23.3 Light-Emitting Diodes
655
(a)
(b)
(c)
400
450
500
550
600
650
700
Fig. 23.20 a Scheme and b image of color conversion Luxeon LED. From [1949]. c Spectrum (solid line) of white
LED with blue LED pumping a yellow phosphor together with eye-sensitivity curve V (λ) (dashed line). Adapted
from [1951]
(a)
0
20
40
60
80
100
(b)
y
0.29 0.30
0.31 0.32 0.33 0.34
x
0.35
0.34
0.33
0.32
0.31
0.30
1
5
20
50
100
I (mA)
F
Fig. 23.21 a Luminous intensity of white LED (NSCW215) vs. dc forward current. b CIE chromaticity coordinates for
various dc driving conditions as labeled. Data taken from [1959]
to provide a flicker-free image to the human eye, e.g. 100 Hz. The intensity of the LED is modulated
via the pulse width, i.e. between 0–10 ms in this case (PWM, pulsewidth modulation).
The direct light from white LEDs is visually appealing and cannot be distinguished from a blackbody
source of matching temperature. But since the spectral power distribution of white LEDs is different
from natural light, objects illuminated by such light source can appear in ‘wrong’ colors. For the
spectrum of Fig. 23.20c, in particular green is reproduced poorly. A quantitative measure for the ability
of a light source to reproduce the colors of an illuminated object faithfully compared with a natural
(blackbody) light source is the color rendering index (CRI).
A major advantage of LEDs for display and lighting applications is their long lifetime compared
to halogen (about 2000 h), xenon (10,000 h) or fluorescent (6000–10,000 h) lights. Philips Lumileds
projects (for white LUXEON
® K2 LEDs) 70% lumen maintenance at given current (1 A) at 50,000 h
for junction temperature T j ≤ 120
◦ [1961]. Similar values are given by OSRAM [1964] for white
655
(a)
(b)
(c)
400
450
500
550
600
650
700
Fig. 23.20 a Scheme and b image of color conversion Luxeon LED. From [1949]. c Spectrum (solid line) of white
LED with blue LED pumping a yellow phosphor together with eye-sensitivity curve V (λ) (dashed line). Adapted
from [1951]
(a)
0
20
40
60
80
100
(b)
y
0.29 0.30
0.31 0.32 0.33 0.34
x
0.35
0.34
0.33
0.32
0.31
0.30
1
5
20
50
100
I (mA)
F
Fig. 23.21 a Luminous intensity of white LED (NSCW215) vs. dc forward current. b CIE chromaticity coordinates for
various dc driving conditions as labeled. Data taken from [1959]
to provide a flicker-free image to the human eye, e.g. 100 Hz. The intensity of the LED is modulated
via the pulse width, i.e. between 0–10 ms in this case (PWM, pulsewidth modulation).
The direct light from white LEDs is visually appealing and cannot be distinguished from a blackbody
source of matching temperature. But since the spectral power distribution of white LEDs is different
from natural light, objects illuminated by such light source can appear in ‘wrong’ colors. For the
spectrum of Fig. 23.20c, in particular green is reproduced poorly. A quantitative measure for the ability
of a light source to reproduce the colors of an illuminated object faithfully compared with a natural
(blackbody) light source is the color rendering index (CRI).
A major advantage of LEDs for display and lighting applications is their long lifetime compared
to halogen (about 2000 h), xenon (10,000 h) or fluorescent (6000–10,000 h) lights. Philips Lumileds
projects (for white LUXEON
® K2 LEDs) 70% lumen maintenance at given current (1 A) at 50,000 h
for junction temperature T j ≤ 120
◦ [1961]. Similar values are given by OSRAM [1964] for white