Filters improve images

Jon Titus, Contributing Technical Editor -- Test & Measurement World, 5/1/2007

Designers may not immediately think of an optical filter as a way to enhance images. But a basic $30 polarizing filter can attenuate specular reflections from nonmetallic surfaces and eliminate the need to tweak lighting.

Color filters such as these screw onto lenses and pass or block light within narrow wavelength bands. Courtesy of Edmund Optics.

Colored filters also enhance monochrome images. “A red filter narrows the spectrum and reduces chromatic aberration,” noted Greg Hollows, vision integration partners coordinator at Edmund Optics. “All lenses exhibit some chromatic dispersion, which means colors across a wide range of wavelengths do not simultaneously focus on the surface of an image sensor. As a result, an image may show some blurring, which decreases a camera’s spatial resolution.”

A color filter also can increase the contrast between objects. Think of blue components on a PCB. When observed through a red filter, these objects appear dark, and the high contrast between them and the PCB lets a vision system locate and measure well-defined edges.

Using red light has another advantage: Silicon image sensors have high responsivity at red wavelengths, so removing light toward the blue end of the spectrum reduces the sensor’s sensitivity only slightly. This red-light sensitivity extends into near-infrared (NIR) wavelengths (750–1100 nm), which the human eye cannot detect.

As a result, NIR light can produce some unexpected results, because it can reflect from surfaces that would appear dark in visible light. So, if your camera produces images that show things you cannot see in visible light, your lens may need a filter that blocks NIR wavelengths. (Manufacturers offer cameras with built-in NIR filters, although you can specify a camera without this type of filter.)

Read additional information about optical filters.

In some cases, though, NIR light can work to your advantage and reveal defects you cannot see under visible light. Metal parts, for example, challenge vision systems when finished and unfinished parts look alike. But if NIR radiation reflects differently from bare metal and finished metal, a camera with good NIR response and equipped with an NIR bandpass filter can distinguish between good and bad parts. The filter blocks non-NIR ambient light.

At the other end of the spectrum, ultraviolet light also inspects things not visible to the eye. “Consider a pharmaceutical package,” said Steve Kinney, product manager at JAI. “Under white light, a portion of the package might appear unmarked, but special inks printed in this area could reflect UV light. So, an inspection station outfitted with a UV light, a UV-sensitive camera, and a UV bandpass filter could read the 'invisible’ information.” Even if someone examined the package with a UV light, they could not read the label.

jontitus@comcast.net

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For further reading

“Interference Filters: The Key to It All,” C&L Instruments. www.fluorescence.com/tutorial/int-filt.htm.

Wagner, Julianne, “Successful Polarization Techniques,” Edmund Optics. www.edmundoptics.com/techSupport/DisplayArticle.cfm?articleid=257.

Optical filters used with machine-vision cameras fall into two general categories; filters that use a colored material and filters that use interference techniques. The former provides a substrate--usually glass--made with compounds that absorb wavelengths within specific ranges. Beverage bottles rely on this type of coloration. Interference filters rely on materials deposited as thin films on glass. The thin films cause light waves to interfere with one another.

Although interference filters produce sharp band edges and can create narrow pass bands, their characteristics change as light rays move away from their optical axis. "If you look at an object straight through an interference filter, the light passes through a film of thickness x," said Greg Hollows of Edmund Optics. "But if light enters the filter at a 45-degree angle, the light passes through the thin film with a path length of 1.414 times x. Because the interference filter relies on a relationship between film thickness and light wavelengths, filter characteristics change with respect to the angle at which light reaches the filter."

Dichroic filters, a subset of interference filters, provide narrow pass bands but do not suffer as badly from directional effects. These filters reflect the unwanted light wavelengths and pass the wanted ones. So, they look like the rejected color on side that faces the light source and they show the passed color on the other side, thus the name dichroic for two colors. A solution of chlorophyll, for example, exhibits dichroic behavior. The solution reflects green light, but the solution appears red when you place it in front of a light source.