Converting cameras for infrared use: When does the filter have to be replaced, and when is simply removing it ok?

Question

When converting a digital camera for infrared use, the general approach is to remove any filters that limit its sensitivity to the visible range of light wavelengths.

Some conversions replace the existing filter by clear glass, some by a dedicated infrared filter, and some seem to just leave the filter off entirely.

I wonder what difference leaving the filter off entirely and replacing it with clear glass of equivalent physical dimensions and refractive index makes. My current suspicion is that cameras with a purely contrast-based autofocus, like most compact and bridge cameras, can do fine without any glass in front of the CCD, but any camera with phase-detection AF needs the clear glass replacement for the autofocus to work properly. Is that correct?

How about protection of the sensor -- is it ok to leave the CCD surface uncovered inside the camera housing?

Answer 1

For phase detection AF to work properly, the camera needs more than just a clear glass replacement for the removed filter, it needs a replacement of a different thickness.

IR rays don't focus on the same point as visible light, which is why lenses have (used to have?) a red dot to tell you how to adjust focus from the visible light distance to the correct distance for IR. So the AF on an unmodified phase detection AF camera will be incorrect for IR. However, during conversion it's possible to put in a replacement for the removed filter that changes the sensor stack's height (the technical term for the filter's thickness) so that the AF focus system works for IR!

Roger Cicala gets into the details of sensor stack height at The Glass in the Path: Sensor Stacks and Adapted Lenses. One company that does conversions and sets the sensor stack height correctly is LDP LLC (MaxMax.com).

Contrast detection AF shouldn't have a problem because it uses the image sensor itself, not separate AF sensors.