Neutral Density Filters
Neutral density (ND) filters are used to attenuate incident radiation without altering its spectral distribution. ND filters are available in two types:
Color Glass Filters
Neutral Density Color Glass Filters - Made from Schott NG Filter glass
- These filters can be AR coated to prevent back reflections.
- They can handle higher input energy, especially if actively cooled.
- Very accurate ND values are possible by controlling the thickness of the color glass.
Metallic Coated Filters
BK7 Optical Glass - For visible to the NIR
Fused Silica - UV substrates allows for an even attenuation from 200-700nm
Some advantages of metallic coated ND filters include:
- Coating on quartz allows controlled attenuation in the UV.
- Coating provides attenuation with greater linearity over a wide spectral range.
- Can handle higher intensity light sources because part of the beam is reflected rather than absorbed.
The film is comprised of a very hard alloy material that is resistant to aging under normal conditions. Use at high temperatures can cause oxidation however, and is not recommended. Any commercial cleaning solution can be used to clean these filters.
NOTE: Use of any of these neutral density filters with high power lasers or other extreme light sources can cause catastrophic failure and is not recommended.