How should I orient my reflective ND or interference filter?

Reflective ND (neutral density) filters and Interference filters will function as specified with either side facing the source. However, we recommend orienting the side with the "mirror-like" reflective coating toward the source. This will minimize any thermal effects resulting from the absorption of the heat by the glass on the other side. Also, having the "mirror-like" side facing away from the source will cause an interference pattern when the source is a coherent beam of light. For filters in general, the coated surface is oriented toward the source. In addition, filters will perform optimally if positioned in a collimated beam of light. This will reduce the angle of incidence and the performance results will closer match that of the filter's design. Interference filters in particular, are extremely sensitive to angle.