# Measuring Light Output

Howdy Yall,
First time poster, long time lurker. I am working on a project right now measuring the "useful" life of a battery...say a AA battery. To relate that to a real world application, I will be powering a light (LED) off the battery. I was initially planning on getting the battery, an LED (unknown specs yet) and a resistor....say 5 ohms. I will be timing how long it takes that LED to "go out". The problem is, what is "go out"? I need to be able to quantify that. Is there a good, CHEAP way to measure light output (intensity?) quantitatively? How about designing a circuit that would cut the LED off when the battery dropped below 0.75 V? That would also work. Any help would be appreciated!

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Danger
Gold Member
How about designing a circuit that would cut the LED off when the battery dropped below 0.75 V? That would also work. Any help would be appreciated!
Welcome to PF. Such a circuit is definitely possible, since cell phones and laptops use them. I'm afraid that I can't help with the design, though.

Drakkith
Staff Emeritus
Could you not simply say that "Go Out" is the moment that you can no longer see light emitted from the LED? How exact do you need to be on this?

Danger
Gold Member
Could you not simply say that "Go Out" is the moment that you can no longer see light emitted from the LED? How exact do you need to be on this?
The problem that I foresee with that is that one's perception of light varies with the circumstances of the observation. The primary culprit in that regard is ambient light, but the variation of sensitivity vs. exposure also enters the scene. I seriously don't think that a strictly biological and therefore variable opinion can be considered accurate.

Drakkith
Staff Emeritus
The problem that I foresee with that is that one's perception of light varies with the circumstances of the observation. The primary culprit in that regard is ambient light, but the variation of sensitivity vs. exposure also enters the scene. I seriously don't think that a strictly biological and therefore variable opinion can be considered accurate.
That all depends on how exact a measurement is required by this experiment. If it needs to be very accurate, then you will need something to measure the battery for you. If it doesnt need to be "too" exact, then an LED in a box with a hole in it for your eye could work.

Danger
Gold Member
If it doesnt need to be "too" exact, then an LED in a box with a hole in it for your eye could work.
That depends upon how acclimatized your eye becomes to the wavelength involved. In such a situation, I would far rather have a digital camera as the observer, with a measurement of the various output channels of the CCD as the arbitrator. Seriously, things have a tendency to disappear it you stare at them for too long.

Drakkith
Staff Emeritus
That depends upon how acclimatized your eye becomes to the wavelength involved. In such a situation, I would far rather have a digital camera as the observer, with a measurement of the various output channels of the CCD as the arbitrator. Seriously, things have a tendency to disappear it you stare at them for too long.
Definitely! A CCD would make a much better observer than the naked eye. I was merely pointing out that if the requirements of the experiment were not too strict, then one could use the naked eye. It would be preferable not too however.

OP, you might be able to find a CCD somewhere or perhaps get one from a digital camera? I'm just guessing here.

Danger
Gold Member
Drakkith, thank you for not taking objection to my post. I realized after putting it up that it might seem contentious, but that wasn't the intention and I'm glad that it wasn't taken so. I was adamant about the point only because I have a lot of personal experience with things vanishing right before my eyes simply because of said eyes becoming accustomed to their presence. (And before some smart-*** chirps up, this was noted well before I started drinking...)

Drakkith
Staff Emeritus
Drakkith, thank you for not taking objection to my post. I realized after putting it up that it might seem contentious, but that wasn't the intention and I'm glad that it wasn't taken so. I was adamant about the point only because I have a lot of personal experience with things vanishing right before my eyes simply because of said eyes becoming accustomed to their presence. (And before some smart-*** chirps up, this was noted well before I started drinking...)
Hah! No problem. I try very hard to remain objective and to understand people posts, especially in light of the Negative Temp thread in the classical physics forum...some people just cant be reached... I've tried to remain objective in that thread, but when everytime you post something you get told "Your objections arent relevant" or "You dont understand" it gets old quick.

Danger
Gold Member
Hah! No problem. I try very hard to remain objective and to understand people posts
If only more people had that attitude...

If you just want to measure the useful life of the battery, couldn't you connect your LED-resistor circuit in parallel with a multimeter (measuring voltage)? And then take a video of the multimeter to see when it drops below 0.75 volts (just so you don't have to stare at a multimeter for 5 hours)?

Andy Resnick
Howdy Yall,
First time poster, long time lurker. I am working on a project right now measuring the "useful" life of a battery...say a AA battery. To relate that to a real world application, I will be powering a light (LED) off the battery. I was initially planning on getting the battery, an LED (unknown specs yet) and a resistor....say 5 ohms. I will be timing how long it takes that LED to "go out". The problem is, what is "go out"? I need to be able to quantify that. Is there a good, CHEAP way to measure light output (intensity?) quantitatively? How about designing a circuit that would cut the LED off when the battery dropped below 0.75 V? That would also work. Any help would be appreciated!
This is an odd project. Batteries are designed to deliver constant voltages over their lifetime, are are specified in terms of the amount of energy they contain. For example, my cell phone battery (Li-ion) claims to contain 1100 mA-h. An alkaline AA battery is rated at 2700 mAh

http://en.wikipedia.org/wiki/List_of_battery_sizes

Even so, if you are dead-set on measuring the lifetime of a battery-LED combination, then hook it up, point the LED at a photocell

http://www.makingthings.com/teleo/teleo/cookbook/photocell.htm [Broken]

and unless you want to sit and take readings every second for hours, plug it into a data acquisition module to record the voltage for you.

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