becomes most obvious when a new lamp is installed in place of a burned out lamp that has shifted in color.
COLOR SHIFT — MEASUREMENT AND CONTROL The appearance of the lamp’s color is described by the measurement of the Correlated Color Temperature (CCT). The normal expected color variation of a population of MVR400 lamps can be as little as 600 Kelvin (K), or as much as 1000 K. All lamps will fall within the industry-wide standard “10 step color oval.” Visually, in simple terms, a one-step color oval increment on the x, y CIE Chromaticity Diagram represents approximately 100 K. metal halide lamps are usually made in small batches that have less color variation than the wider 10-step color oval. If an installation receives lamps from a smaller batch, the color spread will often be less than if the several smaller batches are mixed.
Burn orientation can effectively influence these color variation qualities. Mercury and High- Pressure Sodium lamps may be operated in any burn position and will still maintain their rated performance specifications. Metal Halide and Low-Pressure Sodium lamps, however, are optimized for performance in specific burn positions, or may be restricted to certain burn positions for safety reasons. They are:
U = Universal burning position HBU = Horizontal -15° to Base Up HBD = Horizontal +15° to Base Down HOR = Horizontal ±15° H45 = Horizontal to ±45° only VBU = Vertical Base Up ±15° VBD = Vertical Base Down ±15° If no special burn position is noted, the burn position is universal.
LUMENS — LUMENS LISTED ARE REFERENCE LUMENS Rated average lamp lumens are obtained under controlled laboratory conditions in a prescribed burning position. Initial Reference Lumens refer to the lamp lumen output after 100-hours burning. Mean Reference Lumens refer to the lamp lumen output at the mean lumen point during lamp life. The mean lumen point occurs at 50% rated life for HPS and mercury lamps, and at 40% rated life for metal halide lamps. Lamp performance on typical systems under typical service conditions will vary from the reference lumen ratings.
High Intensity Discharge (HID) lighting systems are subject to a wide range of variations that may affect final lighting levels. As a result, lamp performance on actual systems may vary due to lamp orientation, ambient temperatures, ballast variations, line voltage and other reasons. Care must be taken when choosing a system to consider how these changes can affect your light levels both initially and at the mean lumen point.
LIFE EXPECTANCY AND POSSIBLE LAMP FAILURE Most HID lamps are constructed of an outer bulb with an internal arc tube made of quartz. The arc tube operates under high pressure at very high temperatures — as high as