Walt

From Jack: "The bottom line is it shouldn't matter to the histogram if the IR cut filter is on or not. The histogram responds to any energy that makes it to the sensor, and if it is not blown the exposure is fine."

Jack- Yes, I think this is precisely the point. The IR sensitivity is not detected by a meter, but does make it to the sensor and is thus shown in the histogram. So a highlight (as measured by the meter) that is also reflecting (or emitting) IR is going to put more energy on the sensor and some of that energy is not contributing to the (visible) image. If a black sweater is reflecting IR, it's not going to make a difference; if a white object is reflecting IR it can be the difference between a detailed highlight and a blocked one because the IR contributes to the overload of the sensor. When IR reflectance or emission is from a highlight, the dynamic range of the sensor is effectively reduced by non-image forming light. This is a problem.

Additionally, I am imagining that IR radiates (or spreads) differently than reflected visible light, certainly in the longer wavelengths, and that this degrades the image. I have recently spent some time is an airplane with a Forward Looking Infrared (FLIR) display and spent time taxiing around an airport looking at the displayed image. How this unit compares with the spectral sensitivty of the M8 sensor I don't know--I suspect that the M8 is less sensitive to the longer wave (emitted) IR than the FLIR. But the FLIR image is instructive because it looks surprisingly like the paving stones in the original post--smeared and bloated. Light gray rabbits (running in the grass along side the runway) pop out on the FLIR as blown highlights.

I believe that I have observed these problems clearly after months of not using IR filters, thinking I did not need them for BW. I need them. Blown highlights are very dramatically reduced and clarity in some high key areas is improved. The FLIR has also been instructive in revealing the complexity of what reflects or emits infrared light. It has little to do with the visible brightness or reflectivity of the surface, though visibly bright surfaces do often reflect or emit infrared.

Walt