Why are UV Filters Directional? (or are they?)

Question

I recently had a conversation with someone who collects bugs at night. They use large halogen lights to attract the bugs, and since the lamps produce some radiation in the near UV range (which is undesirable) large UV filter sheets are placed in front of the lamps.

These UV filters claim to be directional -- evidently there is an arrow on them telling the user which side should face the light, and which side should face away from the light. My question is why? What physical property of a UV filter could alter the filtering characteristics based on the side upon which the light is incident?

In attempting to find an answer, I came upon the Baader Planetarium website, which sells these UV filters, which they do claim are directional. Specifically, the website says,

Always put the more reflective side towards the telescope side. To guide you we already put a small arrow on the filter rim, on those filters were the position matters. This arrow indicates which face of the filter should be directed towards the sky (telescope-sided). All cell-mounted filters are already oriented in a way that the most appropriate filter face is facing the sky when the filter would be mounted directly onto the front end of the nosepiece of a camera. If you mount your filter the other way, any reflected light would have a short way to the camera sensor, resulting in a higher risk of getting some kind of back-reflections inside the camera field. Many sensors have highly reflective areas near to the light sensitive area, also the area with the bonding contacts is sometimes highly reflective.

For a large halogen lamp, however, such considerations seem irrelevant -- there are no delicate optics to destroy. Furthermore, the bug collector claims that the UV filters appear identical on both sides.

Is there some sort of reflective coating on one side of halogen lamp UV filters (if so why?) and if not, why are these UV filters directional?

Answer 1

In the case of the lights you want to pass UV, so put the UV side toward the light because if you use the other side then about 4% will be reflected away by the uncoated side. Each side of a glass window reflects about 4% of the incident light (assumes light is incident from an array of angles). I think your bug collector is trying to block the visible and IR parts....

These special filters use what is called a dichroic coating, a thin layer that is matched in thickness to the wavelength of the light. In practice many layers are used to help the filter work at many angles and the desired wavelengths. Transmission >99% can be achieved.

Also: From Thorlabs : Although the filter will function with either side facing the source, it is better to place the coated side toward the source. This will minimize any thermal effects or possible thermal damage that blocking intense out-of-band radiation might cause due to the absorption of the out-of-band radiation by the substrate or colored glass filter layers.