Longpass Optical Filter and Shortpass Optical Filter Technical Information

Longpass and Shortpass Optical Filter Basics

For longpass filters and shortpass filters there are really only four basic parameters that need to be defined they are:

  • Cut-On/Cut-Off Edge—the wavelength of the edge at 50% of the maximum transmission of the filter.
  • Edge Steepness—the slope of the edge between reflection and transmission.
  • Reflection Range—the spectral region in which the filter does not transmit.
  • Physical size and shape.

Longpass and Shortpass Filter Types

A number of different filter designs are available which tend to reflect the sharpness or steepness of the cut-on curve and or the level of transmission and blocking required. Here is a brief description of the available designs;Cut-On/Cut-Off (CF) Typical of the industry standard, providing peak transmission of 85% to 90%, with attenuation typically 99.9% or 10-3. They have a relatively shallow slope between the reflection and transmission regions, defined as a 1-decade 3–5% slope between 85% and 5%T.

ALPHA technology produces slope factors 10–50 times steeper than industry standards, with a 5-decade slope factor defined as a measurement of the distance between 50% and 0.001% transmission (or OD 5). Longpass and shortpass filters are available with edge steepness to the following values: 1% Epsilon, 3% Gamma, and 5% Beta.

3RD Millennium (3RDLP and 3RDSP)
Coatings are ALPHA technology designs on fused silica substrates, resulting in specifications characteristic of ALPHA LP and SP filters, improved filter performance, and surface coating advantages. Manufactured in an automated coating chamber resulting in high volume product precision and high velocity.

Colored Absorption Glass
Manufactured using scientific glasses available from Schott, Hoya, Corning, and others. Filters are configured and polished, and are available with durable anti-reflective surface coatings.
We often use absorption glass filters to extend the attenuation range of our various filters. The best filter solutions frequently combine interference coatings laminated to any appropriate longpass or shortpass color absorption glass.
See Figure 1 for a comparison of longpass filter types.

Industry Cut-On/Cut-Off Steepness Factor

ALPHA Cut-On/Cut-Off Steepness Factor

The lowest cut-on steepness factors indicate the steepest cut-on edge.

Figure 1: Spectral Curves of Longpass Filters - A comparison of theoretical longpass filters including CF, absorption glass, and ALPHA designs at normal incidence (0°). They all have a similar cut-on wavelength at approximately 600nm.

Standard Specifications: Longpass and Shortpass Filters

The following specifications apply to all Omega Optical longpass and shortpass barrier filters unless other custom specifications are requeste

Cut-on Wavelength Tolerance:


±10nm at 50% peak transmission


±3nm at 50% peak transmission

Average Transmission


Cut-on Wavelength Temperature Dependence

0.016nm/°C to 0.026nm/°C

Construction Interference Filters

Either (a) exposed, front-surface hard coating on a single-element substrate, or (b) protected soft dielectric coating in a laminated moisture-resistant assembly

Temperature of Measured Performance


Operating Temperature Range

-60°C to +80°C

Humidity Resistance

Per Mil-STD-810E, Method 507.3 Procedure I

Coating Substrates

Optical quality glass

Surface Quality

80/50 scratch/dig per Mil-O-13830A

Outer Dimension Tolerance

+0, -0.25mm (+0, -0.01”)

Minimum Clear Aperture

4mm less than nominal outside dimension



   Unblocked Filters

Unblocked filters, 1.5mm +0, -0.5

   Longpass Filters

Blocked, 6mm maximum

   Shortpass Filters

Blocked, 10mm maximum

Documentation. Spectrophotometric trace of the attenuation, cut-on, and transmission regions with a resolution of 0.1nm provided.

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