s, which are exceedingly The History and Technology of Analog Video sensitive, respond to brightness only, and thereby describe an image to the brain in shades of gray. Cones, which are concentrated toward the center of the retina, respond to color, specifically, light of three different wavelengths that we know as red, green, and blue. Blending these three colors in the brain derives all other colors. Yellowish Green Green Yellow Green Greenish Yellow Green Yellow Yellowish Orange Orange Bluish Green Reddish Orange Green White 700-740 Greenish Purplish Purplish Purplish Purple Reddish Purple Bluish Purple Figure 5-6. Chromaticity Chart (Michel Bayard). Each rod and cone is connected to an individual fiber in the optic nerve. When we scan an image, the rods and cones send messages to create in the brain a high- resolution light mosaic that the brain interprets as an image. Visible light-color-is but a small interval in the spectrum of electromagnetic radiation. This radiation, which fills the universe, possesses dual characteristics of waves and particles. The particles, called photons, contain energy. For the purposes of video communications, energy is measured in electron volts (eV). The energy of visible light varies from two to four eV. The particles that comprise electromagnetic radiation are assembled in waves. A signal's frequency can be determined by measuring the number of wave-crest cycles that pass a fixed point in one second. Although frequency is measured in hertz, it is not practical to use the traditional system to measure light because the frequencies are too high. Therefore, we discuss these waves in terms of their lengths. Light waves span 380-770 nanometers (10-9 meters), in other words, between 16 and 28 millionths of an inch. We describe these various wavelengths by giving them color names-violet, blue, green, yellow, orange, and red. The exact length of a wave determines its color, but most visible lights we see are blends of three primary Video Communications: The Whole Picture colors-red, 