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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.

Reply to
pcmangler
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It could be; led technology has improved enormously over the years. So yesterdays "ultra-high brightness" is todays dim bulb.

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.

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.

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John Devereux
Reply to
John Devereux

Don't forget a small R in sereis with it or you may have more current than you want by accident.

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.

It means a minimum for a stated ucrrent.

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

Graham

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Reply to
Pooh Bear

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

Reply to
martin griffith

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.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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This is the government our founding fathers warned us about.
Reply to
Paul Hovnanian P.E.

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.

Reply to
mc

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

Reply to
redbelly

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