In contrast to the lumen method of calculation used in the Standard 90.1 models, point-
by-point calculations are a very accurate way to determine the distribution of light in a
space. The target illuminance was 25 footcandles, because the direct component should
contribute to roughly half of the overall illuminance in the space. The remainder of the
horizontal illuminance would be provided by linear fluorescent sources or indirect
metal halide. Table 4 shows the results of 15 calculations, using 5 of the most
applicable types of luminaires at three different ceiling heights.
Table 5. Calculation Results using Downlight Luminaires at Three Mounting Heights
Vertical Lamp
Horizontal Lamp
Horizontal Lamp
PAR38 HIR
PAR38
Fixture
Fixture
Fixture
Halogen InfraRed
Metal Halide
Triple Tube CFL
Triple Tube CFL
Triple Tube CFLs
(Incandescent)
Ceramic, Pulse Start
42 Watt
42 Watt
2 lamp-42 Watt
100 Watt
70 Watt, electronic
Ceiling
12'-0" 16'-0" 20'-0" 12'-0" 16'-0" 20'-0" 12'-0" 16'-0" 20'-0" 12'-0" 16'-0" 20'-0" 12'-0" 16'-0" 20'-0"
Height: (1)
Average
Illuminance (fc)
29.7
26.6
23.9
28.6
25.3
27.9
29.2
25.7
29.9
31.1
28.8
26.6
25.5
24.3
32.5
Lighting Power
Density
0.72
0.72
0.72
0.85
0.85
1.07
0.93
0.93
1.24
2.67
2.67
2.67
0.67
0.67
1.00
LPD
Normalized
2.4
2.7
3.0
3.0
3.4
3.8
3.2
3.6
4.1
8.6
9.3
10.0
2.6
2.8
3.1
Maximum
Illuminance
37.4
34.8
32.1
36.1
34.6
37.9
36.1
34.6
39.7
49.8
35.3
32.5
94.7
49.6
39.3
Minimum
Illuminance
17.8
16.1
15.1
15.1
14.0
16.9
16.8
15.1
18.6
16.6
18.8
15.6
3.8
7.2
20.8
Max/Min
Illuminance
2.1
2.2
2.1
2.4
2.5
2.2
2.1
2.3
2.1
3.0
1.9
2.1
24.7
6.8
1.9
Uniformity
0.6
0.6
0.6
0.5
0.6
0.6
0.6
0.6
0.6
0.5
0.7
0.6
0.1
0.3
0.6
NOTES: (1) Metric equivalencies for ceiling heights are as follows: 12'-0" equals 3.7 meters, 16'-0" equals 4.9 meters, 16'-0"
equals 6.1 meters.
The results of these calculations show that the "Vertical Triple CFL 42 Watt" luminaire
provides the best overall results, and is acceptable for each of three mounting heights.
The power density is low at 0.72 watts per square foot, the fixtures do a good job of
providing the target illuminance of 25 fc, and the uniformity is good at 0.6. The
luminaire is a low glare fixture because the lamp sits up high in the housing of the
fixture, which also results in an efficient vertical distribution. The luminaire has a
dimmable electronic ballast, and the lamp is available in a variety of colors (including
the popular 3000K or 3500K) and has a Color Rendering Index (CRI) of 82.
For additional background information on the calculations and the "Vertical Triple
CFL" see Appendices A, B, and C.
March 2006
Page 13
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