What is IR Pollution and why we notice it more sometimes!

Camera sensors are sensitive to light in the visible spectrum plus IR and UV (non-visible spectrum). Camera sensors do see these invisible sources and convert them into a signal which can create colour pollution.

Some cameras use a Cut Filter to block IR and UV light hitting the sensor and some don’t. For the cameras without IR filtration it’s generally not a problem in standard use as the amount of visible light far outweighs the amount of unwanted light in the non-visible spectrum. Cameras with an IR Cut filter are still generally prone to a small amount of leakage between 700nm and 730nm, which again isn’t an issue in normal use.

So when is it a problem? Problems occur when you reduce the amount of visible light without reducing the amount of non-visible. This increases the overall percentage of IR hitting the sensor thus making the output more red, especially in the blacks. This happens when you use a non IR compensated ND Filter and the stronger the filter the worse the effect becomes as the percentage of IR signal increases. Some modern cameras have an electronic ND filter which can’t be IR compensated so IR pollution is fairly inevitable.

Current shooting styles where the lens is wide open to achieve a very shallow depth of field, require that the operator put a lot of ND in to achieve the correct exposure. High value cameras such as the Sony Venice and the Arri Alexa LF and S35 have built in IR ND Filters so do not exhibit the effect, whilst using the internal filters. Less expensive cameras have Electronic VND or non IR NDs so will show the pollution under these circumstances.