banner engineering blog

Registration Mark Detection: Why Use a Color Mark Sensor with RGB Technology

Banner Engineering 5/12/2017

Registration Mark DetectionSummary: Registration marks identify packaging material and are used for position control and to coordinate the separation and cutting of webs of packaging material. The webs are processed at high speeds and product changeovers can be frequent. The size and location of a registration mark as well as the color, sheen and translucence of the material can significantly affect mark detection. Keep reading to learn more about how color mark sensors with RGB technology detect registration marks, when to use a color mark sensor versus true color sensor, and more.  

Q: What is the difference between a true color sensor and a color registration mark sensor? 

True color sensors can be taught to detect minute color differences (for example, it can tell dark blue from black), while color mark sensors detect a grayscale change in color. True color sensors, like Banner's QC50, reflect light off a target using a white LED and red, green and blue filter elements. A color value is then assigned to the light returning to the sensor based on the amount of light reflecting off the target. With color mark sensors, the sensor uses just an LED and no filters. The sensor distinguishes the target color based on how it contrasts with the background color.

Q: Isn't it better to use a true color sensor rather than a color mark sensor? 

Not always. A large number of color sensing applications can be done quite effectively by detecting a grayscale change in the color mark. In these instances, we recommend using a color mark sensor like the R58E, especially in high-speed applications. A true color sensor is required when sorting out targets that differ only in color; for example, when you need to distinguish between light blue and dark blue. However, when distinguishing a registration mark on a constant background, a color mark sensor is your best choice.

Q: What's the difference between a color mark sensor and a registration mark sensor? 

There is no difference; these terms are synonymous. 

Q: What does 16 grayscale contrast levels specification mean? 

One way to spec a color mark sensor is to understand how many levels of grayscale it can detect. The finer the sensor's resolution, the more levels can be detected. A commercial grayscale printer's chart is divided into 20 segments ranging from black to white, and the shades of gray between. Many sensors are able to detect the first 16 segments starting at white. 

Q: Does Banner's R58E have a spec for grayscale contrast levels? 

The R58E can easily meet the spec for 16 levels of grayscale. Because of the combination of RGB LEDs, the R58E's resolution capability far exceeds grayscale charts. The R58E's capability for subtle contrast changes far surpasses the specification for 16 levels of grayscale. 

Q: Is it true that processing time for the color mark sensor affects speed? 

Speed is more of an issue for a true color sensor rather than a color mark sensor. The R58E from Banner can perform 10,000 actuations per second, which is very fast! 

Q: Why does Banner's R58E color mark sensor use the red, green, and blue combination of LED colors? 

The R58E uses this combination so that it can reliably sense virtually any registration mark/background combination. The RGB (red, green, and blue) combination can be combined to create almost any color. Your computer monitor, for example, which is capable of millions of colors, uses RGB technology. The R58E, using these three colors, is able to determine the color of the registration mark and the color of the background. The sensor will then use the LED that provides the best contrast between the registration mark and the background. 

Q: How do I pick the right LED color for my application? 

With the R58E, you do not need to choose an LED color. The R58E selects the proper LED color automatically during the initial TEACH function. However, when using a monochromatic color mark sensor, resources such as color selection charts are used to select the proper LED color. 

Q: How do I switch between the different LED colors? 

You don't need to manually switch LEDs. The R58E will automatically select LED colors during the TEACH function. 
 
Q: Can I choose the LED colors in the R58E? 

No. The R58E comes with only the possibility of three LED colors: red, green and blue.

Q: Does having 3 LED colors affect the sensor's response time? 

No. After the initial TEACH process, the R58E's response time is the same as comparable registration mark sensors. The R58E has a 50 microsecond response time, which allows 15 microsecond repeatability. 
 
Q: How do I sense a registration mark on clear material? 

Clear materials (e.g. a clear poly web) don't reflect light very well, so when sensing a registration mark on a clear material, position a reflective surface directly behind the clear material to return light to the sensor. The registration mark will block the light from reaching the reflective surface and will be detected by the sensor. To avoid the shine from clear materials, you might consider mounting your sensor at a 15° skew angle. 
 
Q: How can I maximize sensing in my high-sensitivity applications with the R58E? 

Use the dynamic TEACH function to teach the sensor a series of conditions on the fly. The R58E takes multiple samples of a registration mark against its background and is able to automatically set the sensitivity at the optimum level.