Used for subtraction of blue background. All of our astronomical filters have IR blocking "built-in" so IR light does not contaminate today's sensitive CCD detectors. When you use filters from Custom Scientific, you do not need any additional IR blocker filters. One exception is in the thickness-matched Clear filters which transmit all wavelengths from the UV to the far IR; they are clear, high-precision, optical windows designed to transmit all wavelengths of light without having to refocus when switching back to a color filter. Filter has Anti Reflective Coatings.