"Dimitrij Klingbeil" wrote in message
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"Arfa Daily" wrote in message
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"Maria Mustafa" wrote in message
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Hello every one
I want to know the luminance of simple LED green bulb of normal small
size in cd/m2 at 2.5V
Can anyone help
Regards
Maria
That's an impossible question, from the same book of impossible questions
as "How long is a piece of string ?" With a quick glance at two or three
catalogues, I could probably find you twenty different green LEDs with
twenty different specifications. And how big is "normal small" ? 3 mm ?
5 mm ? Tinted encapsulation ? Diffused encapsulation ? Water clear ?
Viewing angle ? Then there's how much current you are going to push
through it. And "cd/m2" is a meaningless measurement. Lumens / m2
possibly, but a single green led is going to return a figure so small for
that, it will be useless. Sorry to be so negative, but there's little in
your question to be positive on ... :-\
Arfa
Hello Arfa
Actually, the cd/m2 unit is not so meaningless - it's the measurement unit
of
luminance (http://en.wikipedia.org/wiki/Luminance).
It wasn't so much the validity of the unit that I was questioning - although
I don't think that quoting candellas referred to square area is a common way
of expressing any kind of luminous efficacy for a 'small' LED - I was more
making the point that when referred to what the OP is asking about, the
result would be so tiny as to not be any kind of a valid parameter for that
type of LED.
You may have heared the
unit "nit" being referred to (apparent brightness of CRT, LCD screens, EL-
foils, or any other flat and rather regular "area light sources"), it's
the
same unit. 1 nt = 1 cd/m2. The integration over the visible wavelengths is
weighted with the human eye perception sensitivity, so it measures how
bright
a luminous area appears to be (independent of the radiating area,
distance,
and viewing angle, for that part see the cos(beta) term in the
definition).
In principle this measurement applies to any area that can emit light, the
active die area of the LED chip including.
The die in a 'small' led will be lucky to be 1 mm2 so that's one millionth
of a sq m. Considering that a 'small' LED is likely to have an output of a
few mCd, the resulting figure - if you could even measure it accurately -
would be so small as to be meaningless ...
I've got no real experience with
actual values, but the German wiki page on luminance has some examples.
(http://de.wikipedia.org/wiki/Leuchtdichte#Beispiele). A white LED is
listed
as 50Mnt, a fluorescent tube as 11knt. The website for a Philips "Altilon"
(http://www.philipslumileds.com/produ...luxeon-altilon)
high-power LED mentions 60Mnt when driven at its rated current of 1000mA,
but this looks like a thermal challenge.
But there, you're talking power LEDs - a whole different kettle of fish ...
Arfa
This LED integrates a copper heat
spreader that has to be mounted with M3 screws to a sizeable heatsink. A
typical "bulb of normal small size" is rather unlikely to drive the chip
at
a current density comparable to the above, so the perceived brightness per
unit area should be less. Green LEDs however will have a noticeably higher
nits rating than white LEDs of the same chip current density because of
the
peak in the human eye response to green wavelengths (and because the white
LED prosphor is never 100% efficient).
It seems like typical LED datasheets mostly skip this parameter, and if
any specify it, those tend to be LEDs that are intended to be mounted in
secondary optics designed for a very high luminous intensity (headlights,
searchlights and the like). There's some (partial) sense in it, since the
luminance is a figure-of-merit parameter when it comes to integrating the
LED into high intensity narrow beam optics, but is a little less useful
otherwise. The other use case where this parameter gets important is the
"sunlight visibility" - the LED visibility under bright sunlight. To be
visible the LED chip needs to have a considerably higher luminance than
the LED and its immediate surroundings would have by way of reflection
when under direct sunlight (this is easier said than done as most LEDs
don't achieve such high luminance values). Green LEDs help in this regard
because of the eye sensitivity and because of the colour contrast, the
sunlight not being green after all.
To the OP: There's a regular on the SE* newsgroups who knows a lot about
light sources - Don Klipstein. He's got a page http://donklipstein.com/
and since he knows a lot more than I do about all sorts of light emitting
things, he might know some more reliable typical values from experience.
Regards,
Dimitrij