Neutral density filters are very useful in a number of applications such as attenuators for broadband spectral sources, partial reflectors for light balancing applications and for use as neutral beamsplitters. Metallic type neutral density filters obtain their optical density by depositing a metal alloy coating onto a specific type of substrate which is determined by the wavelength region of interest. For the Andover standard metallic neutral density filters, the substrate is either optical grade glass or fused silica. Custom substrates are available and will be quoted upon request. Please contact one of our technical sales representatives for assistance. Unlike the all-dielectric or absorption type filters, the metallic type rely on a combination of absorption, reflection and transmission to obtain their proper optical density. Since these films are partially reflecting and absorbing, they will withstand more incident energy than the totally absorbing type; however, their maximum power density is still limited by the fact that they are partially absorbing. If one intends to use these filters as attenuators in laser systems, please check with a technical sales representative for the recommended maximum power density that these filters will withstand.

Optical density (D) is defined as the logarithm to the base 10 of the reciprocal of the transmitted radiant power (T):

If the user requires a specific density that is not an Andover stock item, they may take advantage of the additive effect of these filters and use several filters in series. One must exercise caution in doing this because multiple reflections will occur and cause a reduction in the density value. To avoid this phenomenon, it is recommended that the filters be tilted slightly so that their films are not parallel to each other.

General Specifications
Thickness:	1.5mm
Dimensional Tolerances:	±0.5mm
Clear Aperture:	90% of outside diameter
Surface Quality:	60/40 Per MIL-0-13830B
Coating Quality:	40/20 Per MIL-0-13830B
Coating Adherence:	Per MIL-M-13508C
Humidity:	Per MIL-STD-810C
Max. operating temperature:	+100°C
Mechanical:	Not mounted in metal holder
Substrate Material:	Fused silica for the spectral region from 250nm to 2000nm or glass for the spectral region from 350nm to 2000nm
Optical Quality:	Flatness of 3 to 5 waves per inch and parallelism of 3 arc minutes or better

 

Metallic Neutral Density Transmittance Specifications
		Transmission Deviation From Normal
Optical Density (OD)	Nominal Transmittance (%)	250nm to 350nm	350nm to 700nm	700nm to 800nm	800nm to 2000nm
0.10	79.49	± 8%	± 2%	± 2%	± 8%
0.15	70.79	± 7%	± 2%	± 2%	± 7%
0.20	63.10	± 6%	± 2%	± 2%	± 6%
0.30	50.12	± 5%	± 2%	± 2%	± 5%
0.40	39.81	± 4%	± 1.5%	± 1.5%	± 4%
0.50	31.62	± 4%	± 1.5%	± 1.5%	± 4%
0.60	25.12	± 4%	± 1.5%	± 1.5%	± 4%
0.70	19.95	± 4%	± 1.5%	± 1.5%	± 4%
0.80	15.85	± 4%	± 1.5%	± 1.5%	± 4%
0.90	12.59	± 3.5%	± 1.0%	± 1.0%	± 3.5%
1.00	10.00	± 3.5%	± 1.0%	± 1.0%	± 3.5%
1.30	05.01	± 3.0%	± 1.0%	± 1.0%	± 3.0%
1.50	03.16	± 1.5%	± 0.5%	± 0.5%	± 1.5%
2.00	1.000	± 0.5%	± 0.2%	± 0.2%	± 0.5%
2.50	0.320	± 0.15%	± 0.07%	± 0.07%	± 0.15%
3.00	0.100	± 0.06%	± 0.04%	± 0.15/-0%	0.1% (nominal)
4.00	0.010	± 0.008%	+.01/-.008%	+.01/-0.0%	0.01% (nominal)

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