. The HVS is extremely efficient at adjusting for these color temperature differences so our perception of color remains constant.
Different artificial lamps have distinct colors too. A tungsten lamp has a color temperature of approximately 3,200K and a Hydrargyrum Medium-arc Iodide (HMI) lamp is 6,000K. HMI’s are incredibly powerful lamps and are generally used outside so they can be used to match the midday sun.
Only when we move from a bright summers day and go inside to a tungsten lit room do we get a hint at the differences in color temperature; for a few seconds, the room appears to be slightly yellow or orange as our HVS adjusts.
Florescent lamps can be designed to deliver assorted color temperatures. Generally, a domestic florescent lamp will have a color temperature of 4,400K, appearing blue compared to a tungsten lamp, but they can be equally designed for 3,300K or 6,000K.
In the controlled environment of a studio, the effects of color temperature are usually constant and easy to control. A studio will be fitted with many lamps, but generally are all the same type such as tungsten, or florescent. Cameras are easily matched under the same type of color source, but it’s very difficult to match under differing sources such as tungsten and florescent.
When moving between distinct color sources, camera operators must re-calibrate their cameras to take into consideration the change in color temperature, a process known as white-balancing. Although automated color balance is available in broadcast video cameras, operators will usually operate their camera in manual mode, so any automation doesn’t affect their pictures, and must therefore manually white-balance.
White-balancing is the simple task of holding the camera steady, focusing on an illuminated white piece of paper and then pressing the white-balance button. The process completes in a few seconds and is carried out many times throughout the day.
In live news, where there may not be time to white-balance, the camera operator usually places a piece of blue transparent celluloid over the light above the camera. Known as a “Gel”, this will correct the cameras lamp color, so it matches the outside color temperature. The down side is that the gel absorbs some light and reduces the lamps output, this is easily overcome as the operator will open the iris on the lens to correct for the short fall.
Film is Different
Color correction in Cine cameras using film requiring chemical processing is entirely different. The film stock used is matched to a specific color temperature, such as daylight, or tungsten. Once the stock has been loaded into the camera nothing more can be done to change the color balance.
The Director of Photography (DoP) or Cinematographer is responsible for making sure the colors in the lamps are correctly balanced using the appropriate Gels and the exposure is correct. At the advent of color films, the only time a DoP would know whether the shoot was the correct color and exposure, was when the stock returned from the chemical processing laboratory. At best this would usually happen overnight.
Video-assist, a hybrid of film and video in a camera, allows DoP’s to preview the shots before they are recorded on film to reduce the risk of the color temperature and exposure being wrong. Few directors use film, but instead use video, as it’s far easier to work with and adjust before the scene is shot, potentially saving a great deal of money. However, there are some directors who still insist on using film to help them express their artistic creativity.
Video cameras provide a lot more flexibility for outdoor, indoor and studio recording. Camera operators are constantly aware of color temperature and will white-balance or add Gel to lamps whenever they need to.
Correction Compromises Quality
Bad video recordings can be corrected in the edit suite as the color temperature can be adjusted during post production. However, this is undesirable as quality can be compromised and the process takes valuable time.
As well as getting the best shot, the responsibility of the camera operator extends to getting the best shot at the highest possible quality, and this includes making sure the camera is correctly white-balanced. It’s relatively easy to spot exposure issues as they can be seen in the viewfinder, but the same is not true of white-balance, so the camera operator must always be aware of the color temperature of the ambient light they are operating in.
Be Constantly Vigilant
Color temperature is difficult to observe as the HVS is incredibly efficient at correcting our perception of it. However, camera operators, editors and broadcast engineers must be constantly vigilant, or pictures will take on interesting colors that both compromise quality and disrupt the enjoyment of the viewer.
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