SLR Magic recently released its latest product, the Image Enhancer Pro filter that is targeted at cutting light more evenly across the spectral range.
Digital sensors see outside the range of human vision—with especially big problems seeing into the infrared spectrum in ways that humans can’t, resulting in unwanted distortion. While you might want to use this for a specific effect, it’s much more likely that the extra info in the IR spectrum is showing up in your images in unpleasantly lifted blacks, sometimes with a color cast, provided by fabrics that look pleasant on set but reflect more IR than the eye can see. This problem is typically exacerbated by traditional ND filters, which usually lower the visible spectrum more than the IR spectrum, leaving you with even more ND issues.
Why does this matter? (see here)