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LED brightness?

P

Peter Hucker

Jan 1, 1970
0
I'm wanting to purchase bright LEDs, but am confused by the specs.

Am I correct in the following assumption?

A 100mcd LED with a viewing angle of 30 degrees will light the surface in front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
 
J

John Fields

Jan 1, 1970
0
I'm wanting to purchase bright LEDs, but am confused by the specs.

Am I correct in the following assumption?

A 100mcd LED with a viewing angle of 30 degrees will light the surface in front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
 
P

Peter Hucker

Jan 1, 1970
0
---
No.

100 millicandelas is the total light output, so the area illuminated
by the LED with the larger beamwidth will be dimmer.

I was trying to go by this:
http://en.wikipedia.org/wiki/Luminous_intensity
But there are too many different units!

So what you're saying is the way they quote them in catalogues (mcd) is total light output, whereas luminance (in mcd/sq.m) is what I was thinking of?

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

Dan married one of a pair of identical twin girls. Less than a year later, he was in court filing for a divorce. "OK," the judge said, "Tell the court why you want a divorce."
"Well, your honor," Dan started, "Every once in a while my sister in law would come over for a visit, and because she and my wife are so identical looking, every once in a while I'd end up making love to her by mistake."
"Surely there must be some difference between the two women." the judge said.
"You'd better believe there is a difference, your honor. That's why I want the divorce." he replied.
 
P

Peter Hucker

Jan 1, 1970
0
Thanks, just changed to buying a completely different set :)

Why is there so much difference in specs with LEDs? Aren't they all basically the same technology?
 
E

ehsjr

Jan 1, 1970
0
Peter said:
Thanks, just changed to buying a completely different set :)

Why is there so much difference in specs with LEDs? Aren't they all
basically the same technology?

That's a non sequitur. Tungsten filament incandescent lamps are
all basically the same technology, yet they come in many different
configurations. Same is true of LEDs, which come in various
colors, shapes, sizes, viewing angles, mcd ratings etc.
I suppose that manufacturers decide where they think there is,
or will be, a need, and make things with specs that meet the need.
That would result in the many different LEDs that are available.

Ed
 
F

Fritz Schlunder

Jan 1, 1970
0
front of it at the same brightness as a 100mcd LED with a viewing angle of
15 degrees, but will light a larger area, therefore is giving off more light
in total.
---
No.

100 millicandelas is the total light output, so the area illuminated
by the LED with the larger beamwidth will be dimmer.


Uhh... No...

Candelas aren't a measure of total light output. Candelas are a measure of
lumens per steradian (a steradian being a solid angle measurement). Lumens
are a measure of total light output adjusted for human eye sensitivity
towards different wavelengths. Human eyeballs are vastly more sensitive to
555nm green photons than they are to say red or blue photons for example.

So... A one lumen white light source should in theory be able to light up a
whole room to approximately an equal amount of brightness as perceived by a
human eye as a one lumen blue light source, as a one lumen green light
source, as a one lumen red light source, etc. The one lumen green light
source doesn't have to be as electrically efficient (in terms of total
photons produced per joule input) however since the human eye doesn't need
as many green photons to appear equally bright as other colors.

Unfortunately the vast majority of LED manufacturer's don't yet rate their
LED product's output in lumens, instead they usually provide intensity in
millicandelas and also include a "viewing angle" which is normally defined
as twice the angle of half intensity (or in other words two phi one-half).
Sometimes instead they might give you just the angle of half intensity
instead (especially in the case of infrared LEDs). This is a most
unfortunate state of affairs, because it precludes highly accurate and fair
comparisons between products.

Anyway, the OP's assumption was in fact correct. A 100mcd LED with a
viewing angle of 30 degrees *should* produce more total light output than a
100mcd LED with a viewing angle of 15 degrees. This doesn't absolutely
always have to be true however, since it depends on the beam profile, and it
also depends on how accurate and truthful the manufacturer's specifications
are. By playing with the beam profile, it is possible to make a 30 degree
100mcd LED produce less total light output than a 100mcd 15 degree LED,
without lying in any way on the specifications, but in most cases the 30
degree product should be superior.

If you want the most total light output and highest efficiency product, go
for LEDs that have high mcd ratings as well as large viewing angle ratings.
Unfortunately manufacturer's and especially distributors often make mistakes
on their advertising (usually because distributors don't have a clue what
any of the various units mean either), further muddling the issue and making
accurate direct performance comparisons even more difficult.
 
J

John Fields

Jan 1, 1970
0
Uhh... No...

Candelas aren't a measure of total light output. Candelas are a measure of
lumens per steradian (a steradian being a solid angle measurement).
 
P

Peter Hucker

Jan 1, 1970
0
That's a non sequitur. Tungsten filament incandescent lamps are
all basically the same technology, yet they come in many different
configurations. Same is true of LEDs, which come in various
colors, shapes, sizes, viewing angles, mcd ratings etc.
I suppose that manufacturers decide where they think there is,
or will be, a need, and make things with specs that meet the need.
That would result in the many different LEDs that are available.

The weird thing is that the ones that are 100 times brighter do not use 100 times more power.


--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

A blind man was standing on the corner with his dog when the dog raised his leg and went on the man's trouser leg.
The man reached in his pocket and took out a doggie biscuit.
A busybody who had been watching ran up to him and said, "You shouldn't do that. He'll never learn anything if you reward him when he does something like that!"
The blind man retorted, "I'm not rewarding him. I'm just trying to find his mouth so that I can kick him in the ass."
 
P

Peter Hucker

Jan 1, 1970
0
front of it at the same brightness as a 100mcd LED with a viewing angle of
15 degrees, but will light a larger area, therefore is giving off more light
in total.


Uhh... No...

Candelas aren't a measure of total light output. Candelas are a measure of
lumens per steradian (a steradian being a solid angle measurement). Lumens
are a measure of total light output adjusted for human eye sensitivity
towards different wavelengths. Human eyeballs are vastly more sensitive to
555nm green photons than they are to say red or blue photons for example.

So... A one lumen white light source should in theory be able to light up a
whole room to approximately an equal amount of brightness as perceived by a
human eye as a one lumen blue light source, as a one lumen green light
source, as a one lumen red light source, etc. The one lumen green light
source doesn't have to be as electrically efficient (in terms of total
photons produced per joule input) however since the human eye doesn't need
as many green photons to appear equally bright as other colors.

Unfortunately the vast majority of LED manufacturer's don't yet rate their
LED product's output in lumens, instead they usually provide intensity in
millicandelas and also include a "viewing angle" which is normally defined
as twice the angle of half intensity (or in other words two phi one-half).
Sometimes instead they might give you just the angle of half intensity
instead (especially in the case of infrared LEDs). This is a most
unfortunate state of affairs, because it precludes highly accurate and fair
comparisons between products.

Anyway, the OP's assumption was in fact correct. A 100mcd LED with a
viewing angle of 30 degrees *should* produce more total light output than a
100mcd LED with a viewing angle of 15 degrees. This doesn't absolutely
always have to be true however, since it depends on the beam profile, and it
also depends on how accurate and truthful the manufacturer's specifications
are. By playing with the beam profile, it is possible to make a 30 degree
100mcd LED produce less total light output than a 100mcd 15 degree LED,
without lying in any way on the specifications, but in most cases the 30
degree product should be superior.

If you want the most total light output and highest efficiency product, go
for LEDs that have high mcd ratings as well as large viewing angle ratings.
Unfortunately manufacturer's and especially distributors often make mistakes
on their advertising (usually because distributors don't have a clue what
any of the various units mean either), further muddling the issue and making
accurate direct performance comparisons even more difficult.

Oh great. Now I have two completely different answers, I'll have to buy some of each and see what they really do.
 
P

Peter Hucker

Jan 1, 1970
0
front of it at the same brightness as a 100mcd LED with a viewing angle of
15 degrees, but will light a larger area, therefore is giving off more light
in total.


Uhh... No...

Candelas aren't a measure of total light output. Candelas are a measure of
lumens per steradian (a steradian being a solid angle measurement). Lumens
are a measure of total light output adjusted for human eye sensitivity
towards different wavelengths. Human eyeballs are vastly more sensitive to
555nm green photons than they are to say red or blue photons for example.

So... A one lumen white light source should in theory be able to light up a
whole room to approximately an equal amount of brightness as perceived by a
human eye as a one lumen blue light source, as a one lumen green light
source, as a one lumen red light source, etc. The one lumen green light
source doesn't have to be as electrically efficient (in terms of total
photons produced per joule input) however since the human eye doesn't need
as many green photons to appear equally bright as other colors.

Unfortunately the vast majority of LED manufacturer's don't yet rate their
LED product's output in lumens, instead they usually provide intensity in
millicandelas and also include a "viewing angle" which is normally defined
as twice the angle of half intensity (or in other words two phi one-half).
Sometimes instead they might give you just the angle of half intensity
instead (especially in the case of infrared LEDs). This is a most
unfortunate state of affairs, because it precludes highly accurate and fair
comparisons between products.

Anyway, the OP's assumption was in fact correct. A 100mcd LED with a
viewing angle of 30 degrees *should* produce more total light output than a
100mcd LED with a viewing angle of 15 degrees. This doesn't absolutely
always have to be true however, since it depends on the beam profile, and it
also depends on how accurate and truthful the manufacturer's specifications
are. By playing with the beam profile, it is possible to make a 30 degree
100mcd LED produce less total light output than a 100mcd 15 degree LED,
without lying in any way on the specifications, but in most cases the 30
degree product should be superior.

If you want the most total light output and highest efficiency product, go
for LEDs that have high mcd ratings as well as large viewing angle ratings.
Unfortunately manufacturer's and especially distributors often make mistakes
on their advertising (usually because distributors don't have a clue what
any of the various units mean either), further muddling the issue and making
accurate direct performance comparisons even more difficult.

I think you're right actually - http://en.wikipedia.org/wiki/Luminous_intensity tells us that candelas are perceived power in one direction. Hmmm it made sense the other way too - there are too many bloomin units!

Well if you're right, then I've just ordered the wrong ones. I'll go order the right ones and see for myself which is really brighter.
 
S

Steve Noll

Jan 1, 1970
0
---
No.

100 millicandelas is the total light output, so the area illuminated
by the LED with the larger beamwidth will be dimmer.

Do the manufacturer's specs actually say that the LED outputs are
"total output" ?

CIE develops international specs for measuring visible LEDs. CIE
127-1997 defines measuring LED output, termed "Average LED Intensity",
in (milli) candelas. I make these measurements at the day job. It
requires using an extremely accurate ($) photopic detector with a 1cm
dia round aperture either 100mm or 316mm in front of the LED, which
is equivalent to capturing 6.5-degrees or 2-degrees of the central LED
output. Obviously, this is not total output, nor is it intended to
simulate that. More often than not one isn't concerned about the
total output of a visible LED, but how bright it looks when seen by
eye at a reasonable distance away. CIE is working on a standard for
integrated spheres for use in total output measurement. I believe it
is due out in a few months.

Anyway... the point being you really have to be careful about LED
output specs. If possible buy a few and try 'em in your application.
Sad to say that I've personally seen LED manufacturers using
antiquated equipment to make their measurements, use integrating
spheres to make measurements in units they weren't calibrated for, and
even just make up data. I wouldn't go solely by their printed specs.


Steve Noll | The Used Equipment Dealer Directory:
| http://www.big-list.com
| Peltier Information Directory:
| http://www.peltier-info.com
 
P

Peter Hucker

Jan 1, 1970
0
On Sun, 20 Nov 2005 02:43:07 -0700, "Fritz Schlunder"


Uhh... No...

Candelas aren't a measure of total light output. Candelas are a measure of
lumens per steradian (a steradian being a solid angle measurement).

[grumbles at £8 extra spending]

I am doing. Seen as I'm about to buy a few thousand of the things, I'd rather buy a large selection of single units first to make sure.
 
D

Don Klipstein

Jan 1, 1970
0
I'm wanting to purchase bright LEDs, but am confused by the specs.

Am I correct in the following assumption?

A 100mcd LED with a viewing angle of 30 degrees will light the surface
in front of it at the same brightness as a 100mcd LED with a viewing
angle of 15 degrees, but will light a larger area, therefore is giving
off more light in total.

Correct!

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
---
No.

100 millicandelas is the total light output, so the area illuminated
by the LED with the larger beamwidth will be dimmer.

Candela is not a unit of total light output, but of "beam candlepower".
1 candela illuminates a spot on a surface 1 meter away to the extent of 1
lux.

The units of total light output are "spherical candlepower" (candela
averaged over all directions, including directions into which no light is
sent), and the lumen. 1 lumen is the total light in a "solid angle" of 1
steradian (1/4-pi hemisphere) where the light has an intensity uniformly
of 1 candela. A "spherical candlepower" is 4-pi lumens, the output of a 1
candela omnidirectional source.

- Don Klipstein ([email protected])
 
P

Peter Hucker

Jan 1, 1970
0
Candela is not a unit of total light output, but of "beam candlepower".
1 candela illuminates a spot on a surface 1 meter away to the extent of 1
lux.

The units of total light output are "spherical candlepower" (candela
averaged over all directions, including directions into which no light is
sent), and the lumen. 1 lumen is the total light in a "solid angle" of 1
steradian (1/4-pi hemisphere) where the light has an intensity uniformly
of 1 candela. A "spherical candlepower" is 4-pi lumens, the output of a 1
candela omnidirectional source.

- Don Klipstein ([email protected])

I purchased a range of LEDs and tested them. The mcd rating appears to be TOTAL light output.

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

The teacher in Johnny's school asked the class what their parents did for a living. One little girl said her father was a doctor, another said her mother was an engineer. When it was Little Johnny's turn, he stood up and said "My mom's a whore."
Naturally, after that remark, he got sent off to the principal'soffice. Then, 15 minutes later, he returned.
So the teacher asked "Did you tell the principal what you said in class?"
Johnny said "Yes"
"Well, what did the principal say?"
"He said that every job is important in our economy, gave me an apple and asked for my phone number."
 
D

Don Klipstein

Jan 1, 1970
0
I purchased a range of LEDs and tested them. The mcd rating appears to
be TOTAL light output.

Have you measured total luminous flux of a 15 degree model and a 60
degree model of the same millicandela, and known to actually be so?
Have you ever shone two LEDs of the same MCD, same color/wavelength and
same chip chemistry but greatly different beam wionto a solar cell and
measured short circuit current of the solar cell with a milliammeter?
(Comparison requires matching of spectrum because solar cells have
different spectral response than human vision has.)

Have you ever illuminated a dark room with two LEDs of greatly different
beam width and same MCD (and same color and wavelength and spectral
characteristics - night vision differs from "day vision" in a way that
varies with color/wavelength)?

I have been there and done that!

An ideal 1,000 millicandela beam that is 1,000 millicandela uniformly
within its specified boundary and completely lacking light outside it has
has .842 lumen of light if 60 degrees wide and has only .054 lumen of
light if 15 degrees wide. At least 15 degree and narrower-beam LEDs
usually have enough light outside their specified beams to get total light
output usually more than, sometimes by a factor of 2 more than, 2 times Pi
times (1 minus cosine of half the rated beam angular diameter).

- Don Klipstein ([email protected])
 
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