How Does LED Brightness Vary with Current?

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How Does LED Brightness Vary with Current?

Postby Ivo » Wed Jan 19, 2011 2:53 pm

I'm conducting an experiment which is similar to that below:

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p037.shtml

Instead of using a light-to-voltage converter, I'm using a digital light intensity meter (lux meter). However, do I need to ensure the lights in the room are switched off (i.e. the room is dark?) or would it not make a difference. I'm only studying the relationship between light intensity and current through LED, so does it matter? If so, due to space constraints I won't be able to do it in a dark room. Instead, I might have to use a large cardboard box. But why do I have to line the inside of the box with black felt?

Many thanks in advance guys!
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Project Question: How Does LED Brightness Vary with Current?
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Re: How Does LED Brightness Vary with Current?

Postby barretttomlinson » Thu Jan 20, 2011 3:21 am

Hi,

Your light meter will respond to any light it sees, regardless of where it comes from. Stray light can mess up your results. If you can’t run your experiement in the equivalent of a photographic dearkroom, then using a dark box is a good alternative. The problem is that a cardboard box can have light leaks. The purpose of the black felt is to absorb any stray light that hits it. If you can be absolutely sure your container/box has no light leaks, and if your apparatus generates no stray light that can bounce off the box walls to cause problems, then you don’t need the black velvet - it is just used for insurance against stray light affecting your experiment.

Good luck with your project!

Barrett L Tomlinson
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Re: How Does LED Brightness Vary with Current?

Postby Ivo » Thu Jan 20, 2011 2:12 pm

Hi Barrett,

Many thanks indeed for your informative reply; it has been very helpful. Oh OK, so the black velvet is used to ensure stray light is absorbed and does not affect my readings. By stray light I assume you mean the unwanted reflections to the lux meter from the LED via the inner surface of the box; as well as external light sources leaking into the box.

Just another quick question. I'm expecting that as current through an LED increases, the light intensity of the LED also increases. However, I'm struggling to find an explanation or any definitve relationship between the two variables (e.g. linear relationship and so on). I also presume that this relationship will be different for different colours of LEDs as well, is there any explanation for that as well?
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Re: How Does LED Brightness Vary with Current?

Postby Craig_Bridge » Thu Jan 20, 2011 3:32 pm

You might want to start your quest for knowledge here: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/led.html
And eventually your quest will come across Max Planck's quantum physics work which is covered here http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html#c4

After you start to understand the geometry of the LED and the quantum physics of the photo release mechanism (from the previous suggested links), your quest folds back to understanding the current density and geometry relationships in a semiconductor junction. A search might leads you to articles like http://gautier.moreau.free.fr/articles/asryan96.pdf which involve so much higher math that it will take the nerdiest of engineering nerds a month or more to figure out if they are applicable to your quest and to more general college level text books like http://ecee.colorado.edu/~bart/book/book/contents.htm where you might eventually learn enough from (after taking a detour into learning enough math to do the calculations required) to figure it out yourself. I don't recommend either of these approaches as they involve a LOT of complicated aspects of electrical engineering at the masters, doctorate, and post-doc level.

Why don't you try the Scientific Method? Come up with a hypothesis that you can test, run the experiments, evaluate your data, and see if you guesed right or wrong or if your experiment neither proved or disproved your hypothesis. That is what scientists do.

But first, you do need to try and understand the concepts from the first two links as you can't directly observe these aspects (but that is another area, the uncertainty priciple that Werner Heisenberg discovered).
-Craig
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Re: How Does LED Brightness Vary with Current?

Postby Ivo » Sun Jan 23, 2011 7:20 am

Thanks for your information - a lot of complicated mathematics for me though!

I was thinking whether it would make a difference if I did the experiment in normal indoor conditions (normal lighting conditions), where I would put the LED a set distance apart from the light meter. I would then take the reading when the LED is ON, then I take the reading immediately after I switch it OFF. This is because I may not be able to do this in a dark room. By doing it this way, would it make a difference, if so why is this? Is there some clever physics explanation for this?

I plan to improve the experiment by conducting it in a felt-lined box, but what kind of uncertainties/ systematic errors does this reduce?

Many thanks in advance guys!
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Re: How Does LED Brightness Vary with Current?

Postby robertreavis » Thu Jan 27, 2011 3:44 pm

The hard part is measuring the LED light without any other light to "distract" the meter. It is just like to listen to a quiet whisper in a quiet room or next to a noisy highway.
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Re: How Does LED Brightness Vary with Current?

Postby Ustonn » Thu Sep 27, 2012 4:19 am

Robinson wrote:
Ivo wrote:I'm conducting an experiment which is similar to that below:

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p037.shtml

Instead of using a light-to-voltage converter, I'm using a digital light intensity meter (lux meter). However, do I need to ensure the lights in the room are switched off (i.e. the room is dark?) or would it not make a difference. I'm only studying the relationship between light intensity and current through LED, so does it matter? If so, due to space constraints I won't be able to do it in a dark room. Instead, I might have to use a large cardboard box. But why do I have to line the inside of the box with black felt?

Many thanks in advance guys!

LEDs (light-emitting diodes) are electronic components that convert a portion of the current flowing through them into light.
The principle of LED brightness is to More current = More brightness
The goal of this project is to measure the light output of an LED as a function of current through the LED.



I think they are stable and not vary with curent
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