Generally, no.
It is not recommended to use standard interference filters as dichroic mirrors to reflect blocked wavelengths, although they might seem similar in construction.
While both use thin-film dielectric coatings to manipulate light, using a standard filter in a dichroic setup introduces several significant optical issues. Here is a breakdown of the primary reasons why they are not interchangeable:
1. Angle of Incidence (AOI) Shift
Standard interference filters are almost always designed and specified for use at a normal angle of incidence (0°). Dichroic mirrors, on the other hand, are typically designed to split beams at a 45° angle.
When you tilt a standard interference filter to 45° to use it as a mirror, the optical path length through the thin-film layers changes. This causes a phenomenon known as "blue shift," where the central wavelength and the transmission band shift toward shorter wavelengths.
Additionally, at a 45° tilt, the filter's performance will split based on the polarization state of the light (s-polarization and p-polarization). This polarization splitting ruins the sharpness of the filter's transition edges and creates a distorted, unpredictable transmission and reflection band.
2. Method of Blocking: Absorption vs. Reflection
Even if you account for the blue shift, you run into the issue of how the filter handles blocked light.
- All-Dielectric Hard-Coated Filters: Modern high-end bandpass filters are entirely dielectric and block out-of-band wavelengths purely by reflecting them. In theory, these will reflect the blocked wavelengths, but you still face the AOI shift issues mentioned above.
- Traditional / Metal-Dielectric Filters: Many standard filters utilize absorptive colored glass or metal-dielectric coatings to assist with blocking out-of-band light (often called "out-of-band blocking"). If the filter relies on absorption to block a specific wavelength, it will absorb that light as heat rather than reflecting it, making it entirely useless as a mirror for that wavelength.
3. Ghosting and Second Surface Reflections
Dichroic mirrors are specifically designed to minimize "ghosting." They usually have an anti-reflective (AR) coating applied to the back surface that is optimized for a 45° angle.
A standard interference filter usually has AR coatings optimized for 0°, or sometimes no AR coating on the back surface at all. If you bounce light off it at 45°, the main coating will reflect the blocked light, but the back surface will also reflect a small percentage of the transmitted light. This creates secondary, slightly offset beams (ghost reflections) that can ruin your optical setup.
Summary
If you need to split or combine beams based on wavelength, you should use a purposely designed dichroic mirror or beamsplitter. They are specifically coated to operate effectively at a 45° angle without splitting polarization excessively, and they guarantee reflection of the blocked bands rather than absorption.
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