Neutral Density Filter Selection Guide

Neutral density (ND) filters are used to equally attenuate the intensity of a light beam over a wide wavelength range. ND filters range from colorless to gray in appearance. Metallic neutral density filters provide a relatively consistent degree of attenuation over a wide spectral band. We offer a variety of catalog ND filters for UV/VIS applications including absorptive, metallic on glass or fused-silica, and variable linear or circular, as well as filters for infrared applications.

Figure 1. Fixed and Variable ND Filters. (Click on image to enlarge)

METALLIC NEUTRAL DENSITY FILTERS

The metallic ND filters use a thin coating of inconel on a glass or fused silica substrate. The inconel material contains a number of different elements such as nickel (Ni), chromium (Cr), cobalt (Co), and iron (Fe). Careful control of the alloy composition and vaccum deposition produces coatings which are spectral neutral over a wide wavelength range. The metallic ND filters are generally more neutral comparing to the absorptive type of ND filters. UV Fused silica substrate are useful in the ultraviolet spectrum, while the cost effective B 270 optical glass substrate provides excellent performance in the visible and Near-IR spectrum range. Always use these filters with the reflective side facing the radiation source. The metallic ND filters withstand higher power and thermal shock better comparing to the absorptive type. These filters are spectrally neutral beyond 2 µm.

ABSORPTIVE NEUTRAL DENSITY FILTERS

These ND filters attenuate by absorption (and Fresnel reflection, the constant reflection from the air-glass interfaces). The absorption is iconic in anature so this type of density filter follows the Beer and Bouguer's Laws.
I_T ≈ I_O (1-R)² e ^-kct

Where:
I_T = Transmitted Intensity
I_O = Incident Intensity
R = Surface Reflectance (typically 0.04 or 4%)
e = 2.718
k = Constant
c = Ion Concentration
t = Filter Thickness

Solid glass absorptive ND filters are relatively neutral in the 400 to 700 nm range. They are usable from 350 nm to 2500 nm, but the transmittance is different from that expected from the desnity value. Since most of the incident energy is absorbed, there filters produce fewer problems from multiple reflected beams comparing to the metallic type, but they are intended only for use with lower power sources.

Calculating Transmittance and Optical Density

Optical Density (OD) is defined as the logarithm to the base ten of the ratio of the power of the incident beam to that of the exiting beam. The optical density can be convert to transmittance using the following equations
OD = log₁₀ (I_O/I_T) = -log₁₀( T) ; T = 10^-D

Where:
OD = Optical Density
I_O = Incident Power
I_T = Transmitted Power
T = Transmittance

Neutral Density filters can be stacked in series to produce higher densities. The total optical density is the sum of the individual densities. The total transmittance of a stack of neutral density filters is the product of the transmittance of each filter.

Total Density = D₁ + D₂ + D₃ ...
Total Transmittance = (T₁)(T₂)(T₃)...

NOTE: Do not place ND filters in series with their surfaces parallel. Parallel surfaces may cause multiple internal surface reflections which can combine to cause an increase in tranmittance;; this is especially true of the metallic type filters.

Optical Density (OD)	Transmittance (%)
0.04	91.20
0.1	79.43
0.2	63.10
0.3	50.12
0.4	39.81
0.5	31.62
0.6	25.12
0.7	19.95
0.8	15.85
0.9	12.59
1.0	10.00
2.0	1.00
3.0	0.10
4.0	0.01

Selecting a Neutral Density (ND) Filter

Click Neutral Density Filters to shop or browse all of our standard models, or select a product series below for more information on our products and capabilities.

Neutral Density Filters		Features
	UV Fused Silica Metallic Neutral Density Filters	• Broadband attenuation, UV to near IR • Metallic neutral density coating • Low wavefront distortion • UV grade fused silica • For lower power applications
	Absorptive Neutral Density Filters	• New 12.7 mm diameter size • Broadband attenuation, visible to near IR • Absorptive neutral density glass • 25.4 mm dia and 50.8 mm square sizes are permanently marked with the optical density • For lower power applications
	Optical Crown Glass Metallic Neutral Density Filters	• Broadband attenuation, visible to near IR • Metallic neutral density coating • Cost effective optical crown glass • Optical crown glass • For lower power applications
	Glass Neutral Density Filters and Kits	• 1" diameter ﬁts mirror mounts and ﬁlter wheels • Broad wavelength range—0.4 to 2 µm • Available in convenient kits
	Infrared Neutral Density Filters	• Attenuate IR radiation from 2 to 14 µm (optimized for 2 to 5 µm) • Spectrally neutral throughout usable range • Precision Germanium substrate • High damage threshold, up to GW/cm²
Circular Linear Variable Neutral Density Filters		Features
	Circular Linear Variable ND Filters	• Linear Optical Density Gradient of 0-1, 0-2.0 and 0-4.0 • UV Fused Silica Substrate for Broad Wavelength Use • Available in Three Different Diameters of 25mm, 50mm and 100mm • Antireflection Coating on Second Surface to Reduce Back Reflection
	Circular Variable Metallic Neutral Density Filters	• Linear optical density gradient is circular • Variable neutral density filter • Variable beamsplitter • Withstands moderate energy
Linear Variable Neutral Density Filters		Features
	Linear Variable Metallic Neutral Density Filters	• Linear variable attenuator • Linear variable beamsplitter • Visible to NIR • Individual filters, or matched pairs to eliminate the density gradient over larger apertures