There are many different types of filters in machine vision that can be utilized 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 in order 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 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 then nearly identical to standard optical glass manufacturing.

Colored glass filters are advantageous for a couple of different reasons: they are of relatively low cost when compared to interference filters and, more importantly, they 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 cut-on wavebands, do not have curves that are as sharp or accurate as coated interference filters, and do not have transmission throughput levels (percentages) 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.

Figure 1. Transmission curves for several different colored glass filters.
Infrared (IR) cutoff filters can be either colored glass filters or a type of coated filter that is useful for both monochrome and color cameras in machine vision applications. Since the silicon sensors in most machine vision cameras are responsive to wavelengths up to approximately 1 m, any IR light incident on the sensor that may have been caused by overhead fluorescent lights or other unwanted sources can create inaccuracies on the sensor. 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 filter over the sensor. With monochrome cameras, the presence of IR light will degrade the contrast of the overall image.

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

Figure 2. Transmission curve examples of longpass and shortpass (a) and bandpass and notch filters (b).
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, from hard coated fluorescent filters to dichroic filters to polarization filters. Each coated filter undergoes a unique manufacturing process to ensure the proper performance. Wavelength-selective optical filters are manufactured by depositing dielectric layers on a specific substrate of alternating high and low indices of refraction. The surface quality and uniformity of the substrate establishes the baseline optical quality for 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, as well as providing much sharper cut-off and cut-on bands when compared to colored glass filters.

Figure 3a. Interference filters function based on the distance that light incident upon the filter travels. At the correct angle of incidence, the light waves incident on the filter destructively interfere, disallowing them from making it through the filter. At a different angle, the destructive interference is not as effective, essentially changing the type of filter.
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. Shortpass filters are the opposite, passing shorter wavelengths and blocking longer. Bandpass filters pass a band of wavelengths while blocking longer and shorter wavelengths. The inverse of a bandpass filter is a notch filter, which blocks a band of wavelengths and passes the longer and shorter. Transmission curve shapes for these filter types are shown in Figure 2.

Filters designed for deep blocking (high Optical Density) and steep slopes (sharp transition from blocking to transmission) are used in applications where precise light control is critical. Most machine vision applications do not require this level of precision; typically, any filter with an Optical Density (OD) of 4 or greater is more precise than required and adds unnecessary cost.

Photonics & Imaging Technology Magazine

This article first appeared in the May, 2016 issue of Photonics & Imaging Technology Magazine.

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