Definition:
A neutral density (ND) filter is an optical filter that reduces the intensity of the light across a set range. Essentially, it acts like a pair of sunglasses for your camera or optical instrument, reducing the amount of light entering the lens or sensor across the specified light spectrum.
Common Uses:
- In optical systems filters and detectors can often need their signals balancing, an ND filter in the optical path is a smart way of achieving ‘system balance’
- A known ND filter can be used as component to measure intensity within a system, they can be used at a certain ratio 95R:5T and used to measure the direct beam power by allowing a small measurable ‘leak’ to escape the system and be re-directed to a power meter.
Design and Manufacture:
At Vortex we manufacture custom neutral density filters between 300nm and 6000 nm to any level of transmission. More information can be found on the pages below including case study examples of our previous work.
All of our ND filters are deposited using sputter deposition technology which typically has a process energy 1000 times higher than thermal evaporation. This means that the 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 Neutral Density Filters

Infrared 700–6000nm Neutral Density 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.