These filters allow convenient adjustment of beam attenuation by translating the filter using a linear stage. Optical density varies linearly with position from OD 0.08-2.

To prevent unwanted interference from etalon effects, these filters have a 15 arc min wedge. To minimize back reflections, the filter should be oriented with the coated metallic surface facing the incident beam. Any light reflected from the uncoated second surface will pass through the metallic coating first, which will greatly attenuate unwanted back reflections.

These filters consist of a thin metallic coating of variable thickness. Because attenuation is primarily caused by reflection with only a small amount of absorption, these filters may optionally be used as variable beamsplitters. Variable beamsplitters are useful for applications such as holographic nondestructive examination (NDE). In holographic NDE, a beam from a laser is split into two beams. One beam is expanded using a spatial filter to create a uniform reference beam. The other is used to illuminate the object being tested. A signal beam is formed from the light reflected from or transmitted through this object. The two beams may then be recombined to form detectable interference fringes. Intensity of the signal will vary greatly as a function the reflectance (or transmittance) of the specimen. This can be compensated using a variable beam split ratio.

Two filters can be used such that when stacked, the gradient of the metallic coatings will create an more even distribution. When you have a larger beam diameter, it is typically preferable to have the beam evenly attenuated.