Does the orientation of a dichroic filter affect its spectral transmission?

Yes, the orientation of a dichroic filter significantly affects its spectral transmission. This dependence comes down to two main factors: the angle at which the light hits the filter, and which side of the filter is facing the light source.

Here is a breakdown of how orientation impacts performance:

1. Angle of Incidence (The "Blue Shift")

Dichroic filters are interference filters made of microscopic layers of dielectric materials. They work by causing specific wavelengths of light to constructively or destructively interfere with each other based on the thickness of those layers.

When you tilt the filter and increase the Angle of Incidence (AOI), light travels through the dielectric layers at an angle. This changes the effective optical thickness of the layers. As the AOI increases, the transmission bands and reflection bands shift toward shorter wavelengths. This phenomenon is commonly referred to as a "Blue Shift."

For example, a filter designed to transmit light at 500nm when placed perfectly perpendicular to the light source (0° AOI) might transmit at 490nm if tilted to a 30° AOI. If a precise wavelength is required, the filter must be mounted at the exact angle for which it was designed (typically 0° or 45°).

2. Front vs. Back Orientation (Coated Side vs. Substrate)

Dichroic filters usually consist of a glass substrate with the complex interference coating on one side and an anti-reflective (AR) coating on the other.

While flipping the filter front-to-back might not drastically change the transmission wavelengths in a perfect theoretical model, it matters a lot in practice:

• Thermal Damage: The coated side should always face the incoming light source. The coating is designed to reflect unwanted wavelengths immediately. If the uncoated glass substrate faces the light, the light must travel through the glass before being reflected by the coating. This can cause the glass to absorb energy and heat up, potentially damaging the filter or altering its performance.
• Ghosting and Scatter: If the light hits the glass substrate first, you can get secondary internal reflections (ghosting) between the bare glass surface and the coating, which reduces the overall efficiency and clarity of the transmitted beam.

To help with proper installation, manufacturers usually indicate the coated side with a small caret (an arrow) on the edge of the filter pointing to the coated surface, or they bevel the edge on the coated side.