Indian-head test pattern

The Indian-head test pattern is a test card that gained widespread adoption during the black-and-white television broadcasting era as an aid in the calibration of television equipment. It features a drawing of a Native American wearing a headdress surrounded by numerous graphic elements designed to test different aspects of broadcast display. The card was created by RCA to be the standard image for their TK-1 monoscope, a simple video camera capable of producing only the image embedded within it. The pattern was introduced in 1939 and over the following two decades became a fixture of television broadcast across North America in 525-line resolution and (often in modified form) abroad in 525- and 625-line resolution until it was made obsolete by the rise of color television in the 1960s.

Features and use

The Indian-head test pattern was created by RCA at its factory in Harrison, New Jersey. Each element of the card was designed to measure a specific technical aspect of television broadcast so that an experienced engineer could, at a glance, identify problems. The card contains elements used to measure aspect ratio, perspective, framing, linearity, frequency response, differential gain, contrast, and brightness. The grid and circles were used for perspective, framing and linearity. The tapered lines (marked with 20, 25, 30, and 35) were used for resolution and frequency response. The thin lines marked from 575 to 325 on one side and 300 to 50 on the other side referred to lines of resolution. The gray bands emerging from the center off to the lower right and upper left were for differential gain, contrast, and white level. The pattern began with the Indian-head portrait created in August 1938 by an artist named Brooks using pencil, charcoal, ink and zinc oxide. For about a year, the portrait (which contains several identifiable shades of gray from Zone VIII texture in the white feathers to Zone II texture in the black hair) was the entire test pattern, but in 1939 the portrait was incorporated into the current pattern of calibrated lines and shapes.

Television stations would produce the image of the Indian-head test pattern in two ways. First, they would use a monoscope in which the pattern was permanently embedded, which was capable of producing the image with a high degree of consistency due to the device's simplicity. The monoscope tube is constructed similarly to a small cathode-ray tube (CRT), but instead of displaying an image, it scans a built-in image, producing a video signal. The tube has a perfectly proportioned copy of the test pattern master art (or a modified variant with the station ID replacing the Indian-head portrait, such as those used by KRLD-TV, WBAP-TV and WKY-TV) inside, permanently deposited as a carbon image on an aluminum target plate or slide. The target plate is sequentially scanned with a focused beam of electrons, which were originally called cathode rays. When the electron beam strikes the carbon image areas, the carbon resists current flow, and the resulting lower electron current flow is adjusted to appear as video black. When the electron beam strikes the metallic-aluminum image areas, there is less resistance with higher current flow, and the resulting higher electron current flow is adjusted to appear as video white. This image was used to calibrate monitors in the station. Second, stations would use a cardboard-mounted lithograph of the test pattern (typically attached to a rolling easel in each TV studio); videographing the lithograph would create a second image that could be compared against the monoscope-created control image. The test pattern was useful for the calibration of home television sets as well as television studio equipment, so the image was routinely broadcast outside hours of programming. The Indian-head test pattern was built into the RCA "monoscope" tube, a 2F21, which acted as a complete replacement for the TV camera. (It was often accompanied by an audio test tone for the purposes of calibrating aural system frequency response measurements.)

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