654
23 Electricity-to-Light Conversion
Fig. 23.17 Historic
development of the flux (in
lumen) and cost (in $/lm)
for semiconductor LEDs.
Data from [1949]
10
10
10
10
10
10
10
3
2
1
0
-1
-2
-3
1970
1980
1990
2000
Year
flux
cost
Fig. 23.18 Luminous
performance of various
LED materials in
comparison with other light
sources. Adapted
from [1957]
With an UV LED that is itself invisible (and must be shielded so no UV radiation leaves the LED),
phosphors with various colors can be pumped (Fig. 23.19c). The mix of phosphors determines the
white point.
Using a blue-emitting LED based on (In,Ga)N material, phosphors (similar to those used in fluorescence lamps) can be pumped. Blue light is converted into green, yellow or red light such that the
resulting total spectrum appears white to the human eye. Also, a broad range of other colors can be
designed (color on demand), e.g. pink or particular corporate colors.
The color of a white LED depends on the operation conditions. In Fig. 23.21a the intensity vs. dc
driving current characteristic of a white LED is shown. In Fig. 23.21b the chromaticity coordinates
are shown for various dc currents. A change of wavelength with forward voltage occurs for the blueemitting (In,Ga)N material due to filling of low-energy states (Fig. 23.22). In order to avoid this effect,
the LED is driven with pulses of a fixed current amplitude and a repetition frequency that is high enough
(a)
(b)
(c)
Fig. 23.19 Different strategies to generate white light with LEDs. a Additive mixing of R, G, and B LEDs, b blue LED
and yellow phosphor, c UV LED (invisible) and R, G, and B phosphors. From [1949]
23 Electricity-to-Light Conversion
Fig. 23.17 Historic
development of the flux (in
lumen) and cost (in $/lm)
for semiconductor LEDs.
Data from [1949]
10
10
10
10
10
10
10
3
2
1
0
-1
-2
-3
1970
1980
1990
2000
Year
flux
cost
Fig. 23.18 Luminous
performance of various
LED materials in
comparison with other light
sources. Adapted
from [1957]
With an UV LED that is itself invisible (and must be shielded so no UV radiation leaves the LED),
phosphors with various colors can be pumped (Fig. 23.19c). The mix of phosphors determines the
white point.
Using a blue-emitting LED based on (In,Ga)N material, phosphors (similar to those used in fluorescence lamps) can be pumped. Blue light is converted into green, yellow or red light such that the
resulting total spectrum appears white to the human eye. Also, a broad range of other colors can be
designed (color on demand), e.g. pink or particular corporate colors.
The color of a white LED depends on the operation conditions. In Fig. 23.21a the intensity vs. dc
driving current characteristic of a white LED is shown. In Fig. 23.21b the chromaticity coordinates
are shown for various dc currents. A change of wavelength with forward voltage occurs for the blueemitting (In,Ga)N material due to filling of low-energy states (Fig. 23.22). In order to avoid this effect,
the LED is driven with pulses of a fixed current amplitude and a repetition frequency that is high enough
(a)
(b)
(c)
Fig. 23.19 Different strategies to generate white light with LEDs. a Additive mixing of R, G, and B LEDs, b blue LED
and yellow phosphor, c UV LED (invisible) and R, G, and B phosphors. From [1949]