Please explain IR cut filter

[bike_rider 0]

I've been reading the posts on LPR systems based on non-LP cameras. A couple of the more successful setups make a reference to things like "

Lens has a 52nn screw-in 850nm IR cut filter. "

 

Can somebody explain this to me? I'm familiar with the IR filters that are automatically moved in true D/N camera, but that doesn't seem to be what these comments are about.

 

Thanks

 

Update, I think the correct term is IR pass filter. Is this right?

[SBTVideoman 0]

Camera filters can be IR CUT or IR PASS depending on application.

 

An IR CUT filter passes visible light but blocks all wavelenghts longer than visible red, meaning no infra-red light is passed.

 

An IR PASS filter will typically pass near infrared light, but may block long-wavelength infrared. These typically pass wavelenghts around 830 - 940nm which is the wavelengths common IR illuminators work at.

 

Now - to the application you discuss, licence plate recognition....and why an IR CUT filter may be useful. Any camera looking at the front of a car will typically have the cars headlights in the field of view. Most cars these days employ halogen headlights, which pump out lots of infrared in addition to the visible light. If you use a camera which is very sensitive to IR (eg an ExView or SuperHAD B&W camera, or an IR-pass colour camera or a Day/Night camera with mechanical filter), you may find that such a camera will flare out abnormally when confronted with car headlights, because it "sees" too much of the IR component. That's where screwing an IR CUT filter into the lens may help the issue, because now the camera will only be seeing visible light.

 

Obviously a good camera should not have a problem....but having the option in the lens to screw on an additional filter is useful.

 

Hope that makes sense.

[bike_rider 0]

Thanks,

[VST_Man 1]

I think you meant to say IR pass for the LPR cameras. LPR's with IR work via the IR Pass allowing the reflected IR to pass. IR cut would block the reflected IR and produce no LPR reflection........License Plates use a special reflective paint............IR luminates it and the camera with a IR Pass filter allows the reflected IR in.

 

IR cut filters on Day/Night cameras cut IR during the day and allow IR at night. Color reproduction during the day is harder to reproduce than a simple B&W night pic.

 

also..most IR luminators with black lense covers are actually IR Pass filter glass...they block visible light and pass IR...........no visible LED red glow or "light" from bulb IR

 

a bit confusing...........

[bike_rider 0]

If I go down this path, I think an IR pass filter is what I'd be looking for.