Neutral Density Filters

Also known as gray filters, neutral density filters decrease the intensity of light without affecting spectral response or the relative spectral distribution of energy. The main purpose of using neutral density filters is to reduce the amount of light that passes through the optical system, especially in light control applications for measuring instruments and exposure control in imaging. Other applications include NIR (near infrared) spectroscopy, machine vision and analytical chemistry.

General Types of Neutral Density Filters

Neutral density filters achieve attenuation in two ways: reflection and absorption of light. Absorptive neutral density filters rely on absorption of unwanted light and require calibration for precise experiments.When used in multiple stacks, any desirable light intensity level can be achieved without affecting the spectral response of light from the source. In contrast, reflective neutral density filters reflect unwanted light while allowing a fixed percentage of light to pass through. These filters use a metallic coating applied onto a flat polished substrate such as glass or quartz. Reflective filters have transmissions with a very flat response over a wide spectral range making them ideal for most general purposes.

JML metallic neutral density filters attenuate light through reflection, not absorption.

Precision Neutral Density Filters

JML neutral density filters, which are spectrally neutral or flat from approximately 400nm to 1200nm, offer outstanding parallelism and exceptional surface characteristics. Precision polished for demanding research or OEM applications, the filters are made of BK7 Grade A fine annealed glass for general laboratory usage, or opticalquality quartz or UV fused silica substrates for maximum thermal stability. Fused silica, or synthetic quartz, has excellent mechanical and optical characteristics. It is an extremely hard and highly inert material, which makes it very suitable for high temperature processing. In addition, fused silica can be predictably polished to a very low micro-roughness.

A metallic coating, which consists of a thin film of inconel protected by an overcoat of silicon monoxide on one side, provides a nearly constant broadband attenuation value, or optical density, over a broad spectral range — from visible light through near infrared wavelengths. Optical density (OD) is defined as the base 10 logarithm of the reciprocal of transmittance (T):

OD = log (1/T) or T = 10-D

Typically, OD value is a function of glass type and thickness of the filter. Although optical densities can be combined additively in stack to achieve values of transmittance or density not otherwise available, we do not recommend stacking metallic neutral density filters due to potential effects from multiple reflections. However, it may be possible to arrange the neutral density filters at a slight angle of incidence so that multiple reflections do not occur in the direction of the optical path.

On BK7 or quartz and fused silica substrates, JML produces a variety of standard neutral density films with optical densities that include 0.1, 0.3, 0.5, 1.0, 2.0 and 3.0. The film or coating can be customized to fit most transmission requirements. One restriction is that we do not recommend that this type of coating be used with high power lasers due to the absorptive properties of the metallic materials.

Metallic Linearly Variable Neutral Density Filters

These neutral density filters provide linearly variable attenuation or beam-splitting in the visible and near infrared range. The optical density varies linearly across the length of the filter. Attenuation is achieved by a thin film of metallic coating inconel protected with an overcoat of silicon monoxide on one side of the filter. To eliminate unwanted fringe patterns and cavity feedback, the BK7 and fused silica substrates have a slight wedge between the faces. In addition, the second polished side is broadband AR (anti-reflective) coated for the visible range.

Metallic Circularly Variable Neutral Density Filters

Widely used in spectrophotometers and instruments that require variable attenuation, the circularly variable neutral density filter linearly attenuates light as the filter is rotated. The filter simplifies attenuation and beamsplitting from visible light to near infrared. It achieves this by gradually varying around a 270° circle. A large, finite aperture can be attenuated by counter rotating two filters in series. Attenuation is achieved by a thin inconel film, which allows the attenuator to withstand moderate laser energy as well as act as variable beam-splitters. A broadband AR (anti-reflective) coating on nonmetalized areas of the filter minimizes losses and avoids ghost images. The coatings are optimized to be spectrally neutral from 450-700nm, however, they will operate from 400-1100nm with only minor losses.

Click Here to view Neutral Density Filters products.