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

Features

Circular, Continuously Variable, Reflective Neutral Density Filter
Optical Density Range: 0 - 2.0 or 0 - 4.0
Ø25 mm, Ø50 mm, and Ø100 mm Sizes
Three Back-Side Coating Options:

Uncoated (240 - 1200 nm)
AR Coated for the 350 - 700 nm Range
AR Coated for the 650 - 1050 nm Range

Mounted and Unmounted Versions

Thorlabs' Round Continuously Variable Metallic (i.e., reflective) Neutral Density Filters provide linear, adjustable attenuation via rotation. They are composed of a UV fused silica glass substrate and a metallic Inconel coating through a full 270°, ensuring a flat spectral response from the ultraviolet to the mid-infrared. These filters have optical densities ranging from either 0 to 2.0 or 0 to 4.0. For the variation of optical density with angle, please refer to the Specs tab.

The mounted versions of our continuously variable neutral density filters come with angular graduations and are mounted on a rotating axle. The mount has been carefully designed to minimize the effect on the clear aperture of the optic. Each mounted filter includes adapters for both 8-32 and M4 mounting holes, making the mounted filter compatible with Thorlabs' Ø1/2" TR Series Posts and accessories.

The optical density, OD, is defined by the equation:

Optical Density Equation

Hence, a higher OD corresponds to lower transmission and greater reflection of the incident light, while a lower OD corresponds to higher transmission and less reflection. For example, an OD = 2 filter will attenuate the transmitted beam to 1% of the incident intensity.

The Inconel coating on these filters is a very hard metallic alloy that is resistant to aging under normal conditions, but will oxidize at elevated temperatures. It is therefore recommended that the neutral density filters be used at temperatures below 100° C.

These filters are not intended to serve as laser safety equipment. For lab safety, Thorlabs offers an extensive line of safety and blackout products that significantly reduces exposure to stray or reflected light.

General Specifications
Substrate Material	UV Fused Silica
Diameter Tolerance (A & B)	Outer Diameter: +0.00/-0.25 mm Inner Diameter:+0.25/- 0.00 mm
Thickness Tolerance (C & D)	±0.25 mm
Coating Radial Angle (E)	±1^o
Spectral Range	240 - 1200 nm
Front ND Coating	Inconel (NiCrFe)
Irregularity	<λ (per cm²)
Surface Quality	60-40 Scratch-Dig
Wedge	<3 arcmin
Optical Density Tolerance	±5% (At Both Extremes)
Linearity of Density	±5%
Chamfer	0.2 mm x 0.2 mm
R_avg (AR-Coated Optics)	<0.5%

Item Specifications

Unmounted Round Continuously Variable Metallic ND Filters
Item #	Back Coating	Optical Density^a	A (mm)	B (mm)	C (mm)	D (mm)	E	F (mm)
NDC-25C-2	Uncoated	0 - 2.0	25	7.5	8	23	270°	2
NDC-25C-4	Uncoated	0 - 4.0	25	7.5	8	23	270°	2
NDC-50C-2	Uncoated	0 - 2.0	50	7.5	8	48	270°	2
NDC-50C-2-A	A Coated (350 - 700 nm)
NDC-50C-2-B	B Coated (650 - 1050 nm)
NDC-50C-4	Uncoated	0 - 4.0	50	7.5	8	48	270°	2
NDC-50C-4-A	A Coated (350 - 700 nm)
NDC-50C-4-B	B Coated (650 - 1050 nm)
NDC-100C-2	Uncoated	0 - 2.0	100	7.5	8	98	270°	2
NDC-100C-4	Uncoated	0 - 4.0	100	7.5	8	98	270°	2

Mounted, Round Continuously Variable Metallic ND Filters
Item #	Back Coating	Optical Density^a	Outer Diameter	A (mm)	B (mm)	C (mm)	D (mm)	E	F^b (mm)
NDC-25C-2M	Uncoated	0 - 2.0	1.25"	25	7.5	8	23	270°	2
NDC-25C-4M	Uncoated	0 - 4.0	1.25"	25	7.5	8	23	270°	2
NDC-50C-2M	Uncoated	0 - 2.0	2.5"	50	7.5	8	48	270°	2
NDC-50C-2M-A	A Coated (350 - 700 nm)
NDC-50C-2M-B	B Coated (650 - 1050 nm)
NDC-50C-4M	Uncoated	0 - 4.0	2.5"	50	7.5	8	48	270°	2
NDC-50C-4M-A	A Coated (350 - 700 nm)
NDC-50C-4M-B	B Coated (650 - 1050 nm)
NDC-100C-2M	Uncoated	0 - 2.0	4.5"	100	7.5	8	98	270°	2
NDC-100C-4M	Uncoated	0 - 4.0	4.5"	100	7.5	8	98	270°	2

^a The optical density is specified at 633 nm.
^b Unmounted thickness.

Optical Density as a Function of Angle

The optical density is a linear function of the angle:

OD = mθ

where OD is the optical density and θ is the angle in degrees. The value of m depends upon the optical density range of the particular ND filter:

For filters with a OD range of 0 - 2.0, m = 0.00741.
For filters with a OD range of 0 - 4.0, m = 0.0148.

This formula assumes 0° at the point where the coating begins and 270º at the end of the coating (i.e., the point of highest optical density).

These curves illustrate the angular dependence of the AR coating that is on the back side of some of our ND filters. Please see the product groupings below for more information on the coated ND filters.

Click to Enlarge

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Posted Comments:

Poster: tcohen

Posted Date: 2012-12-06 16:23:00.0

Response from Tim at Thorlabs to Ian: Thank you for contacting us. It is possible that you are seeing the temperature dependence of the filter. As your source heats the optic it is increasing the OD until an equilibrium is reached. We will gather some data on this and I will contact you to keep you updated.

Poster: tcohen

Posted Date: 2012-12-06 16:14:13.087

Response from Tim at Thorlabs to Chek: Thank you for your feedback. Data on the 2.0 Inconel coating that we generated on N-BK7 has showed a flat OD out to your wavelength. I will contact you with this data.

Poster: ian.block

Posted Date: 2012-11-24 07:28:11.45

We have noticed that this component has an initial time-dependent response. With a 300mW ~1mm diam. 660nm incident beam, the transmission drops in an exponential (vs time) fashion by nearly 50% over ~60s after which it stabilizes. With a 20mW ~1mm diam. 633 incident beam, the behavior is significantly less pronounced but nonetheless observed. The behavior is repeatable (and thus there seems to be no damage to the material) but this is extremely inconvenient for certain applications. Is this normal?

Poster: chek.cheung

Posted Date: 2012-11-13 22:35:57.567

What is the optical density range in the wavelength from 1500nm-2200nm, if i choose the OD=2.0 in stock?

Poster: jlow

Posted Date: 2012-10-04 09:40:00.0

Response from Jeremy at Thorlabs: The NDC filters are reflective ND filters. For OD of 1.0, the damage threshold is 1000 W/cm2 (532 nm, CW, Ø0.019 mm beam spot), and 0.025 J/cm2 (532 nm, 10 ns, 10 Hz, Ø0.566 mm beam spot).

Poster: rpeterson2205

Posted Date: 2012-10-03 15:17:37.0

I need to know damage thresholds when comparing ND products.

Poster: bdada

Posted Date: 2011-09-12 16:19:00.0

Response from Buki at Thorlabs: These continuous neutral density filters do not have a high damage threshold. A reflective neutral density filter was tested and was damaged at the following power: 0.05J/cm^2, at 532nm, 10ns pulses, and 10Hz rep rate. These are most suited for low power applications. Please contact TechSupport@thorlabs.com if you have additional questions.

Poster: kchow

Posted Date: 2011-09-09 13:49:20.0

I would like to get some infomation on damage threshold for these filters please. kam

Poster: cgruenewald

Posted Date: 2011-09-05 18:18:39.0

What is the maximum power which can be irradiated onto these filters? I have a 300mW 532nm laserspot with 1 mm diameter (cw). Will it destroy the filter?

Poster: jjurado

Posted Date: 2011-08-16 09:54:00.0

Response from Javier at Thorlabs to onjoya: Thank you very much for contacting us. We will send you information regarding the transmission of these continuously variable ND filters with respect to angle shortly.

Poster: onjoya

Posted Date: 2011-08-12 16:29:29.0

Im new to our lab and cant find the manual for our ND filter. Can anyone tell me how to convert the angle (number on the scale) to transmitted intensity?

Poster: jjurado

Posted Date: 2011-06-23 19:09:00.0

Response from Javier at Thorlabs to kristinwustholz: Thank you very much for your feedback. The beam shape is most likely being affected by dirt in the optic. The cleaning procedure for this ND filter is very similar to that of a regular lens. There are a few methods that you can use to clean the optic, such as washing it with distilled water and optical quality soap, using the drop and drag method using a fast drying solvent, or cleaning with an applicator. For more info, you can follow our optics cleaning tutorial: http://www.thorlabs.com/tutorials.cfm?tabID=26066 I will contact you directly for further assistance.

Poster: kristinwustholz

Posted Date: 2011-06-21 10:17:21.0

Weve found that rotating the ND filter produces changes in the beam shape of our laser at the sample. Therefore, we cannot adjust the ND filter once an experiment has started. Otherwise, the beam shape and alignment is perturbed. Any suggestions? There also appears to be some smudges on the surface of the optic.. what is the recommended cleaning procedure for the NDC-50C-4? Customer Email: kristinwustholz@gmail.com This customer would like to be contacted.

Poster: apalmentieri

Posted Date: 2009-10-28 16:33:35.0

A response from Adam at Thorlabs: Paul, the 1200nm maximum range is due to the coating not the fused silica. They can be used outside of the 1200nm range, but we cannot provide any guarantee on the specifications. If you would like, we can offer these with the back side AR coated. I will email you to get more information.

Poster: paul

Posted Date: 2009-10-28 15:14:36.0

The spec show wavelength range of 380 - 1200 nm. Is 1200 nm the max wavelength due to coating or fused silica? We need out to 1320 nm. Can these be made of another glass than fused silica as it has an absorption dip there. Can these be made of BK7. Can the back side be AR coated to reduce transmission variations due to interference when used with a laser.

Poster: klee

Posted Date: 2009-06-23 11:30:47.0

A response from Ken at Thorlabs to carddb: The PRM1Z8 (replacment for PRM1-Z7) can hold Ø1" optics so you can simply drop in a unmounted Ø25 mm NDC filter and lock it in place with a retaining ring (included with PRM1Z8). Our optics department is also investigating the possibility of having a motorized version of these filters. I will forward your suggestion to them.

Poster: carddb

Posted Date: 2009-06-23 10:43:49.0

At diffent times there has been a need for a motorized rotational stage for the round variable filter. It might be as simple as providing an adaptor to the PRM1-Z7 stage. Let me know if this is possible. Thanks Darrell Card

Poster: Laurie

Posted Date: 2008-11-12 17:12:21.0

Response from Laurie at Thorlabs to dale.otten: Thank you for your feedback. In this particular case, these items were purposely designed so that the coating only covers 3/4 of a circle (the design can be seen fairly well on the pictoral view shown under the specs tab). The reason is that some applications may benefit from being able to position the filter such that the beam doesnt experience any attenuation (other than what will naturally occur due to the ~4% surface reflections.

Poster: dale.otten

Posted Date: 2008-11-12 16:59:05.0

http://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=1393 I am wondering what this means in the overview by the Ind. coating is "coated through a full 270`". Why not a full 360`? Is this a typo, or are these ND filters only coated 3/4 of the way around?

Poster: acable

Posted Date: 2007-12-22 12:32:53.0

The price box descriptions need rewriting so the differences in each model show instead of all the characturistics that are the same. Perhaps just replacing Optical Density with OD would be enough.

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Round Continuously Variable Metallic Neutral Density Filters

Features

Item Specifications

Optical Density as a Function of Angle