Unlike many photographic films, the CCD or CMOS of a digital camera is inherently susceptible to infrared contamination, even when the manufacturer attempts to reduce this problem by adding an infrared absorbing filter on top of the image sensor. Because these filters do not have a sharp cutoff at the infrared wavelengths there has to be a compromise in order not to exclude visible red wavelengths. This results in some infrared wavelengths being allowed to pass. While minor infrared contamination does not normally cause much of a problem for visible images, when using conventional neutral density filters especially denser grades, this issue is multiplied by the filter factor
The Hot Mirror filter reduces the amount of infra-red light entering the lens while having no affect on the wavelengths of light in the visible spectrum. Digital sensors are especially sensitive to IR light, which can cause aberrations and colour shifting in the image. The hot mirror filter is especially useful when using a neutral density filter because the ND filter will increase the ratio of IR light to visible light. The Hot Mirror ND filter accomplishes both tasks in one combination filter, freeing up an extra filer stage in your matte box.
Is there anyone using IR or hot mirror filters that can clarify? How significantly do these filters improve your image?