Variable ND Selection Guide
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ND Filter Type	Round Step	Round Continuous	Cage Compatible	Rectangular Step	Rectangular Continuous	Variable Absorptive ND Filter

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The Variable Absorptive Neutral Density Filter can be mounted on a post via the bottom-located 8-32 tap. Four 4-40 taps on both the front and back of the housing allow the filter to be integrated into a 30 mm cage system.

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Beam Path Through Variable Absorptive ND Filter

Features

• Variable Optical Density Between 0.8 and 3.0
• Ø8.5 mm Maximum Input Beam Size
• 8-32 Tap for Post Mounting
• Compatible with 30 mm Cage Systems
• SM05-Threaded Aperture on Both Sides of Housing
• NG5 (Schott Glass) Substrate
• Ideal for Low-Power Applications

Thorlabs' Variable Absorptive Neutral Density Filter can provide an optical density (OD) between 0.8 and 3.0. The filter consists of two NG5 Schott Glass prisms which can be translated using the adjuster knob on the side of the housing. Varying the position of the prisms with respect to one another varies the amount of the substrate through which the beam travels, thereby changing the effective OD of the filter. See the diagram to the right for an illustration of the beam path. Unlike our Variable Beamsplitter/Attenuators, this filter does not induce polarization effects on the beam.

Rotating the adjustment knob clockwise yields a larger OD; the prism position indicator (located on the front of the housing) moves to the right as the OD increases. Each tick mark on the indicator is equivalent to a 0.2 change in OD.

The filter has a clear aperture of Ø10 mm. However, because the beam will be displaced horizontally by 1.5 mm, the input light beam should be Ø8.5 mm or smaller. When the beam enters through the front of the filter, the beam will be displaced to the left.

The anodized aluminum housing is 4.97" x 1.03" x 1.76", not including the adjustment knob. It can be mounted on a Ø1/2" post using the 8-32 tapped hole on the bottom. To mount the filter on a metric post, use an AS8E4M 8-32 to M4 adapter. The filter can be integrated into a 30 mm cage system with the four 4-40 tapped holes on both the front and back of the housing. The aperture on each side of the housing is SM05 (0.535"-40) threaded for compatibility with our Ø1/2" Lens Tubes.

Optical Density

Optical density (OD) indicates the attenuation factor provided by an optical filter, i.e. how much it reduces the optical power of an incident beam. OD is related to the transmission, T, by the equation

where T is a value between 0 and 1. Choosing an ND filter with a higher optical density will translate to lower transmission and greater absorption of the incident light. For higher transmission and less absorption, a lower optical density would be appropriate. As an example, if a filter with an OD of 2 results in a transmission value of 0.01, this means the filter attenuates the beam to 1% of the incident power. Please note that the transmission data for our neutral density filters is provided in percent (%).

Please note that this product is not designed for use as laser safety equipment. For lab safety, Thorlabs offers an extensive line of safety and blackout products, including beam blocks, that significantly reduce exposure to stray light.