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Light Source (LED's): Doubling intensity versus doubling sources

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mdwebster

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Greetings,
I'm trying to figure out this optical intensity question. I'm looking at ways to increase the range of an IR emitter. Would it be better to put in a second emitter or to double the intensity of a single emitter (via using more current or using a higher efficiency emitter).

Intuitively, it seems that multiple lights wouldn't increase, say, the half-brightness range as much as doubling the intensity of a single source, but I'm not sure. It seems that multiple sources just gives you the opportunity to increase your half-angle intensity by situating the LED's at slightly offset angles from one another.

But ... I'm not sure, so I thought I'd ask ... :)

Thanks for any help,
Mike
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Be more specific about exactly what you're trying to do, and also what supply rail you have available.

BTW, it's common to have two IR LED's in remote controls, wired in series (as in my tutorials) so you get extra power at no extra cost.
 

mdwebster

New Member
I'm just trying to figure out, in general, if having two LED's will increase your range as much as doubling the light intensity of a single LED.
I'm working with 5V and maybe 400mA of available current. Looking at one spec sheet here in front of me, going from 200mA to 400mA will increase the radiant intensity (in mW/steradian) from 300mW/sr to 500mW/sr.

Would having two LED's at 200mA really give me 600mW/sr if they were both lined up on the same axis?

I guess it just seems counter-intuitive to me for some reason, hence my question. I guess it can be boiled down to this: Are two lights exactly as bright as one light which has twice the intensity?

I appreciate the time in any case.
Thanks,
Mike
 

3iMaJ

New Member
If you're in the far field (> 10 lambda) then doubling the intensity of one diode is the same as using 2 diodes. I'm not considering the device losses. So, my assumption is by doubling the input current, voltage, or whatever doubles the radiation intensity.

Oh, and the diodes must be transmitting the same waveform, so another obvious assumption is coherent transmitters.

On a side note: This is exactly the same type of problem where dealing with an array of transmit elements. Generally the assumption is isotropic transmit elements, but it turns out the resulting array pattern is the array_factor * single_element directive pattern. Where the array factor is just due to how the patterns of isotropic elements add in the far field.

I would also like to point out that your elements cannot be influencing the fields of one another. That is to say that there is no coupling between your transmitters.
 
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3iMaJ

New Member
I've attached a few simulations to give you an idea of how far to place the elements apart. I've assumed isotropic elements, as I don't know the gain patterns of your elements.
 

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Hero999

Banned
Use as many LEDs in series as you can and use a low duty cycle so you can overdrive them and get even more light without having to buy more of them.

With visible LEDs, increasing the current and lowering the duty cycle doesn't give you anything but for infra-red data transmission it does as the receiver responds to peak power not the average power.
 

mdwebster

New Member
Thanks a lot for the pointers, 3imaj. It led to some interesting reading in a field I wasn't familiar with. These emitters are completely non-isotropic. Their half-angle intensity (FWHM) is only 10 degrees.
The frequency is 890nm, and it's a 5mm LED, not an SMT, so I can't see getting any better than ~5mm from center to center which is ~5 lambda. In all likelihood, they'd be spaced at least one LED width apart to ease manufacturing constraints, so they'd be in far-field territory from one another already.

Thanks too to Hero, I'm already using a low duty cycle / relatively high-current design. This may well belong to another thread in the electronics forum, but my biggest problem so far is, as I ramp the current up, the turn-off time suffers. It takes 10 times longer to turn off than when turning on and I get pretty ugly ringing as it does so. I'm using a BJT for the drive transistor. Does this sound like a saturation problem? Would putting a schottky from base to collector prevent full saturation?
 

3iMaJ

New Member
If you can get a closed form solution of your diode's beam pattern I can give you an idea what the total beam pattern will look like. Even if you can get an approximation that would probably be good enough.
 

Hero999

Banned
You could try using a MOSFET if the switching time is an issue.
 
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