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Transition to Digital: Colorimetry

In the 17th century, two scientists devoted their time to the study of light: Isaac Newton and Christian Huyghens. While trying to fend off falling apples, Newton carried out many experiments and found that white light is a homogeneous mixture of an infinite number of colors ranging from red to yellow and green through blue to violet. He demonstrated this concept by directing daylight through a narrow slit onto a triangular glass prism. The ray that emerges from the prism is a divergent beam of light containing no trace of white but, instead, a continuous sequence of colors. (See Figure 1.) He also demonstrated that the white light could be re-created by passing the divergent beam through a second prism. He assumed that light was composed of colored particles whose paths were deviated differently by his prism. Huyghens carried out other types of experiments and deduced that such phenomena as interference could only be explained by assuming that light was an oscillatory wavelike phenomenon. These two scientists could not reconcile their opinions concerning the nature of light.

In the 19th century, various studies led to a better understanding of light. The studies culminated with the work of James Clerk Maxwell, who introduced the concept of electromagnetic waves consisting of an electric field travelling in space with a related magnetic field. The two fields are at an angle of 90 degrees. (See Figure 2.) The concept of electromagnetic radiation grouped together many natural phenomena, as shown in Figure 3. As can be seen, the segment of the spectrum visible to the human eye occupies approximately one octave, from 380nm to 760nm (1nm = 10 superscript -7cm). A perceived color is associated with every wavelength. Maxwell's studies predicted the use of radio electromagnetic waves for the transmission of information - a prediction that culminated with Heinrich Hertz's experiments and Guglielmo Marconi's radio transmissions. These studies and practical applications demonstrated to the scientific world the wavelike nature of light as part of the wider electromagnetic spectrum.

Further experiments resulted in the discovery of the photoelectric effect where light shining on a photo sensor generates an electrical current. This phenomenon could only be explained by the existence of light particles called photons, which impart their energy to atoms and force electrons to travel in an electrical conductor. Reluctantly, scientists of both camps had to concede that the electromagnetic radiation has a dual nature: corpuscular and wavelike. Luminaries Max Planck and Albert Einstein predicted this.

Human visual system characteristics The human visual system (HVS) perceives light by associating colors (or hues), a perception artifact, to frequencies. The eye has maximum sensitivity at the green color and lesser sensitivity at red and blue.

Color perception is associated with a specific set of 6 million to 7 million cells called cones. Studies indicate that there are specialized types of cones responding to red or 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, as well as picture detail, is due to a second type of cell called a rod. 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 film) images. Among the early preoccupations of the dream factories (Hollywood and others) was the correct reproduction of colors. In 1932, a group of scientists under the umbrella of CIE (Comite International de l'Eclairage or International Lighting Committee) developed a bi-dimensional (x, y) representation of the visible colors, the so-called CIE diagram, shown in Figure 4. This diagram allows users 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 white light as a set of x and y values describing a point in the central area of the diagram. Various standards define 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 adding any two could generate none of the three. The process of generating various colors using three primary color sources is called additive color mix. All light sources generate additive colors.

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) 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 luminance equation: This involves the specification of the matrix coefficients related to the R, G and B primary signals.

- The color-difference equations: This involves the specification of the matrix coefficients related to the R, G and B primary signals.

- The transfer characteristics: The transfer characteristic of the CRT is inherently nonlinear. To achieve an overall linear transfer characteristic, the nonlinearity of the CRT is compensated for elsewhere in the system. Historically, the compensation is carried out in the camera and is referred to as gamma correction. This results in red, green and blue signals predistorted to match the reference characteristic of the CRT. More recent standards specify complex mathematical expressions that are applied to linear R, G and B signals to compensate for defined CRT nonlinearities. The transfer standards will eventually have to be revisited to reflect the appearance of non-CRT display technologies featuring linear transfer characteristics.

The ITU-R.BT.470-4 (NTSC 1953) defines parameters of the NTSC color television system adopted in 1953 for transmission in the U.S. These parameters reflected the CRT technologies in existence at the time. Early versions of the PAL and SECAM systems used the same parameters. Later CRT technologies used phosphors with different chromaticities requiring 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 very similar to those used for PAL and SECAM. SMPTE 240M reflects the parameters chosen for the legacy analog HDTV standard using a total of 1125 lines with 1035 active lines. The ITU-R.BT.709 standard is the currently preferred version.

Signals using the legacy NTSC 1953 standard differ considerably from the newer standards that have smaller differences. The implementation of the DTV standard will require a fair amount 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.