Optical Filters

Bandpass Filters

How to read the curve: Look for a transmission peak bounded by blocking regions at shorter and longer wavelengths.

Key Terms

Center Wavelength

The wavelength at the center of the passband, commonly abbreviated as CWL.

FWHM

The width of the passband measured at 50% of its maximum transmission.

Peak Transmission

The maximum percentage of incident light transmitted by the filter.

Related type: Multi-bandpass filters provide two or more transmission bands in one filter.

Notch Filters

How to read the curve: Look for a narrow drop in transmission surrounded by transmitting regions.

Key Terms

Stopband

The wavelength range blocked by the filter, often centered on a laser wavelength.

Blocking Method

Light may be blocked through absorption or reflection.

Other Names

Notch filters may also be called band-stop or band-reject filters.

Longpass Filters

How to read the curve: Transmission is low on the short-wavelength side and rises as the curve crosses the cut-on wavelength.

Key Terms

Cut-On Wavelength

The wavelength that defines the transition from blocking to transmission.

Edge Steepness

The width of the transition between the blocking and transmitting regions.

Edge Measurement

Edmund Optics measures the transition from 10% to 80% transmission.

Example: A 1% edge slope at 500 nm corresponds to a 5 nm transition width.

Shortpass Filters

How to read the curve: Transmission is high on the short-wavelength side and falls as the curve crosses the cut-off wavelength.

Key Terms

Cut-Off Wavelength

The wavelength that defines the transition from transmission to blocking.

Edge Steepness

The width of the transition between the transmitting and blocking regions.

Edge Measurement

Edmund Optics measures the transition from 10% to 80% transmission.

Example: A 1% edge slope at 500 nm corresponds to a 5 nm transition width.

Dichroic Filters

How to read the curve: One spectral region is transmitted while the complementary region is reflected.

Key Terms

Angle of Incidence

Dichroic filters are frequently designed for a 45° angle of incidence.

Spectral Behavior

Designs may provide longpass or shortpass transmission.

Common Application

Separating excitation and emission wavelengths in fluorescence imaging systems.

Also called: Dichroic mirrors or dichroic beamsplitters.

Neutral Density Filters

Reduce light transmission by a specified amount across a wavelength range.

Key Terms

Optical Density

A logarithmic measure of attenuation, commonly abbreviated as OD.

Transmission

Each whole-number increase in OD reduces transmission by a factor of 10.

Filter Types

Absorptive filters absorb light within the glass, while reflective filters use a coating to reflect light.

Example: OD 1 transmits 10% of incident light, while OD 2 transmits 1%.