Dichroic Mirrors

Dichroic mirrors, also known as dichroic beamsplitters, are essential optical components in the optoelectronics industry. Their defining characteristic is the ability to exhibit significantly different reflection and transmission properties at different wavelengths of light. This unique capability makes them indispensable for applications such as optical filtering and beam splitting.
The functionality of a dichroic mirror is based on the principles of thin-film interference and reflection. These mirrors consist of multiple layers of dielectric coatings with precisely controlled thickness and refractive index. When light strikes the mirror, the coatings cause constructive interference for specific wavelengths, reflecting them, while other wavelengths experience destructive interference and are transmitted through the substrate. This allows for highly efficient separation or combination of light based on its color or wavelength.
The performance of a dichroic mirror is defined by several key parameters:
Reflectance (R) & Transmittance (T): These metrics indicate the efficiency of the mirror in reflecting or transmitting light within its designated wavelength bands. High-performance mirrors typically achieve values greater than 95%.
Cut-off Wavelength: This is the specific wavelength that marks the transition between the reflection and transmission bands.
Angle of Incidence (AOI): Dichroic mirrors are designed to operate optimally at a specific angle, most commonly 45°.
Dichroic mirrors are primarily categorized into two types:
Longpass Mirrors: Transmit longer wavelengths while reflecting shorter ones.
Shortpass Mirrors: Transmit shorter wavelengths while reflecting longer ones.
These components are widely used in fluorescence microscopy to separate excitation and emission light, in laser systems for beam combining, and in projection displays to manage RGB color channels.