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Choosing the Right Filter for Machine Vision Imaging

Published: June 14, 2016

Why a GAUSSIAN TRANSMISSION CURVE is ideal for use with LEDs

Logo_MIDOPTThe spectral output from any single color LED light source used in any vision application typically follows a normal Gaussian or “bell shape” profile. For a machine vision bandpass filter to be most effective, the position, height and width of the passband should approximate this bell-shaped curve (Figure 1a).

Selecting a filter that is too broad can allow more unwanted ambient light to pass through the filter (Figure 1b), while selecting a bandwidth that is too narrow can result in loss of a significant portion of the desired illumination and overall darkening of the image (Figure 1c). Either can result in a sharp reduction in contrast.

MidOpt_Gaussian-Curve_fig1-web (2)

Similarly, in industrial fluorescence applications, it is often not desirable if the filter passes most of the light in those regions where the illuminant or fluorescent emission provide only a very weak signal, particularly when ambient light or the excitation light source are capable of overwhelming the application.

Contrary to this thought, some bandpass filter providers state that they design their filters to have a “high and flat transmission profile”, claiming that this will provide more even illumination. Instead, this shape leads to undesired effects. (Figure 2). With these filters, the tail ends of the Gaussian-shaped LED spectral output curve (where only a very weak signal is provided) are allowed to dictate the fit of the passband, and far too much ambient light can be allowed to pass relative to the very small amount of LED light emitted in these spectral regions.



When wide-angle lenses (≤12mm FL) are used and the passband shifts downward (also referred to as short-shifting or blue-shifting), lower wavelength ambient light can quickly overwhelm the application out toward the edges of the image (Fig. 3). The amount of light transmitted by the filter at any given wavelength should be matched to the relative spectral output of the light source.

To minimize the effects of angular shifting, MidOpt developed its StablEDGE™ Filter Design which employs absorptive filter glass to form the leading edge of the filter passband, thus ensuring there is no shifting in this region.

MidOpt_Gaussian-T-Curve_fig3-web (1)

Minimize the Effects of Short Shifting

MidOpt StablEDGE™ optical filters are specifically designed to be less susceptible to effects from angular shifting seen when optical filters are placed in front of short focal length (<12 mm) camera lenses.

This feature is becoming increasingly important as today’s trend in machine vision imaging progresses towards more compact inspection layouts, which utilize less space – forcing the camera and lens closer to the subject. As a result, short focal length lenses are now more widely used than ever before.

Using a traditional coated interference filter in these more compressed configurations results in contrast loss toward the edges of the image. Because of the angle imposed by the field of view (FOV) of the lens, the passband shifts and allows short wavelength ambient light to overwhelm the subject. Light from LED or laser diode lighting is also cut off. In contrast, peak transmission of MidOpt’s StablEDGE™ filters is not significantly altered, and effects due to short shifting are minimized.

MidOpt_StablEDGE_Angles-webStablEDGE™ filters take advantage of absorptive filter glass to form the leading edge of the filter passband. This assures no shifting in this region, even when the lens FOV exceeds 100 degrees. Filter glasses also offer far superior lower wavelength blocking of ambient light, sharp transition slopes and unmatched durability.

StablEDGE_curves_3MidOpt’s StablEDGE™ Filter cut-off slopes utilize interference filter coatings, however the cut-off slope is positioned to be sufficiently broad, and the Gaussian passband profile ensures that excessive ambient light is not allowed to degrade image contrast. Thus, shifting will not significantly encroach into peak transmission, assuring angular insensitivity over the desired range.

Among all machine vision filter manufacturers, MidOpt is unique in incorporating StablEDGE™ technology across a full range of products. StablEDGE™ designs are less angle-of-incidence sensitive, inherently more rugged, environmentally stable, and can be found in MidOpt’s BP and BN (Bandpass) series and most LP (Longpass) series filters.

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