Definition:
A narrow bandpass filter is an optical filter designed to transmit a specific narrow range of wavelengths within the electromagnetic spectrum while blocking all other wavelengths.
Design and Manufacture:
At vortex we can manufacture any custom narrow bandpass filter 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 narrow bandpass filters are deposited using sputter deposition technology which typically has a process energy 1000 times higher than thermal evaporation. This means that all our coatings are highly durable in challenging environments as well as very stable with temperature change. More information about this can be found below.
More information about our capabilities can be found through the links below

Visible 300 - 700nm Narrow Band Pass Filters

Near Infrared 700 - 2500nm Narrow Band Pass Filters

Mid Infrared 2500 - 6000 nm Narrow Band Pass Filters
Spluttering Vs Evaporation
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 a guide as to how well the coating is bonded to the substrate. The test involves pressing ‘Scotch’ tape against the coating and then pulling it off, it should stay firmly attached and show no signs of removal.
Humidity Resistance Test:
The humidity resistance test assesses the performance of the coating in an atmosphere of 95% relative humidity for a 24 hour period. There should be no change in optical performance or physical integrity.
Mild Abrasion Test:
The mild abrasion test evaluates the resistance of the coating to surface wear. The filter is subjected to a dry rubbing cloth for 50 strokes. After a clean with acetone, there should be no evidence of marking visible when viewed with the naked eye.
Severe Abrasion Test:
The severe abrasion test is similar to the mild abrasion test but involves more aggressive abrasion conditions to simulate harsher wear scenarios. The coating is subjected to 10 strokes of abrasion with the Mil Spec abrader which is impregnated with grit. This test is particularly important for filters used in rugged environments or applications with high mechanical stress.
Cleanability Test:
The cleanability test ensures the filter can be cleaned with standard laboratory solvents such as propan-2-ol (IPA) and acetone. The coating should survive 10 minutes immersion in each solvent.
Water Solubility Test:
The filter is immersed in water for 24 hours and it should comfortably survive this with no signs of damage or delamination.
Narrow Bandpass Filter Key terms explained.
Centre Wavelength:
This is the centre point of the filter; it is calculated by adding together the two half maximum points together and dividing them by two. See diagram below.
Half Power Point:
This is sometimes referred to as the 50% point. It is the point at which the transmission is half of the peak transmission (If Peak transmission was 80% the half power point would be at 40% transmission).
Bandwidth/Full Width Half Maximum (FWHM):
This is the width of the filter at its middle point. This is calculated by subtracting the lower/first half transmittance wavelength from the second/higher transmittance wavelength. See diagram below.
Peak Transmission:
This is the point where the filter reaches its highest transmittance percentage.
Transmission Band:
This is the wider range at which a high level of transmission is required. Normally a wavelength range where transmission has to be above a certain level of transmission.
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. This should be the detectors sensitivity range plus a small safety margin.
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%.
Cut off Wavelength:
This is the exact opposite of the cut on wavelength. This refers to the wavelength at which a filter stops transmitting 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.
Slope of a filter Right Hand side:
The right-hand slope is calculated as follows, (ʎ 5% of Peak T in nm-ʎ 80% of Peak T in nm)*100/ʎ 80% 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.
Narrow Bandpass Filter Terms Explained Graph
