Camera luminance using IR LEDs

camera luminance

Hi all,

I am interested in designing lightning for CMOS camera sensor using IR LEDs.
My circuit is battery operated, so the current consumption is an issue.
Can anybody tell me what LEDs is best to use, i.e which LEDs will give me best luminance for as little current as possible.

Whatever leds you use use are really going to drain your battery quite quickly unless you are using high capacity cells. Best to look up specifications for best luminance at different current levels, probably not a great deal in it between manufacturers. If you are using it for monitoring, have you thought about using a battery operated PIR to switch on the camera/leds when movement is detected. You would need some sort of simple timer to keep it on for a short while after a detection event.

Sure, i thought about it, but my problem is I don't know which LEDs will be powerfull enough to generate light for the sensor-at a resonable distance, and still not drain my battery too much, what Il need is some background to choose the correct LEDs.

I'll second GrandAlf in his opinion that there is not a great deal of difference in LED efficiencey between different semiconductors. But it also depends on how efficiency is defined in your particular case. Here are some avenues that might let you pick the right LEDs for the camera sensor:

1. To second GrandAlf again, turn the LEDs on only when there is something interesting for the camera to look at.

2. What's the view angle of the camera? Put the lens on the LEDs so that the beam doesn't "spill" outside the view field of the camera.

My best advice would be to get some LEDs and lens and to experiment.

From my limited experience ( watching night time wildlife ) 850nm LEDs work better than 940nm. You can see them glow slightly which might be an issue depending on your application.
You will need to use a number of LEDs to get adequate illumination over any distance. I use 50 LEDs at about 400mA to cover the back yard to a distance of about 15m. Even so the image is dim using a B/W CCD with a small lens.
I find that the LEDs give a pool of light that matches quite well to the field of view of a 16mm lens and a 1/3inch CCD.
Use as large a lens as you can to capture what light there is, ( C or CS mount lenses are far better than the miniature board mount lenses ) and go for B/W if you can. The bayer matrix colour filters are not completely transparent to IR so cut down the IR getting into the sensor.
To reduce the amount of illumination you need get the LEDs as close to the target as possible. See any book on photography for how to illuminate the subject with minimal lighting.
You could try using perhaps a passive IR sensor to only turn on the camera and/or illumination when needed, this may not of course be suitable for you application.

Thank you all for your replies, you are very helpfull.
Do any of you have expiriance with high power LEDs in the visible light spectrum, such as they put in phone cameras, for example lumiled's LUXEON high power LEDs?

You can also consider supplying the LED from a current source. In such configuration a serial resistor for the LED current limiting is unnecessary so the battery will be used more efficiently.

Thank you AndrzejM, I did see a couple of App notes regarding current souce in visible light flash, some manufacturers suggest switching power supplies for this, does this seem necessary to you?

Ronnie said:

some manufacturers suggest switching power supplies for this, does this seem necessary to you?

Click to expand...

Current through the LED must be limited. The most common solution is to form a kind of constant current source (or current limiter) by supplying the LED through the series resistor. If your voltage source is very stable the voltage drop on series resistor could be low and the power losses are small. If voltage source is unstable, to keep the LED current within the limits the series resistor should have higher value and lost more power.

Instead of loosing the power in current limiting resistor it is more effective to store the energy in inductance and than hand it over to LED (this is the way the switched current sources work).

Just calculate power losses in resistor and analyse if it is worthy to convert them into the light. (current source is not 100% efficent device but 80- 90 % can be riched).