IR Cut Filter and IR Pass Filter

[sounds 0]

I hope someone could explain more in detail the main difference between the two different type commonly used in IR camera.

 

Thank you in advance

[VST_Man 1]

http://www.jr-worldwi.de/photo/index.html?ir_filter.html

[CameraGimp 0]

I'm not sure what two filters you are referring to. All colour cameras should have two optical filters fitted in front of the imager. One is called the IR cut filter and the other is an optical low pass filter (OLPF). An IR camera doesn't have a filter. The two filters on a colour camera are usually put together in one block. It is the cyan piece of glass (actually crystal) you will see in front of the imager on colour cameras.

 

The IR cut is there to stop IR light getting to the imager. That link is really good and gives lots of info but to put it simply an IR cut filter will stop wavelengths above about 750nm reaching the camera. Look at it as a pair of sunglasses for IR light. If these weren't fitted colour cameras would see IR and although this may sound like a good thing (better sensitivity) it has a major negative effect on colour reproduction.

 

To understand you need to think about how we see colour, white light shines on a green wall, the wall absorbs all the light except the green wavelengths which bounce off the wall so we see a green wall. Now if the human eye could see IR we would see the green light plus IR light. The wall would no longer be green.

 

For us in CCTV this becomes important for day/night cameras. For true colour you need the IR cut filter. For better sensitivity you don't want the filter, it's a pair of sunglasses remember. A good day/night will move the filter mechanically. A cheap day/night will just not have a filter, in which case the colours will be off, leaves are classic, the appear almost white.

 

The OLPF is just weird.

[sounds 0]

Thanks to CameraGimp and VST_Man for your replies. Both of you technically answered my questions. Thanks.