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Unformatted text preview: Luminous Intensity and Apparent Brightness of Two LED’s Two light-emitting diodes (LED’s) are viewed in the laboratory. The LED’s are
both 1000 mm from the observer (indicated by the eye at the right in the ﬁgure). The ﬁrst is a SiC LED emitting at 480 nm (blue). Its axis is oriented at an angle of 30°
with respect to the horizontal. The axis of the device is normal to the emitting surface.
The second is a GaP:N LED emitting at 565 nm (yellow—green). Its axis is oriented at an
angle of 60° with respect to the horizontal. Each of the LED’s has a total output optical
power of 5 mW emitted into a Lambertian distribution. Each of the LED’s has an output
area of 1 mm x 1 mm. Quantitative measurements of the LED’s are made by replacing the eye with a photodetector that has an area of 4mm2 and is positioned 1000 mm from the
LED’s. Calculate, showing all work, the luminous intensity of each LED at the photode—
tector. Express your answers in lumens/stemdian accurately to four signiﬁcant ﬁgures. Which appears the brighter? Put your answers in the spaces provided.
Luminous intensity of the ﬁrst LED (blue) 2 lm/sr. Luminous intensity of the second LED (yellow—green) = lm/sr. Luminous Intensity-and Apparent Brightness of Two LED’s Differential area on surface of sphere d A = ('r d6)(r sin 0 d¢) Differential solid angle d9 = d—4 = sin0d0d¢ 7.2 Total power emitted by LED :0 7r/2 21r
Pt 2 f / IROCOSOsin6d6d¢
0 ¢=0 '27r <15 . 2
327L219 "/ 2 IRO 0 0 = 7r IRo
Radiant intensity at 0 2 0° P
130 = 7:- : 1.5915mW/sr Radiant intensity at 9 IR = [1200030 I R; 1.37828 mW/ s1" 6 = 30° IRz 0.79575 mW/sr 0 = 60° Luminous Efficiencies A = 480nm LE = 0.139
/\ = 565nm LE 2 0.9735 Luminous intensity at 0
IL. = IRi LE,- (680%) (16%)
In = 0.13028 lm/sr less bright
In 2 0.52677 lm/sr brighter r ...
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- Spring '08
- Physical quantities, light-emitting diodes, Solid angle, Luminous Intensity, Radiant intensity