What are ND filters?
ND or Neutral Density
filters uniformly attenuate the intensity of light over a broad
spectral range. This is achieved by either absorption using light
absorbing glass or by a combination of absorption and reflection
using a thin-film metal coating.
Which type of ND filter should I use?
Absorption ND filters are
uncoated, gray tinted glass that attenuates entirely by absorption of
the light. The degree of attention is determined by the thickness of
the glass. They are not transmissive in the UV region and are
generally only effective in the visible portion of the spectrum. As
the absorption takes place in the filter glass these filters are less
prone to surface damage than reflection filters. However as the
degree of attenuation is a function of glass thickness it is not
always possible to achieve the attenuation required for a given
thickness and these filters tend to have greater batch to batch variability.
Reflection ND filters
attenuate by a combination of reflection and absorption. Made with
Inconel® metallic coatings on glass substrates, this type of
neutral density filter is much more uniform/neutral than the
absorption glass filter and can be manufactured for a wider spectral
range. See below for a comparison. Reflective ND filters can be used
into the UV region of the spectra provided a fused silica substrate
is used. The degree of attenuation is not so thickness critical.
How are they used?
ND filters are used in
numerous applications where the amount of light needs to be evenly
reduced. Some applications are;
filters reduce the intensity of the light source without affecting
the spectral range of the source. They work like sunglasses, reducing
the light intensity but without changing the colours. In fluorescence
microscopy ND filters restrict the excitation intensity to minimise
photo bleaching effects.
When instruments are used to measure the amount of light being
transmitted to a detector some form of calibration can be required.
ND filters offer such a method. The introduction of traceable
standard ND filters into the light path reduces the amount of light
reaching the detector.
Laser systems: ND
filters find applications in several high-precision laser
experiments. This is because the power of a laser cannot be adjusted
without changing other properties of the laser light (e.g.
parallelicity of the beam). Moreover, most lasers have a minimum
power setting at which they can be operated. To achieve the desired
light attenuation, one or more neutral density filters can be placed
in the path of the beam.
purpose of standard photographic neutral density filters is to allow
the photographer greater flexibility to change the aperture or
exposure time, allowing for more control, particularly in extreme circumstances.
What level of attenuation can be achieved?
Neutral density filters
attenuate spectral regions selected from 250 to 2500nm. The level of
attenuation can be specified from optical density (OD) 0.04 to 4.0
where OD is defined as -log10 (T). For a table of ND vs %Transmission click
Overall attenuation may be
increased by stacking neutral density filters serially in the light
path. For example, a 0.5 (OD 0.5) neutral density filter placed in
series with a 0.3 (OD 0.3) neutral density filter will produce total
attenuation of OD 0.8 (10-0.8) or 16% transmission. A six-filter set
of 0.1, 0.3, 0.5, 1.0, 2.0, and 3.0 optical densities can achieve a
graduated range of spectrally neutral attenuation from OD 0.1 to 6.9.
What is OD?
OD or Optical Density is a
measure of how much light is blocked by a filter.
Optical Density-OD =
A filter with 1%
transmission has an OD of 2
How are ND filters specified?
An ideal neutral density
filter reduces light of all wavelengths or colours equally but as we
can see from the chart above this is not possible. ND filters are
normally specified by reference to their blocking (OD) value at a
wavelength of 550nm. An ND1 filter has an OD value of 1 and a
transmission value of 1% at 550nm. All our ND filters are supplied
with a spectral curve to enable filter performance across the
transmission range to be validated. For a full specification of our
ND filters please click here.
How do I buy ND filters?
From Glen Spectra, where else!
For prices and details of
our standard filters please click here.
To request a quotation click
To contact Glen Spectra
please click here.
If the filter ND level or
size is not covered by our standard listing then please contact us
for details on custom ND filters. Click here.