Dichroic (Interference) Filters

Dichroic filters or bandpass filters are made of polished optical glass that is coated with multiple layers of dielectric thin films that transmit specific wavelengths of light while reflecting undesired wavelengths at a particular angle of incidence.

Coatings are applied in alternating thin layers of high and low index refraction materials. Light coming into the filter at a specific angle comes in contact with the first index layer and some of the light is reflected and some passes through based on its wavelength which is determined by the index layer. As light travels through these multiple alternating high and low index layers at different speeds, the reflected light either stays in phase (constructive interference) or is reduced by being out of phase (destructive interference) through phase shifts that narrow the final emitted light to a very specific wavelength band. The thickness of the index layers are responsible for the phase shifts and are specifically controlled as well as the number of layers applied to the glass surface to obtain the correct wavelength emitted from the filter.

High and low index layers are often bundled together into stacks (cavities) which are separated by an all oxide layer such as SiO (cavity spacer) which is then followed by a dielectric reflective stack (cavity) that reflects the specific wavelength band produced by the high and low index layers.

Photo credit: Florida State Research Foundation

Unlike absorptive filters which transmit or absorb large wavelength regions, the use of optical coatings described above allow dichroic filters to significantly narrow the wavelength range in which they perform. In fact, dichroic filters with a large number of optical coating stacks (cavities) can narrow the wavelength of light passing through the filter to only a few nanometers.