The inverse of a bandpass filter is a notch filter, which blocks a band of wavelengths and passes the longer and shorter. Bandpass filters pass a band of wavelengths while blocking longer and shorter wavelengths. Shortpass filters are the opposite, passing shorter wavelengths and blocking longer. Longpass filters are designed to block short wavelengths and pass long wavelengths. Many types of hard-coated filters exist, such as bandpass, longpass, shortpass, and notch filters, each with a specified blocking range and transmission range. At a different angle, the destructive interference is not as effective, essentially changing the type of filter. At the correct angle of incidence, the light waves incident on the filter destructively interfere, disallowing them from making it through the filter. Interference filters function based on the distance that light incident upon the filter travels. 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.įigure 3a. 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. Wavelength-selective optical filters are manufactured by depositing dielectric layers on a specific substrate of alternating high and low indices of refraction. Each coated filter undergoes a unique manufacturing process to ensure the proper performance. In addition to colored glass filters, there are a range of coated filters, from hard coated fluorescent filters to dichroic filters to polarization filters. Coated filters typically offer sharper cut on and cut off transitions, higher transmissions, and better blocking than colored glass filters. Transmission curve examples of longpass and shortpass (a) and bandpass and notch filters (b). For instance, daylight blue filters can be used for color balancing when polychromatic light sources and color sensors are used. There are a multitude of other types of colored glass filters. With monochrome cameras, the presence of IR light will degrade the contrast of the overall image. ![]() For this reason, many color imaging cameras come standard with IR-cut filter over the sensor. On a color camera, IR light will create a false color on the sensor that can degrade overall color reproduction. 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. 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. Transmission curves for several different colored glass filters. Note that the filters feature wide cut-on wavebands and have relatively shallow slopes describing their transmission functions.įigure 1. Figure 1 shows the transmission curves for several common colored glass filters. 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. The dopants vary based on which wavelengths are considered for transmission, and the manufacturing process is then nearly identical to standard optical glass manufacturing.Ĭolored 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. ![]() Colored Glass FiltersĬolored 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. Although there is a wide variety of filters, almost all can be divided into two primary categories: colored glass filters and coated filters. It is important to understand the different technologies behind the various types of filters in order to understand their advantages and limitations. 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.
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