Shortpass Dichroic Mirror

September 20, 2026|Team Syronoptics

A Shortpass Dichroic Mirror (also known as a Shortpass Dichroic Beamsplitter) is a specialized optical component designed to transmit light with wavelengths shorter than a specific cut-off point while reflecting light with wavelengths longer than that point.

Unlike absorptive filters (which block unwanted light by turning it into heat), dichroic mirrors use thin-film interference to separate light, making them highly efficient and suitable for high-power applications. They are typically designed to operate at a 45° Angle of Incidence (AOI).

Mechanism of Action

Shortpass dichroics function based on optical interference. The glass substrate is coated with multiple layers of dielectric materials with alternating refractive indices.

Transmission (Pass Band): For wavelengths shorter than the cut-off, the layers create destructive interference in the reflection direction, allowing the light to pass through the substrate.
Reflection (Stop Band): For wavelengths longer than the cut-off, the layers create constructive interference, causing the light to reflect off the surface (usually at 90° relative to the incident beam).

Key Specifications

When selecting or specifying a shortpass dichroic mirror, the following terms are critical:

Cut-off Wavelength (λ_cut-off): The specific wavelength where the component transitions from transmitting to reflecting (defined as the 50% transmission point).
Transmission Band: The spectral range where transmission is high (typically T_avg > 85% or 90%). This is the "short" wavelength region.
Reflection Band: The spectral range where reflection is high (typically R_avg > 95% or 98%). This is the "long" wavelength region.
Edge Steepness (Transition Width): The spectral distance (in nm) required to transition from the pass band to the stop band. High-performance dichroic feature steep edges (e.g., <5nm) for precise color separation.
Angle of Incidence (AOI): The angle at which the mirror is designed to be used. Most dichroics are optimized for 45°. Using them at different angles will shift the cut-off wavelength.

Common Applications

1. Fluorescence Microscopy

Shortpass dichroics are essential for separating excitation light from emission light.

Setup: A shortpass dichroic reflects long-wavelength excitation light toward the sample and transmits short-wavelength fluorescence emitted by the sample to the detector. (Note: This is less common than Longpass setups but used for specific fluorophores).

2. Laser Beam Combining

Used to combine two different laser beams into a single path.

Example: Combining a Green Laser (532 nm) and a Red Laser (640 nm).

Mirror: 600nm Shortpass.
Action: The Green beam (shorter) passes through; the Red beam (longer) reflects off the front. The result is two beams traveling on the same axis.