Color and contrast

Sept. 15, 2005
Photoelectric sensors can now differentiate among hues on packages and products to help speed production.

Jeff Allison
Twinsburg, Ohio

Typical color, contrast, and luminescence sensors all shoot out a light beam and collect reflections through a single lens. The close proximity of the target to the sensor, resulting in a sharp reflection angle, makes the single lens configuration possible. Photoelectric sensors working with targets farther away generally use two different lenses for transmission and to collect reflections. Luminescence sensors and some contrast sensors incorporate a semitransparent mirror to beam light through the lens. The mirror makes for more efficient use of space in the sensor package. The three LEDs used in color sensors typically shine directly through the lens without the aid of a mirror.

Examples illustrate how color and contrast sensors can check quality on packaging lines: Examining glue beads on flaps, noticing the presence or absence of a semitransparent safety seal, and verifying a cap insert operation.

Most engineers are familiar with photoelectric sensors. Here a source, often an LED, beams light at a target. A photodiode senses the reflection as an indication the target is there. An output from the sensor changes state to signal the event.

There are a lot of refinements to the basic photoelectric sensor that come in handy for such nuances as black targets, white targets on light backgrounds, and so forth. One of the most interesting refinements is what's called a color sensor. Its claim to fame is an ability to generate a signal when it detects a specific color and an ability to ignore even the slightest variations of the same color.

The sensors generally detect these subtle effects through use of three different-colored LEDs, red, green, and blue. In operation, the sensor would first go through a teaching mode where it would pulse each of its three LEDs and note the amplitude of the reflection coming back from a target.

Of course, these sensors can have additional features for handling special cases. Sensors able to detect extremely slight differences in colors will do so by taking more samples of the light reflected back from the target. The additional averaging comes at the expense of a more lengthy response time.

Some high-precision color sensors emit a white light to evaluate a more complete spectrum of the reflections. Color sensors are typically single-channel (single color) sensors; in other words, they detect only one color at a time. But more sophisticated models offer multiple channel programming to identify multiple colors.

Photoelectrics called contrast sensors sometimes also use the same three LEDs found in color sensors. Contrast sensors respond to different degrees of brightness between the target and the background. Use of multiple LEDs lets these devices better distinguish light contrasts from all colors. Some contrast sensors, though, use only a single LED. This limits their ability to distinguish contrasts from certain colors.

There is also a special type of contrast sensor called a luminescence sensor. Its principle of operation is the same, but it emits ultraviolet (UV) light. It detects markings that a standard photoelectric contrast sensor wouldn't see. Examples include marks on irregular backgrounds such as grainy wood and markings that are otherwise invisible.

In application, luminescence sensors work with targets that contain materials called luminophores, which react specifically to UV light by emitting a specific wavelength of visible light. The sensor's receiver looks for this specific reflected visible light, not UV light. If the target is not inherently luminescent, luminophores may be added to make it visible with UV. But numerous common materials need no help from additives to glow under UV light. They include some oils, epoxies, greases, inks, glues, chalks, and detergents.

Even engineers who are aware of contrast sensors sometimes pigeonhole them as being useful only for detecting registration marks on moving webs. Such is not the case, however. In reality, contrast sensors can sometimes serve in applications that would seemingly require more expensive systems such as bar-code readers or industrial vision. A few examples illustrate these ideas.

Contrast sensors can easily see whether a glue flap contains enough adhesive. They do so by noting the reflectivity of bare packaging media versus the same media with a bead of glue. Sensors can detect even a bead of clear epoxy just a few millimeters wide. And luminescence sensors can handle the case where a clear bead of glue sits on a clear plastic or Mylar surface.

Interestingly, contrast sensors can be useful in some kinds of bar-code applications. There is no substitute for a bar-code reader when the circumstance calls for reading-in data from the bar-code. But many situations don't need the data. They only need to verify the bar-code is, in fact, present on the package, or that it contains printed information such as an expiration date. A contrast or color sensor can give this verification and costs much less than any bar-code reader.

The contrast sensor would be used when the printed mark of interest is lighter or darker than the package. A color sensor can handle situations where the package and bar-code are different colors.

There is a special role for luminescence sensors in confirming the presence of a plastic wrapper on pharmaceutical bottle caps. This seal ensures the final product hasn't been opened and shows any evidence of tampering. The sealing method of choice is often a clear plastic film. The difference in reflectivity between the two is too slight for even a contrast sensor. Fortunately, these wraps generally react to UV light though the containers do not. This lets luminescence sensors detect many of the films used in tamperproof overwraps.

One obvious application for color sensors is in routing material to different packing stations based on package color. An example is that of paint canisters having color-coded lids. Some color sensors can be programmed to recognize up to 10 separate colors and actuate a different output for each.

Among the main tasks of color and contrast sensors is quality checking on packaging lines. For example, a luminescence sensor can often confirm an instruction leaflet has been inserted into a carton. This is possible because some papers are inherently luminescent while carton material generally is not. Paper that doesn't glow under UV can easily be made luminescent by an invisible marking. Pharmaceutical lines often make use of this technique because inserted leaflets can contain-safety information that is required by law. A missing leaflet means the product is out of regulatory compliance.

In the same vein, color sensors can confirm the presence of pills or other products in clear blister packaging. They can also, of course, check pill color to assure the right pill is in the right pack. A blister pocket without a pill reflects differently than one with a pill in place, allowing the package to be readily flagged.

Print quality is another area that color sensors can help gauge. As caps on bottles are printed with a logo, variances in the printing process may darken or lighten the shade of the printed color. A contrast sensor with an analog output is useful for gauging the color quality. The analog current output can be proportional to how light or dark the print marks are, thus indicating a fading color or one that's too bright.

A lid or cap may have a plastic insert inside for better sealing. A contrast sensor can confirm the presence of the insert. The contrast sensor sees one type of reflection when the insert is present, another when the insert is absent. This same principle could also be used to detect protective packaging material (i.e., foam or plastic bubble webs). Contrast sensors that employ laser rather than LED sources can provide an extended sensing range if need be. They can see the presence and absence of inserts from several feet away.

Similarly, laser-contrast sensors can confirm that a pallet of boxes has been fully shrinkwrapped. They can differentiate between the brown cardboard and the glossy clear wrap over the boxes because of the different reflectivities.

Finally, contrast sensors that are configured with fiber-optic tips can be helpful in confirming the alignment of labels and other features. For example, consider a wrap-around label on a container. Two fiber-optic tips could be aimed at opposite borders of a properly positioned label. Then only one sensor would see the label if it were askew.

Of course, neither would sense anything if the label was completely missing. A standard retroreflective photoelectric sensor could serve as a trigger in this case to activate the contrast sensors when the container is in position.

Pepperl+Fuchs, (330) 425-3555,

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