Dichroic filters, also known as thin-film interference filters, play a vital role in optical systems that require precise wavelength separation, beam combining, or spectral control. They are commonly used in fluorescence microscopy, machine vision, medical imaging, projection systems, and laser applications. Choosing the right dichroic filter is essential to ensure optimal system performance, high transmission efficiency, and minimal signal loss.
Key Factors to Consider When Selecting a Dichroic Filter
1. Wavelength Range and Spectral Performance
Determine the operating wavelength range:
Passband (Transmission Region): The wavelengths you want to transmit.
Stopband (Reflection Region): The wavelengths you want to reflect or block.
Transition Edge Sharpness: Defines how abruptly the filter switches between transmission and reflection—important for multi-channel fluorescence systems.
2. Angle of Incidence (AOI)
Dichroic filters are angle-sensitive. Their spectral performance shifts with the angle of incident light:
Typical AOI: 45° for beam-splitting applications
Normal Incidence (0°): Used for inline spectral separation
Custom AOI: May be required for unusual optical paths
3. Transmission and Reflection Efficiency
Look for:
High transmission (>90%) in the passband
High reflection (>95%) in the stopband
Low optical loss to maintain system brightness and SNR (signal-to-noise ratio)
4. Laser Damage Threshold (LIDT)
For laser-based systems, ensure the dichroic filter can handle high power.
Look for high LIDT coatings
Specify whether you use CW or pulsed lasers
5. Coating Type
Hard coatings: Best durability, long lifetime, high laser resistance
Soft coatings: More economical, but less stable over time
6. Optical Quality
Check:
Surface flatness (e.g. λ/10)
Surface quality (e.g. 40-20 scratch-dig)
Wavefront distortion