Selection Guide for Neutral Density Filters
Selection Guide for Neutral Density Filters		Absorptive
Uncoated (400 - 650 nm)	Mounted
Uncoated (400 - 650 nm)	Unmounted
Uncoated (800 - 2600 nm)	Mounted
Uncoated (800 - 2600 nm)	Unmounted
AR-Coated (350 - 700 nm)	Mounted
AR-Coated (350 - 700 nm)	Unmounted
AR-Coated (650 - 1050 nm)	Mounted
AR-Coated (650 - 1050 nm)	Unmounted
AR-Coated (1050 - 1650 nm)	Mounted
AR-Coated (1050 - 1650 nm)	Unmounted
Reflective
N-BK7 (350 - 1100 nm)	Mounted
N-BK7 (350 - 1100 nm)	Unmounted
UV Fused Silica (200 - 1200 nm)	Mounted
UV Fused Silica (200 - 1200 nm)	Unmounted
ZnSe (2 - 12 µm)	Mounted
ZnSe (2 - 12 µm)	Unmounted
Variable Reflective
Neutral Density Filter Kits

Features

Available with Ø1/2" (12.7 mm) or Ø25 mm
SM1-Threaded (Ø25 mm) or SM05-Threaded (Ø1/2") Mount Engraved with Part Number and Optical Density
Attenuation in the Visible and NIR Spectral Regions (See Graphs tab)
Optical Densities Ranging from 0.1 to 4.0

Thorlabs' Mounted N-BK7 Reflective (Metallic) Ø1/2" and Ø25 mm Neutral Density (ND) Filters provide uniform attenuation over the broad spectral range from 350 nm to 1100 nm. All Ø25 mm filters are mounted in an SM1-threaded (1.035"-40) cell, making them compatible with components such as Ø1" lens tubes. All Ø1/2" filters are mounted in SM05-threaded (0.535"-40) cell; making them compatible with components such as Ø1/2" lens tubes. Our reflective ND filters have optical densities ranging from 0.1 to 4.0. Refer to the Specs tab above for detailed information about the average transmission obtained with each of our reflective neutral density filters. The Graphs tab contains plots of the transmission and reflection for each filter for a broad wavelength range.

These filters consist of a N-BK7 glass substrate with a metal (Inconel) coating. Inconel is a very hard metallic alloy that ensures flat spectral response from the UV to the mid IR and is resistant to aging under normal conditions; however, it will oxidize at elevated temperatures. To prevent oxidation, Thorlabs recommends using these ND filters at temperatures below 100 °C.

The Ø25 mm mounted N-BK7 reflective ND filters are also offered in filter kits. These kits come in metal box with labeled foam inserts to organize the optics. These boxes with inserts are also separately available for storage of individually-purchased mounted filters. These filters are also available unmounted. Other reflective ND filters in Thorlabs offering include UV fused silica filters and variable ND filters, as well as a complete line of absorptive ND filters in several sizes; the table above contains links to all of the available options.

Please note that Thorlabs Neutral Density filters are not designed 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 the stray or reflected light.

General Specifications
Filter	NDxxA	ND5xxA
Substrate	N-BK7 Glass
Diameter	25.0 +0.0 / -0.5 mm (unmounted)	12.7 +0.0/-0.25mm (unmounted)
Mount	Engraved SM1L03	Engraved SM05L03
Thickness	1.0 ± 0.25 mm (unmounted)
Clear Aperture	90% Outer Diameter
Surface Accuracy	<5λ	<2λ
Surface Quality	40-20 Scratch-Dig
Parallelism	<3 arcmin

Mounted Item #	Optical Density (@ 633 nm)	Nominal Transmission
ND01A ND501A	0.1	79%
ND02A ND502A	0.2	63%
ND03A ND503A	0.3	50%
ND04A ND504A	0.4	40%
ND05A ND505A	0.5	32%
ND06A ND506A	0.6	25%
ND10A ND510A	1.0	10%
ND13A ND513A	1.3	5%
ND20A ND520A	2.0	1.0%
ND30A ND530A	3.0	0.10%
ND40A ND540A	4.0	0.01%

Item #	Damage Thresholds
ND503A ND03A	1000 W/cm² (532 nm, CW, Ø0.051 mm)
ND503A ND03A	0.025 J/cm² (532 nm, 10 ns, 10 Hz, Ø0.493 mm)
ND510A ND10A	1000 W/cm² (532 nm, CW, Ø0.019 mm)
ND510A ND10A	0.025 J/cm² (532 nm, 10 ns, 10 Hz, Ø0.566 mm)
ND520A ND20A	1000 W/cm² (532 nm, CW, Ø0.049 mm)
ND520A ND20A	0.025 J/cm² (532 nm, 10 ns, 10 Hz, Ø0.493 mm)

Optical Density

The optical density, OD, is defined in terms of transmission T by the following equation:
Optical Density Equation
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.

For Detailed Plot Information
	For detailed individual plots for each part, as well as Excel files with plot data, click on this icon in the product tables.
An excel file with transmission and reflection data for all N-BK7 Reflective ND filters is also available.

	Transmission	Reflectivity
OD 0.1-0.6	Click to Enlarge	Click to Enlarge
OD 1.0-2.0	Click to Enlarge	Click to Enlarge
OD 3.0-4.0	Click to Enlarge	Click to Enlarge
OD 3.0-4.0 Visible Wavelengths	Click to Enlarge

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Posted Comments:

Poster: jjurado

Posted Date: 2011-08-18 09:22:00.0

Response from Javier at Thorlabs to meier: We have tested the laser induced damage threshold (LIDT) of these reflective neutral density filters with a pulsed 532nm source, and arrived at a value of 0.025J/cm^2 (10 ns pulse width, 10 Hz rep. rate). However, the LIDT cannot reliably be scaled for a 1 s pulse using this value. In this case, a CW threshold is more appropriate. We specify a CW damage threshold of 0.75 W/cm^2 for these ND filters. With a 0.8W beam and 0.25mm^2 diameter, the resulting power density is 320W/cm^2, which greatly exceeds the CW spec. I will contact you directly for further support.

Poster: meier

Posted Date: 2011-08-17 16:44:47.0

Hi, question about the damage threshold: will the filter be damaged by a "short" laser pulse (532 nm) of about 1 second with a power of approx. 0.8W on an area of 0.25mm^2 ?

Poster: bdada

Posted Date: 2011-07-21 10:32:00.0

Response from Buki at Thorlabs: Please refer to the “Threading Specs” tab in the web page linked below to get the specifications for the SMx series Thorlabs threads. http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=3307&pn=SM1L03#3388

Poster: jan.grawe

Posted Date: 2011-07-21 10:20:47.0

Hello, I need the full ppecs of the SM1 thread in order to design a mating part. Could you sent it to me? Best regards, Jan Grawé

Poster: jjurado

Posted Date: 2011-05-10 09:54:00.0

Response from Javier at Thorlabs to Felice: Thank you for contacting us. We would recommend using our reflective ND filters for use in the IR region. Even though their performance is most uniform in the 350-1100 nm range, they can still be used up to ~2 um. See Graphs tab above for reference. I will contact you directly with more information regarding the reflectance of these filters.

Poster: fg2251

Posted Date: 2011-05-09 23:07:58.0

Hi I need neutral density filter to attenuate fs pulse in IR 1.1-2 micron coming from an OPA (tens of microJ) Should I buy the ND series or the Absorbitive series NE? Also can you provide the transmittance in IR region? up to 2 micron? Thanks Felice

Poster: apalmentieri

Posted Date: 2010-03-01 10:52:52.0

A response from Adam at Thorlabs to lyu: We can make 1/2" diameter reflective filters for you. The price would be depended upon the quantity you are looking for as these would need a reflective inconel coating. I will email you directly to find out your quantity so we can get you an accurate price quote.

Poster: lyu

Posted Date: 2010-03-01 09:46:48.0

can you make diameter 1/2 inch reflective ND2 (BK7) filters for me? If can, how about the price? Thanks in advance. Yu

Poster: Tyler

Posted Date: 2009-01-26 11:36:08.0

A response from Tyler at Thorlabs to shshim: I hope that the custom spacers are working out. If we can help in any other way, please let us know.

Poster: shshim

Posted Date: 2008-11-21 13:00:09.0

Do you have any angled adapter to mount the reflective neutral density filters? Currently, I am using a cage filter wheel to hold 6 reflective ND filters. I fear that the reflected beam bounce back into the laser. The cage filter wheel doesnt allow to tilt the filters. I wish you have a simple angled ring that sits between the filter and the mount. I found your angled mount (SM1L03T), but this cannot be mounted to the filter wheel. I hope you have a good solution for this issue. Thanks!

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Mounted N-BK7 Reflective Neutral Density Filters

Features

Optical Density