by chris_vk3aml » Mon Sep 29, 2008 11:32 am
Hello from Australia ---
Be careful when assuming that the number of milliCandela gives you
an absolute measure of the amount of light emitted by the LED chip.
Often, it does not. The fact is that different LEDs have different radiation
angles, an effect varied by changing the distance of the chip behind the
surface of the LED's encapsulent lens. In this way, the same given source
chip might have only 1000 mCd measured axial intensity in the far-field
with Lambertian (cosine-law) radiation; that is with no effective lens in
front of the chip, or with the chip mounted at the centre of a
hemispherical dome.
If, however, the LED chip were placed further back from its encapsulent
lens - and some LEDs do this - the radiation half-flux angle could be
reduced to, say, 5 degrees, and the far-field milliCandela measurement
might go to 20,000 or even 40,000 mCd. A few suckers might grab
these LEDs, assuming that they're "brighter", however THE AMOUNT OF
LIGHT EMITTED BY THE CHIP IS THE SAME - the output is just differently
disposed in space! (I could go into the measurement of emitted cone
angles by explaining the concept of steradian measures here, but for
those who know, it's hardly necessary).
A better rough measure of the LED's light output is given by two factors:
(1) LED coversion efficiency.
(2) Maximum permissable continuous LED current.
The product of these two roughly give the chip's light output, but the
"optical gain" of the encapsulent lens is quite an unrelated matter!
The chip's maximum electrical dissipation figures in milliwatts may
be a better measure.
A lot of the limitations are now set by which LED producing firm has
access to recent patents on chip and design improvements, but there
are two outstanding players in the field who have attracted imitator
manufacturers, especially in China. These imitations are invariably
inferior to the branded products. The outstanding branded products
are made by:
CREE has produced white LEDs which apparently have just about
the highest conversion efficiency of any LEDs, they are frequently
used in multi-led torches - however they do not have access to
LUMILED's (the "Luxeon's") patents on high-current designs.
LUMILEDS (see their website) have produced the LEDs with absolutely
the world's highest light output - operating in the order of 5 watts,
rather than the typical 100 milliwatts of standard LED designs.
If you wanted to plug an optimum LED into that "Televisor Kit" without
any modification of the driving circuit, I would reccomend the CREE
LED product; however if you can build a heftier modulator and power
supply, a single white Luxeon of the 3-watt type would be far better.
Beware of "milliCandela ratings". They seem to be designed for those
with little knowledge - which can, as always, be a dangerous thing!
All the best to all,
Chris Long VK3AML.