- Usable over wide wavelength range
- Reflective or absorptive filters
- Combination of multiple filters possible
Neutral density filters are used to attenuate the intensities of laser beams and other light sources down to a level appropriate for a CCD. Very often they are employed as a last level of attenuation. Neutral density filters characteristically exhibit homogeneous attenuation levels over very wide wavelength ranges. Because of this they are usable for many different laser wavelengths.
Absorptive filters for the visible and NIR ranges attenuate the light by absorbing part of it — they become warm.
Reflective filters for the UV-wavelength range attenuate the light by reflecting the unwanted part of the light intensity.
|Optical Density||Transmission||Attenuation Factor|
|1,0||10 %||10 x|
|2,0||1 %||100 x|
|3,0||0,1 %||1.000 x|
|4,0||0,001 %||10.000 x|
|5,0||0,0001 %||100.000 x|
|6,0||0,00001 %||1.000.000 x|
The attenuation level of filters is generally given as optical density - OD.
Transmission - T = 10^-OD
Attenuation level = 1/T = 10^OD
Optical density: OD = -log T
Aside from the attenuation levels shown above we can also provide finer subdivisions of the optical density levels.