Section 8: Filter Technology
targets illumination cameras microscopy / objectives filters / accessories liquid lens / specialty telecentric fixed focal length resource guide 124 +44(0) 1904 788600 | Edmund Optics® There are many different types of filters in machine vision that can
be used to improve or change the image of the object under inspection.
It is important to understand the different technologies behind
the various types of filters to understand their advantages and limitations.
Although there is a wide variety of filters, almost all can be
divided into two primary categories: colored glass filters and coated
interference filters.
Colored Glass Filters
Colored glass filters are incredibly common in machine vision, and
are created by doping glass materials with elements that selectively
change their absorption and transmission spectra. The dopants vary
based on which wavelengths are considered for transmission, and the
manufacturing process is nearly identical to standard optical glass
manufacturing. Colored glass filters are advantageous for a several
reasons: they are of relatively low cost when compared to interference
filters and, more importantly, do not demonstrate any shift in
wavelength transmission when used with wide angle lenses or at an
angle. However, colored glass filters also typically feature wide cuton
wavebands, do not have curves that are as sharp or accurate as
coated interference filters, and do not have transmission amounts as
high as interference filters. Figure 1 shows the transmission curves for
several common colored glass filters. Note that the filters feature wide
cut-on wavebands and have relatively shallow slopes describing their
transmission functions.
Infrared (IR) cut-off filters can be either colored glass filters or a
type of coated filter that is useful for both monochrome and color
cameras. Since the silicon sensors in most machine vision cameras
are responsive to wavelengths up to approximately 1μm, any IR light
incident on the sensor from overhead fluorescent lights or other unwanted
sources can create sensor inaccuracies. On a color camera,
IR light will create a false color on the sensor that can degrade overall
color reproduction. For this reason, many color imaging cameras
come standard with IR-cut filters. With monochrome cameras, the
presence of IR light will lower the contrast of the overall image but
IR-cut filters are generally not built-in, unlike with color cameras.
There are a multitude of other types of colored glass filters. For
instance, daylight blue filters can be used for color balancing when
polychromatic light sources and color sensors are used.
Coated Interference Filters
Coated filters typically offer sharper cut-on and cut-off transitions,
higher transmissions, and better blocking than colored glass filters.
In addition to colored glass filters, there are a range of coated filters,
that include hard coated fluorescent filters, dichroic filters, and polarizing
filters. Each coated filter undergoes a unique manufacturing
process to ensure proper performance. Wavelength-selective optical
filters are manufactured by depositing dielectric layers on a specific
substrate of alternating high and low refractive indices. The surface
quality and uniformity of the substrate establishes the baseline optical
quality of the filter, along with setting wavelength limits where
the transmission of the substrate material falls off. The dielectric
layers produce the detailed spectral structure of a filter by creating
constructive and destructive interference across a range of wavelengths,
and by providing much sharper cut-off and cut-on bands
compared to colored glass filters.
Many types of hard coated filters exist, such as bandpass, longpass,
shortpass, and notch filters, each with a specified blocking range
and transmission range. Longpass filters are designed to block short
wavelengths and pass long wavelengths.
(Continued on page 125)
RGB Color Filters
Figure 1
T (%)
80
60
40
20
0
R-60
G-533
B-440
LB-120
350 400 450 500 550 600 650 700 750 1000 λ (nm)
Figure 1: Transmission curves for several different colored glass filters.
For more information on various types of colored filters that
are available, visit www.edmundoptics.co.uk/filters
For recommended Imaging Filters, see pages 131-133.
For a complete listing of EO Filters,
visit www.edmundoptics.co.uk/filters
Section 8.1: Types of Filter Technology
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