F L O O D L I G H T I N G F U N D A M E N T A L S
- Inverse Square Cosine Law
Footcandle levels are ultimately dependent upon the projected distance, aiming angle, and luminous intensity of a lamp/reflector combination. A cursory understanding of the equation which relates these variables provides insights to proper design technique and distribution selection.
I N V E R S E S Q U A R E C O S I N E L A W
E=I cos (x)/D2 -or-
Illuminance (Footcandles, fc) = Luminous Intensity (Candelas, Cd) * cos (incident angle X) / Distance2
Holding other variables constant, as the projected distance increases from the luminaire to the surface being illuminated, a greater amount of luminous intensity (I, Candela) is required to sustain an equal illuminance (E, Footcandle) level.
I= E D2
COS (x)
5fc (10')2
°= 577 candela
COS (30 )
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| E=5 fc | 5 fc (20')2 | |
| X=30° |
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57 |
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E=5 fc
X=30°
cd I=2,310
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Holding incident angle X constant at 30˚ while increasing the projected distance D from 10' to 20' requires an increase in candela from 577 to 2310 respectively to sustain an equal 5 footcandles (fc) of illumination at the target point.
The incident angle as measured from a luminaire’s directed intensity to the target surface normal also plays a significant role in determining illuminance values. Holding other variables constant as the incident angle from the target surface to the projected aiming line increases, so does the amount of luminous intensity (I, Candela) required to sustain an equal illuminance value.
I= E D2
COS (x)
5fc (10')2
°= 577 candela
COS (30 )
E=5 fc
X=30°
cd 577 I=
30°
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D |
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I= | E D2 |
COS (x) |
5fc (10')2
°= 707 candela
COS (45 )
E=fc
X=45°
I=707 | cd |
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45° | = |
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Holding projected distance D constant at 10' while increasing incident angle X from 30˚ to 45˚ requires an increase in candela from 577 to 707 respectively to sustain an equal 5 footcandles (fc) of illumination at the target point.
3
V I S I O N F L O O D A r c h i t e c t u r a l F l o o d L u m i n a i r e
