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Adjusting LED brightness ?

Hi,

I'm trying to work out how to put a red & a blue LED into a project,
but make sure that they have equal brightness. I planned on using a
500R pot on each, but on looking at the specs on suppliers websites,
the brightness figures seem wildly different for red & blue - e.g. a
5mm blue LED is quoted as 1,300 mcd typical, yet even a high brightness
type red is only quoted at 30 mcd. Surely the blue can't really be 43
times brighter ??!!!

Also, I'm confused by the specs on the blue - it quotes a minimum
figure of 880 mcd for luminous intensity. How can it have a minimum ?
- surely it would fade all the way to nil if connected to a pot ?

How can I make sure I get the right types that will allow me to adjust
the brightness in similar ranges ?

Many thanks,

Kev.
 
J

John Devereux

Jan 1, 1970
0
Hi,

I'm trying to work out how to put a red & a blue LED into a project,
but make sure that they have equal brightness. I planned on using a
500R pot on each, but on looking at the specs on suppliers websites,
the brightness figures seem wildly different for red & blue - e.g. a
5mm blue LED is quoted as 1,300 mcd typical, yet even a high brightness
type red is only quoted at 30 mcd. Surely the blue can't really be 43
times brighter ??!!!

It could be; led technology has improved enormously over the years. So
yesterdays "ultra-high brightness" is todays dim bulb.
Also, I'm confused by the specs on the blue - it quotes a minimum
figure of 880 mcd for luminous intensity. How can it have a minimum ?
- surely it would fade all the way to nil if connected to a pot ?

I expect this is a "minimum" at a particular current. It is saying
that if you put the specified current through it, you will get *at
least* the quoted mcd. Of course if you reduce the current you will
reduce the brightness. You can assume that the brightness is
proportional to the current you put through it, as long as you stay
within its rating.
How can I make sure I get the right types that will allow me to adjust
the brightness in similar ranges ?

Is it just for indication? Many of the newer leds you will see are
designed for *illumination* of objects. Just pick two leds with quoted
mcd in the same ballpark (but not 30 vs 1,300). You can the fiddle
with the operating current if you like.
 
P

Pooh Bear

Jan 1, 1970
0
Hi,

I'm trying to work out how to put a red & a blue LED into a project,
but make sure that they have equal brightness. I planned on using a
500R pot on each,

Don't forget a small R in sereis with it or you may have more current than you
want by accident.
but on looking at the specs on suppliers websites,
the brightness figures seem wildly different for red & blue - e.g. a
5mm blue LED is quoted as 1,300 mcd typical, yet even a high brightness
type red is only quoted at 30 mcd. Surely the blue can't really be 43
times brighter ??!!!

Beware comparing figures without checking the view angle. A narrow beam of light
( blue leds are often narrow angle ) will be brighter for the same led die
because the total light output is 'concentrated' over a smaller angle. Also, the
human eye's sensitivity varies with wavelength.
Also, I'm confused by the specs on the blue - it quotes a minimum
figure of 880 mcd for luminous intensity. How can it have a minimum ?
- surely it would fade all the way to nil if connected to a pot ?

It means a minimum for a stated ucrrent.
How can I make sure I get the right types that will allow me to adjust
the brightness in similar ranges ?

Make sure they are all the same view angle. You'll have to compare by eye
though.

Graham

http://www.grandwell.com/vw_angle.htm
 
M

martin griffith

Jan 1, 1970
0
On 17 May 2006 11:43:48 -0700, in sci.electronics.design
Hi,

I'm trying to work out how to put a red & a blue LED into a project,
but make sure that they have equal brightness. I planned on using a
500R pot on each, but on looking at the specs on suppliers websites,
the brightness figures seem wildly different for red & blue - e.g. a
5mm blue LED is quoted as 1,300 mcd typical, yet even a high brightness
type red is only quoted at 30 mcd. Surely the blue can't really be 43
times brighter ??!!!

Also, I'm confused by the specs on the blue - it quotes a minimum
figure of 880 mcd for luminous intensity. How can it have a minimum ?
- surely it would fade all the way to nil if connected to a pot ?

How can I make sure I get the right types that will allow me to adjust
the brightness in similar ranges ?

Many thanks,

Kev.
Check the Linear site, they have a lot of led controllers

"3A Converter Drives LEDs with 500:1 Dimming" for instance, from
Linear Technology Magazine • March 2006

Lot better than tweaking a pot


martin
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Hi,

I'm trying to work out how to put a red & a blue LED into a project,
but make sure that they have equal brightness. I planned on using a
500R pot on each, but on looking at the specs on suppliers websites,
the brightness figures seem wildly different for red & blue - e.g. a
5mm blue LED is quoted as 1,300 mcd typical, yet even a high brightness
type red is only quoted at 30 mcd. Surely the blue can't really be 43
times brighter ??!!!

Also, I'm confused by the specs on the blue - it quotes a minimum
figure of 880 mcd for luminous intensity. How can it have a minimum ?
- surely it would fade all the way to nil if connected to a pot ?

How can I make sure I get the right types that will allow me to adjust
the brightness in similar ranges ?

You need to check with a lighting wizard on this, but the mcd figures
quoted are the intensities of the emitted light at a standard radius and
at some angular offset from the emitter's axis. So, for the same total
luminous flux (optical power) emitted, the LED with a more tightly
focused beam will have a higher peak mcd figure. Likewise, the minimum
mcd must be quoted at some angle off the LEDs axis, but if different
manufacturers use different standards, the measurements can't be
compared.

Candela is also corrected based on a model of human visual perception.
An IR LED, no matter what its optical power output, has zero cd
intensity. Because of this, the intensity figures for different
wavelengths may be misleading, from a power input point of view. Green
would be the most efficient, as it falls closest to the peak of visual
sensitivity.
 
M

mc

Jan 1, 1970
0
I'm trying to work out how to put a red & a blue LED into a project,
Different people's eyes respond differently, and you cannot make them equal
for everyone unless you allow each user to adjust them. This is
particularly true for red LEDs, which are near the end of the visible
spectrum.

As for the minimum brightness, that means the LED is guaranteed to produce
at least that much light when powered with the specified current.
 
R

redbelly

Jan 1, 1970
0
Hi,

I'm trying to work out how to put a red & a blue LED into a project,
but make sure that they have equal brightness. I planned on using a
500R pot on each, but on looking at the specs on suppliers websites,
the brightness figures seem wildly different for red & blue - e.g. a
5mm blue LED is quoted as 1,300 mcd typical, yet even a high brightness
type red is only quoted at 30 mcd. Surely the blue can't really be 43
times brighter ??!!!

Also, I'm confused by the specs on the blue - it quotes a minimum
figure of 880 mcd for luminous intensity. How can it have a minimum ?
- surely it would fade all the way to nil if connected to a pot ?

How can I make sure I get the right types that will allow me to adjust
the brightness in similar ranges ?

Many thanks,

Kev.

I would just buy a red and a blue that have the same viewing angle,
then use a breadboard setup to figure out what resistances give roughly
equal brightness for the two.

Mark
 
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