Neutral Density Filters
A neutral density filter is designed to equally attenuate (reduce) the transmission over a specified waveband or wavelength, either by absorption or reflection.
The optical density (OD) of the filter is controlled by the material, its thickness, and often an additional coating dependent on the type.
There are different types of neutral density (ND) filters that Knight Optical can provide including:
- Absorptive filter glass which attenuates light by absorption. These are robust, and due to the manufacturing process they are not hindered by the angle of incidence. As these work by absorption, care should be taken to avoid overheating. Schott (or equivalent) filter glass is used and polished to the thickness required for the desired OD value at a specified wavelength or wavelength range.
- Metallic ND filters are manufactured by depositing a thin metallic layer on a standard glass material such as borosilicate or UV fused silica. These ND filter types attenuate via reflection, avoiding the possible overheating issue that is possible with filter glass and Wratten ND filters.
- Wratten ND filters offer a low cost alternative to the above ND filters. Like ND filter glass they attenuate light via absorption but they tend to have more fluctuations with wavelength and a narrower working wavelength range.
Knight Optical have a variety of ND filters as part of our stock catalogue ready-to-order.
Our ND filter glass comes in a variety of different dimensions, including 12.5mm and 25mm diameter diameters and 50x50mm squares. These can be ground down to smaller dimensions when requested.
Our Wratten Filters are Wratten 2 type and are available as 50x50mm squares on our website. We also have 300x100mm sheets, which can be cut down to custom sizes for a quick turnaround.
Custom orders for neutral density filters are always welcomed, and is something that Knight Optical excel at. Contact us today and we will work with your on your requirements.
Currently Knight Optical offer fixed ND filters, we do not have variable or graduated ND filters available.
All filters are tested within our metrology laboratory using our state-of-the-art equipment, this includes our Agilent Cary Spectrophotometers which can test the transmission and absorption of these filters. We can provide test data when requested, including .CSV and ASCII data for the spectral properties.
- Photography to reduce the amount of light entering the camera. This is especially useful for long exposure landscape shots.
- Laser beam attenuator to reduce the intensity of the laser before it reaches a detector.