Color Rendering Index (CRI)

A Color Rendering Index provides a means of comparing the effect a light source has on the color of an object between a reference source of the same color temperature (CCT). Lamps that have a high CRI rating provide better color rendering.

Induction Lighting Technology

Describes the apparent color of the source and the degree of warmth or coolness it emits.

Efficacy

The expression of efficiency in conveying power (watts) into light (lumens). It is expressed as lumens per watt.

Kelvin

A unit of measure to describe the color temperature of lamps. Low temperatures refer to lamps with a large red component which generally creates a feeling of warmth. High temperatures refer to lamps will generally contain more blue and the space will appear visually cooler.

Lumen

Refers to the unit of luminous flux. One lumen is the amount light striking a one-square-foot area.

Initial Lumen

The measured luminous output of a brand new light lamp.

Mean Life

The rated life of light sources is a measurement of the average hours of use for a lamp. The failure of a lamp occurs when it will no longer start or keep burning. Over any specific group of lamps, some will fail sooner than others, while some may keep burning. The rated life of lamps is defined in the IESNA Lighting Handbook, 9th Ed. as "that time when 50% of a large group of lamps are still in operation." For HID lamps the procedure for determining rated life uses a "burn cycle" of 11 hours of burning and one hour off. While other "burn cycles" will produce different results, this 11on/1off pattern closely resembles the pattern for outdoor lighting. Typically, outdoor lighting is switched on at dusk and off at dawn, so the average burn period is around 12 hours each day, corresponding to 4,380 hours over each calendar year (actual hours are slightly less, around 4,200).

Metal Halide (MH) lamps are sensitive to position, and the lumen output, efficacy and rated life are different for vertical or horizontal lamp positions. A 100W MH lamps has 15,000 hours life in the vertical position and 11,250 hours in the horizontal position. This means the horizontal rated life is only 75% of the vertical lamp rated life for these 100W MH lamps. High Pressure Sodium (HPS) lamps are not sensitive to burning position. The rated life is 24,000+ hours, which means that at 24,000 hours the testing ended with two-thirds of the lamps still burning.

Using 4,200 hours as a typical annual burn total, the corresponding maintenance period for lamps can be estimated. For these 100W MH lamps, the values are 3.6 for vertical position and 2.7 for horizontal position, and for HPS 5.7 years. Lamp life for other wattages of MH sources will be different than for these 100W lamps in this example, with smaller wattage lamps having equal or shorter life and higher wattage lamps having longer life, up to 20,000 hrs. HPS lamp life is consistently 24,000+ hours.

Mean Lumen

The average light output of a lamp over its rated life. Based on the shape of its lumen depreciation curve, mean lumens are measured at 40% of rated lamp life for fluorescent and metal halide lamps. For mercury, high-pressure sodium and incandescent lamps, mean lumen ratings refer to lumens at 50% of rated lamp life.

Rated Lumen

The amount of light expected from a particular version of the manufacturer's lamp under ideal and closely constrained conditions.

Scotopic/Photopic (S/P) Ratio

This measurement accounts for the fact that of the two light sensors in the retina, rods are more sensitive to blue light (scotopic vision) and cones to yellow light (photopic vision). The scotopic/photopic (S/P) ratio is an attempt to capture the relative strengths of these two responses. S/P is calculated as the ratio of scotopic lumens to photopic lumens for the light source on an ANSI reference ballast. Cooler sources (higher color temperatures lamps) tend to have higher values of the S/P ratio compared to warm sources.

Using 4,200 hours as a typical annual burn total, the corresponding maintenance period for lamps can be estimated. For these 100W MH lamps, the values are 3.6 for vertical position and 2.7 for horizontal position, and for HPS 5.7 years. Lamp life for other wattages of MH sources will be different than for these 100W lamps in this example, with smaller wattage lamps having equal or shorter life and higher wattage lamps having longer life, up to 20,000 hrs. HPS lamp life is consistently 24,000+ hours.

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