Neutral density filters attenuate, split or combine beams in a range of irradiance ratios with little dependence

on wavelength.
FOCtek offers two kinds of neutral density filters, which are neutral grey glass type filters and metallic coated type filters made of optical glass such as N-BK7, H-K9L Fused Silica and so on.
The thickness of neutral grey glass type filters determin the transmission, however, transmission is determined
by coating thickness  in the metallic coated optical material type.

 Neutral Density Filter Specifications

Attribute Specification
Neutral Density Glass Type Metallic Coating Type
Material Optical Neutral Density Glass N-BK7, Fused Silica
Typical Size (mm) Ф25.0, 50.0 x 50.0 Ф25.0 x 2.0, 50.0 x 50.0 x 2.0
Size Tolerance (mm) ±0.2
Parallelism 3 arc min.
Flatness (per 25mm@633nm) 2λ (General), λ/2(High Precision)
Surface Quality 80/50 (General), 20/10 (High Precision)
Optical Density 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 1.0, 2.0, 3.0, 4.0, etc. 0.1, 0.2, 0.3, 0.5, 0.8, 1.0, 2.0, 3.0, 4.0, etc.
Optical Density Tolerance ±5%

Note for Interference Filter:
(1). Typical diameter is phi 10.0mm, 15.0mm, 20.0mm, 25.4mm, other size at shape of quadrate, triangular, 
       or other polygonals are available upon request.
(2). Typical size tolerance is ±0.2mm, better precision is available upon request.
(3). Typical parallelism is 3 arc minutes, better parallelism is available upon request
(4). Typical flatness is 2λ, high precision such as λ/2 is available upon request.
(5). Typical surface quality is 80/50, better precision such as 60/40, 40/20, 20/10 is available upon request.
(6). Optical density (D) is defined as the base 10 logarithm of the reciprocal of transmittance (T): 
       D=log(1/T) or T=10(-D), Besides the typical optical density listed as above, others are available upon request.



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