Definition:

Long pass filters are optical filters designed to transmit longer wavelengths while blocking out unwanted shorter wavelengths. Long pass filters can be positioned anywhere in the light spectrum with very precise cut on wavelengths to suit any application.

Design and Manufacture:

At Vortex we can manufacture any custom long pass filters between 300nm to 6000 nm. Click on the relevant wavelength range below for more information about our capabilities, applications and case study examples of our previous work.

All of our long pass filters are deposited using sputter deposition technology which typically has a process energy 1000x higher than thermal evaporation. This means that all our coatings are highly durable in challenging environments as well as thermally stable. More information about this can be found below.

More information about our capabilities can be found through the links below

Visible 300 - 700nm Long Pass Filters

Near Infrared 700 - 2500nm Long Pass Filters

Mid Infrared 2500 - 6000 nm Long Pass Filters

Spluttering Vs Evaporation

Historically Evaporation has been the go-to technology for the deposition of optical coatings, however Vortex use Sputter deposition. The difference in film structure is explained in the two diagrams below. The sputtering process means that the particles arrive at the substrate with substantially more surface energy (1000x greater than evaporated molecules), they are able to move around and adopt a very densely packed structure. Evaporated particles form a more open structure, which on exposure to air will absorb moisture from the atmosphere, this means such coating will shift considerably more with temperature variation and are generally less durable.

Traditional Thermal Evaporation

  • Voids in coating.
  • Poor environmental performance.
  • High shift with temperature change.

Open structure of traditionally evaporated coatings with gaps and voids.

Sputter Deposition

  • Voids eliminated.
  • Excellent environmental performance.
  • Extremely low temperature shift.

Densely packed sputtered coating with no gaps
or voids.

Coating Durability

We carry out the following MIL-C-48497A tests on samples from every coating run to ensure all coatings are durable and will stand the test of time.

Adhesion Test:

The adhesion test is an indicator of how well adhered the coating is to the substrate. The test involves pressing 3M scotch tape against the coating and then pulling it off, there should be no coating removal.

Humidity Resistance Test:

The humidity test assesses the durability of the coating when subjected to 24 hours humidity. The filter is exposed to 95% relative humidity, there should be no delamination or damage to the coating or change in optical performance. Sputtering deposition is key to ensuring high levels of durability and physical integrity in humid environments.

Mild Abrasion Test:

The mild abrasion test evaluates the resistance of the coating to surface wear. The filter is subjected to the 50 cycles of dry rubbing with a loaded cheesecloth abrader tool. After cleaning there should be no change in appearance or coating performance. This test helps determine the filter’s ability to withstand everyday handling.

Severe Abrasion Test:

The severe abrasion test is similar to the mild abrasion test but involves 10 cycles of rubbing with a more aggressive abrader, again after cleaning the coating should show no damage or change in performance. This test is particularly important for filters used in tough environments or applications with high mechanical stress.

Cleanability Test:

The cleanability test assesses the ease of cleaning the filter surface with a range of lab standard cleaning agents such as acetone and isopropanol. In assembly filters and coatings will require cleaning, the test helps ensure the filter is comfortable with this type of handling.

Water Solubility Test:

Surviving 24 hours in water without deterioration ensures the coating is also suitable for cleaning with water-based cleaning agents without compromising performance.

Long Pass Filter Key terms explained.

Peak or Minimum Transmission:

This is the point or range where the filter must have transmission above a specified level.

Transmission Band:

The range at which a specified level of minimum transmission performance is required. This can be a short as one single wavelength or as wide as 1000’s of nanometres.

Passband Average Transmission:

This can be defined as the average transmission, in the passband, between the two points where transmission exceeds 80% of the peak value.

Blocking Range:

This is where the filter needs to have a low transmittance percentage to prevent the detector from picking up any signals from stray light.

Cut on Wavelength:

This refers to the wavelength at which a filter starts to transmit light. This can be specified at any percentage but is often between 1 and 10%. The diagram below shows it at 5%.

Slope of a filter Left Hand side:

The left-hand slope is calculated as follows, (ʎ 80% of Peak T in nm-ʎ 5% of Peak T in nm)*100/ʎ 5% of Peak T in nm, see the diagram below.

Ripple:

This normally refers to the allowed variation between maximum and minimum in the pass band of a filter. For example, ripple variation to be< +/-5% form the passband average.

Long Pass Filter Terms Explained Graph