IR emitter for license plate recognition?

W

wrybread

Pulling my weight
Sep 12, 2018
257
121
San Francisco
I'm looking for an IR emitter for license plate recognition. The plates are pretty close, only about 30 feet away. And if possible I'd like one that's outside the human visible spectrum so people don't notice it.

Any tips by chance?
 
You have to get one that matches the IR frequency that your camera "sees" -- so it may have that telltale red glow to it.
 
Most cams are optimized to see 850nm IR, the wavelength with the dim red glow that humans can see; most cams can see 940 but its effect is reduced so more intensity of 940 is required as compared to the 850. That overall intensity requirement translates to needing more of them in a given area, I would imagine.

I've read estimates that most cams designed for 850 are as much as 80% less sensitive to 940 than 850, meaning you'd need as much as 5x more luminosity of 940 vs. 850....YMMV :cool:

TL;DR: most cams can see 940nm (the wavelength with no dim red glow to humans) but it'll take a LOT more of it as compared to 850nm, the one with the dim red glow.
 
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Hmm. It's for Dahua IP cameras, not sure of the model name at the moment, but maybe that helps. I'm just trying to make the plate pop, not trying to illuminate anything else, if that makes a difference.
 
All dahua cams are in the 850 wavelength, but as mentioned it can see the 940 but would need a lot more. And you will still get the red glow if you looked directly at it.

At 30 feet the onboard IR is more than enough to make a plate pop.

At night, we have to run a very fast shutter speed (1/2,000) and in B/W with IR and the image will be black. All you will see are head/tail lights and the plate. Some people can get away with color if they have enough street lights, but most of us cannot. Here is a representative sample of plates I get at night with a 1/2,000 shutter and 8 FPS of vehicles traveling about 45MPH at 175 feet from my Z12E that is on the 2nd story soffit with no street lights. Camera is 35 feet above street at this location.

1674886306020.png

At 30 feet, the plate would be even more visible.


Also, keep in mind for LPR, it is all about making the plate as large as possible, so a 2.8 or 3.6mm camera will not work. At 30 feet you need a varifocal set to 12-13mm to be able able to read plates. Your field of view should be not much larger than a car.
 
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Also, keep in mind for LPR, it is all about making the plate as large as possible, so a 2.8 or 3.6mm camera will not work.
Click to expand...

My camera works really well for LPR, in the day I get probably 95% of plates read correctly using openALPR, just looking to add more lighting to make it a bit better at night. The onboard emitters are pretty anemic.
 
wrybread said:
My camera works really well for LPR, in the day I get probably 95% of plates read correctly using openALPR, just looking to add more lighting to make it a bit better at night. The onboard emitters are pretty anemic.
Click to expand...
Can you post a representative picture of your nighttime plate captures along with a screenshot of your camera settings?
 
Post a daytime image and the camera model, along with your camera settings for shutter, gain, etc at night.

If you are using a fixed lens or have a varifocal not optically zoomed so that the field of view is not much larger than the vehicle, it will struggle at night and adding more IR may not be enough.

You can probably get away with some digital zoom during the day that OpenALPR is doing, but not at night.
 
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Wonder if they spread enough. All of the little non-IR lasers like that I've had still were just a small dot wayyyy out in the distance.
 
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I had a cheap laser level with a removable diffuser cap. I forget what it was there for, but if you put a piece of acrylic or an old eyeglass lens in the beam path, it would probably diffuse the beam a little.

I noticed with the laser level that when it hit the reflective coating on modern license plates, the plates would light up in a way that normal painted objects didn't. That makes me think a cheap diode laser might work without modding. But as the engineers say, one test is worth a thousand theories.
 
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