That's not right. The IR cut filter is needed to ensure correct colour reproduction. It has nothing to do with correct exposure unless you wanted to guarantee correct greyscale reproduction on objects that reflect IR.

If an IR cut filter was needed to prevent over exposure how would any monochrome or IR sensitive cameras work in the day?

It's for color reproduction, if outdoors colors look correct you have an IR filter. If they look all strange and crappy then you might not, IR filters exist that are kinda half and half this is what all those IR bullet cameras have basically they have a notch wavelength that they will partially pass the the DSP in the camera will attempt to account for this during the daytime. The notch matches the output of the LEDs and was probably chosen to be not as naturally occuring.

The filter is about 1 F stop so it damn sure does effect exposure and depending on the ciruitry limitations you c an wind up with an overexposure situation in high lighting.

Yes, that is what my understanding was. Plus, they are also used to avoid the slight blur caused by the longer wavelengths (near IR and longer) not focusing on the image plane where the visible wavelengths focus. So IR cut filters serve these two purposes.

Regarding the exposure point the other poster raised, I am not sure that is correct. The exposure is calculated by the amount of light falling on the CCD. It should not matter if any filter decreases the amount of light passing through the lens, the exposure setting algo. should adapt accordingly. The only situation where any illumination reducing filter can affect the exposure is when the camera is already working in very low light environment and filter just decreases the light below the minimum threshold. Otherwise it should not really matter as fas the exposure is concerned.

BTW, can somebody put up examples of a 'day scene' with and without the IR cut filter showing the color change? It would be really appreciated.

Thanks.

Google it

An IR filter – or IR cut filter - is a color filter blocking the infrared light. There are several good reasons for using an IR-cut filter. Using a color camera to achieve realistic colors in white light requires an IR-cut filter. The color spectrum seen by the human eye is quite limited compared to the spectrum seen by a CCD camera. Especially, in the near infrared region of the spectrum the difference in sensitivity is significant. This is important to know since many light sources, including the sun, emit infrared light. A CCD color camera in daylight without an IR-cut filter will therefore see a significant amount of infrared light resulting in strange colors. Another reason for using an IR-cut filter is the limited color correction for many lenses. It is difficult to design imaging optics covering both the visible spectrum and the near infrared spectrum at the same time. Therefore, many lenses have different depth of focus for the visible and the infrared spectrum. Anyway, the IR-cut filter cuts away a significant amount of the overall collected light and thereby affects the sensitivity in a negative way. In general, color cameras are one factor less sensitive compared to monochrome (depending on the CCD chip). This is primarily due to the IR-cut filter.

Sorry but exposure will not be affected by a filter.

Pixels in a CCD gather charge. They do not know and cannot differentiate between white light, blue light, pink light, IR or anything else. If it puts a charge in the pixel that is all they care about.

DSP's create colour as colour CCD's (which are exactly the same as mono CCD's except for this) have coloured filters over pixels and they calculate colours and brightness from these four pixels.

IR light of any wavelength is just charge to a pixel. Any camera that will automatically adjust exposure will do this just as well for natural light as it will for IR light. So although a colour camera with a notch filter may be one f stop more sensitive than one with an IR cut it's exposure algorithm will operate exactly the same. The only exception is if the light levels are so high that the shutter speed or AI lens fstop cannot cope but that is very unlikely in any camera made in the last ten years. I fully accept the colour balance will be thrown out the window but not exposure. The camera will see a bright spot of IR light exactly as it would a bright spot of natural light and adjust for it.

Again, how can a monochrome camera work otherwise. They use the same CCD except for the YMCK filters over the top but they do not over expose during the day.

IR cut filters only affect colour balance and grey scale reproduction. The camera becomes more sensitive but the camera can cope with that in exactly the same way as it would if you put a better lens on it.

Perhaps the issue is the poor greyscale reproduction? A camera that can see IR may show a dark object as light grey. It may look black to us but if it reflects IR it won't be black to a camera.

You're assuming that a camera that lacks a cut filter also has an auto iris lens to compensate for the extra light. On lower end cameras this isn't the case. As such, the lack filter allows for a light source that the human eye can not normally see. This is why cheap bullet cameras look washed out. Quite often they don't have a way of closing the lens to compensate for the extra day time lighting.

Thus creating a view that looks over exposed.

No I am not. The cheapest CCD camera has an electronic iris that is more than enough for daylight with or without an IR filter.

You need to look at the effects of removing an IR filter on greyscale reproduction because I can assure you IR sensitivity does not effect exposure in the way you think. Ok there is more light falling on an imager when you remove the ir filter but the effect is no different from switching to a faster lens, the sun coming out from behind a cloud or someone turning lights on, the cameras auto exposure circuits will adjust and it can cope with sunlight without an filter. Electronic iris is proven and there are 1/2" IR exviews installed outside with manual iris lenses without exposure problems.

I think you are getting over exposed confused with washed out. We know that an IR sensitive camera sees things differently to us or a colour camera, we know IR messes up colour but it messes up other things too.

Look at the leaves on a tree with a colour camera and turn the colour off on your monitor. The leaves will go from dark green to dark grey. View those same leaves with a monochrome camera that can see IR and the leaves will look a lot paler. This isn't washed out because the camera can't cope. They are reflecting IR and that is how they look to something that can see IR. If a monochrome camera appears to have less contrast it is because the scene it is viewing has less contrast but it is not washed out because the IR is swamping the exposure circuit, it is coping as it should. If only we could see IR that is how it would look.

Hope that makes sense.