The electromagnetic spectrum visible to the human eye occupies approximately one octave, from 380nm to 760nm (1nm = 107nm). (See Figure 1.) The way the human visual system perceives light is by associating colors (or hues), a perceptual artifact, to frequencies. The eye has a maximum sensitivity at the green color and lesser sensitivity at red and blue.
Figure 1. Spectrum of electromagnetic radiations
Color perception is associated with a specific set of 6 million to 7 million cells, called cones, situated in the eye retina. Studies indicate that there are specialized types of cones responding to red, green or blue stimuli. The cone cells have a low sensitivity, resulting in achromatic (no color) perception at low light intensities. They also have a low sensitivity to picture detail.
Low light intensity, achromatic light perception and picture detail are due to a second type of cells, called rods, also situated in the eye retina. There are between 110 million and 130 million rods. Rods have a high sensitivity and afford a high resolution of picture detail.
The CIE color diagram
The 20th century witnessed an explosion in the recording and reproduction of still (photographs) and moving (television and movies) images. Among the early preoccupations of the dream factories (Hollywood and others) was the correct reproduction of colors.
Figure 2. CIE color diagram showing location of primaries and outlines of NTSC 1953 and SMPTE 170M color triangles
In 1931, a group of scientists under the umbrella of Commission Internationale de l'Eclairage (CIE), or International Lighting Commission, developed a bi-dimensional (x,y) representation of the visible colors, the so-called CIE diagram, shown in Figure 2. This diagram allows the user to specify colors by assigning values to x and y variables. All visible colors are confined inside a horseshoe-shaped area. Saturated colors occupy positions on the curve. Lower saturation colors occupy positions nearer to the center of the display.
In addition to defining colors, the CIE diagram identifies the white light as a set of x and y values describing a point in the central area of the diagram. Various standards define the white using different pairs of x,y coordinates related to the temperature to which a black body has to be raised to generate the specific white.
Colorimetry standards in color television
Color television relies on the light properties that control the visual sensations known as brightness, hue and saturation. All visible colors of the spectrum can be generated by a proper combination of three primary colors. The definition of a group of three primary colors is that none of the three could be generated by adding the other two. The process of generating various colors using three primary color sources is called additive color mix. All light sources generate additive colors.
Table 1. Various colorimetry standards used in TV production
The photographic reproduction of colors is based on the subtractive color process. Here a white light illuminating a colored surface results in all wavelengths being absorbed, except one, which is reflected and identifies the color of the object.
The television primary colors are red with a wavelength of 700nm, green with a wavelength of 546.1nm and blue with a wavelength of 435.8nm. Any other set of primary colors could have been used, but the choice was determined by the ease with which (in 1953) relatively efficient red-, green- and blue-colored phosphors could be manufactured. The x,y coordinates of the chosen phosphors delineate a “phosphor color triangle.”
Several television colorimetry standards coexist and are detailed in Table 1. These standards define the following:
- The x,y coordinates of the color primaries and of the reference white. This involves the specification of the x and y coordinates representing the primary colors and the reference white.
- The transfer characteristics. The transfer characteristic of the CRT is inherently nonlinear. The transfer function is approximately exponential and commonly referred to as “gamma” curve. Gamma is mainly a light reproduction function of the CRT. In order to achieve an overall linear transfer characteristic, the nonlinear-ity of the CRT is compensated for elsewhere in the system. Figure 3 shows how a nonlinear CRT display is compensated for by a pre-correction of the original signal. Historically, the compensation is carried out in the camera and is referred to as gamma correction. This results in red, green and blue signals pre-distorted to match the reference characteristic of the CRT as follows:Gtransmit = Gpickup1/γ = E´G
Btransmit = Bpickup1/γ = E´B
Rtransmit = Rpickup1/γ = E´RThe NTSC standard defines a gamma of 2.2, while the PAL and SECAM standards define a gamma of 2.8. It is important to note that ITU-R BT.601 doesn't specify the x,y coordinate nor the gamma. Some organizations use the NTSC values, and others use the PAL values. More recent standards, such as ITU-R BT.709, specify complex mathematical expressions, which are applied to linear R, G, B signals to compensate for defined CRT nonlinearities. This has, as a side effect, a positive effect on the noise influence on the reproduced picture. The human eye is more sensitive to noise in the dark areas, where the gamma behavior of the CRT reduces the visibility. The transfer standards will eventually have to be revisited in order to reflect the appearance of non-CRT display technologies featuring linear transfer characteristics.
- The luminance equation. This involves the specification of the matrix coefficients related to the E´G, E´B and E´R primary signals
- The color-difference equations. This involves the specification of the matrix coefficients related to the E´G, E´B and E´R primary signals.
The ITU-R.BT.470-4 (NTSC 1953) defines parameters of the NTSC color television system, adopted for transmission in the United States in 1953. These parameters reflected the CRT technologies in existence at the time.
Figure 3. Correction of CRT nonlinear transfer curve
Early versions of the PAL and SECAM systems used the same parameters. Later CRT technologies used phosphors with different chromaticities. This necessitated a review and update of the colorimetry standards. The ITU-R.BT.470-4 (PAL B,G) used different parameters. The NTSC specifications were reissued in 1995 as SMPTE 170M and used parameters similar to those used for PAL and SECAM. The ITU-R BT.601 standard specifies the parameters of the SDTV component digital format. Note that the color-difference scaling factors are quite different from those used for NTSC and PAL. This is due to the fact that these signals have a peak-to-peak signal amplitude equal to that of the luminance signal unlike NTSC and PAL. The ITU-R.BT.709 standard is the specified version for HDTV.
Signals using the legacy NTSC 1953 standard differ considerably from the newer standards, which have smaller differences. The implementation of the DTV standard will require a fair number of format conversions. In order to avoid color changes in the process, the input and output signal format colorimetry parameters will have to be considered and recalculated as required.
Michael Robin, a fellow of the SMPTE and former engineer with the Canadian Broadcasting Corp.'s engineering headquarters, is an independent broadcast consultant located in Montreal, Canada. He is co-author of Digital Television Fundamentals, published by McGraw-Hill, and recently translated into Chinese and Japanese.
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