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Thin Film Glossary

Source: Alluxa, Inc.
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The following defines a number of glossary terms related to thin film optical filters and their use.

Angle of Incidence (AOI) 

The angle at which a filter is tilted relative to the primary ray. For example, an AOI of 0° means the primary ray is perpendicular to the plane of the filter.

Bandpass Filter 

A filter that transmits a band of wavelengths while blocking the wavelengths on either side of the passband.


The level of rejection of light outside the passband.

Borofloat® (Borosilicate Glass)

A high temperature substrate material with high chemical resistivity against water, neutral, acidic and saline solutions.

Center Wavelength (CWL) 

The center of the passband for a bandpass filter. Another common definition is the average of the 50% of peak of each edge of a bandpass filter.

Cone Angle 

The range of angles of light incident on the filter about the AOI.

Cone Angle Average 

Average of transmission or reflection across the cone angle


A filter that transmits one band and reflects a different band

Edge Filter

A filter that transmits a band of wavelengths and blocks only low side (long wave pass) or high side (shortwave pass) of wavelengths.

Fused Silica

Substrate material composed of amorphous silicon dioxide.

Long Wave Pass (LWP) 

An edge-type filter that transmits longer wavelengths and blocks shorter wavelengths.

Notch Filter 

A filter that blocks a narrow range while transmitting wavelengths on either side of the blocked wavelength range.

Optical Density (OD) 

The negative logarithm (base 10) of transmittance (T). (T on a zero to unity scale)


The range of wavelengths that are desired to be transmitted.


The orientation tolerance allowed between a feature of interest and a reference datum. For plano-plano optical filters, the term describes how parallel the two main surfaces of the optic are to one another.


Variations in transmission in the passband.

Shortwave Pass (SWP) 

An edge-type filter that transmits the shorter wavelengths and blocks higher wavelengths.

Spectral Slope 

Rate of change of the spectrum from a passband to a blocking band.

Transmitted Wavefront Error (TWE) 

Distortion of the wavefront of a plane wave as it transmits through a filter, generally given in units of the HeNe wavelength of 633 nm.