The efficacy of light sources is a measurement of the light output produced for each Watt of electrical energy used - the unit is lumens per Watt. The value can be for either initial, mean or maintained conditions.
Efficacy is sometimes calculated using the rated lamp lumens and rated Watts of the lamp - but this is a misleading simplification when High Intensity Discharge (HID) lamps are considered, because every HID lamps requires a ballast which itself consumes electricity. For example, from the Advanced Lighting Guidelines 2001 (Sec 6.6.2, Table 6-9: Vertical Burn Pulse Start Metal Halide Lamps), the efficacy of 70 to 100 W Metal Halide (MH) lamps is reported to be between 60 and 75 "mean" lumens per Watt.
The actual efficacy value for 100W metal halide lamps is significantly different. Using comparable data from (the leading manufacturer of MH lamps) Venture 2000 catalogue (p 21), the "mean" lumens from a pulse start metal halide lamp depends on whether the lamp is in a vertical or horizontal position. The values are listed as 5900 for vertical and 5300 for horizontal position. Venture's ballasts for such lamps (p 22) have input power ranging from 118 to 128W, depending on the quality of the ballast considered. The corresponding efficacy values are 46 to 50 lumens per Watt for a vertical position and 41 to 45 for horizontal lamps.
For comparison, the Advanced Lighting Guidelines 2001 reports (Sec. 6.6.3) the efficacy of High Pressure Sodium (HPS) sources to be between 70 and 120 "mean" lumens per Watt. From the same Venture 2000 catalogue (p 131) the ballasts for 100W HPS lamps consume 118 to 132 Watts. From the General Electric 1999 catalogue of lamps (p 3-15), the "mean" lumens is listed as 8550 for any burning position. The corresponding efficacy values are between 67 and 75 lumens per Watt.
Table 1: "Mean" Efficacy of HID Sources Source/Description "Mean" Lumens Input Watts Efficacy MH 70-100W A.L.G. "60-75" 100W vertical 5900 118-128 46-50 100W horizontal 5300 118-128 41-45 HPS 70-100W A.L.G. "70-120" 100W (any position) 8550 118-132 67-75
The comparison of efficacy can be made for initial or mean conditions. Since the Lamp Lumen Depreciation for MH and HPS sources have different rates over time, the relative performance between MH and HPS changes depending on the conditions.
Rated lumens and initial efficacy data are relatively easy to obtain and compare, and Table 2 shows comparisons of the initial efficacy for mid-wattage HPS and MH pulse start lamps.
Table 2: Comparison of Initial Efficacy of MH Pulse Start to HPS Wattage MHPS/HPS 100 vert. 95% 100 horiz. 85% 150/175 86% 250 78% 400 89%
Comparing the initial and mean efficacy values for the 100W sources indicates that although the "mean" for MH is at only 40% of rated life, the drop in efficacy is from 76 to 50 for the vertical MH and from 69 to 45. This is a loss of one-third of the rated lumens in the first two-fifths of life. For the HPS 100W lamp, the mean is at 50% or rated life and the drop in efficacy is from 81 to 72, a drop of about 10% over half the rated lamp life.
Table 3: Comparison of Initial and Mean Efficacies for MH Pulse Start and HPS Source/Description Initial Mean % Mean Mean/Initial (lms/W) (lms/W) MHPS 100W vert. 76 40% 50 66% 100W horiz. 69 40% 45 65% HPS 100W 81 50% 72 90%
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