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current is shown in Figure 3. This graph
shows typical behaviour which, not
unexpectedly, is approaching linearity
but should not be taken as applying
universally, since packaging plays a
large role in light output.
From the equation for forward
current, I
f
, it is evident that it
decreases with an increase in
junction temperature, and light output
also decreases.
The light quanta emitted fall in fairly
narrow wavelength bandwidths. Each
light quantum carries an energy of
hc/
where h is a constant (Planck’s
constant), c is the speed of light, and
is wavelength.
Inherently, LEDs provide colour
so that for white light two methods
present themselves:
>
mixing the light of blue, green and red
LEDs; or,
>
using a phosphor capable of producing
effectively white light through the use
of short-wavelength LEDs (about 470
milli-microns).
In the latter method short
wavelengths excite yellow colour in the
phosphor, and the balance of blue light
not absorbed mixes with the yellow to
produce white light.
DC POWER FOR LEDS
The flicker fusion threshold of the
human eye requires in practice a DC
power source for LEDs.
This is the case even though an AC
voltage could be applied (bearing in
mind that reverse voltages should be
limited by clamping at a low reverse
voltage level).
LEDs fire and switch off essentially
without delay (unlike incandescent
lamps or fluoros powered by
electronic ballasts) operating at
frequencies of 20kHz or higher. This
makes for uncomfortable viewing at
AC power frequency.
However, by virtue of DC power being
the natural choice, higher ’system’
efficiencies can be obtained.
Although DC power is generally more
efficient than AC, the difference is not
great. Furthermore, it is a theoretical
consideration inasmuch as it cannot
practically be employed other than for
a fixed distribution voltage.
Yet by eliminating needless
conversions al low voltage levels,
worthwhile energy savings can be
made in practice.
As replacements for dichroic filters,
LEDs come with a bridge rectifier
circuit as a built-in feature so that
they can make use of the same 12V AC
transformers. In some cases an attempt
is made to provide better impedance
matching between the LED luminaire
and transformer.
However for commercial installations,
red
blue
white
160
120
80
40
-20
20 40 60 80 100 120
0
0
Junction temperature ( C)
1100
1000
900
800
700
600
500
400
300
200
100
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Figure 2: Light output as a function of junction temperature.
Figure 3: Light output as a function of forward current.
BY
PHIL
KREVELD