KTS versus CSS for mark detection in print applications

In the printing process, a very accurate register mark detection is essential for the quality of the print.

The color of the register marks matches the color of the print register, so the register marks have different colors, the background is usually white.

During the printing process, the paper is well guided, there are no distance fluctuations and no waves.

This can lead to the assumption that a color sensor could be a good solution, such as the CSS High Speed.

In the following example it is shown why this is not the case:

For the comparison, the CSS High Speed with 13 mm sensing distance is used
CSS-WBF114115AA10Z

against a standard KTS with the same housing and 13 mm sensing distance
KTS-WB9114115AA90Z

In this example, there is a blue register mark on a white background.

Light Spot size versus switching threshold

    CSS-WBF114115AA10Z

• light spot: 2 mm x 4 mm
• Jitter: 18 µs - good for low end quality print
• teach-in: simple 1-point
• threshold: referring to color match value (CMV) 900
• ON: if the light spot almost completely covers the register mark
• OFF: just when the light spot starts leaving the register mark

Result: the switching period can be very short due to the fact that the size of register mark is close to the size of light spot. In case of higher web speed, the mark detection can be lost!

   KTS-WB9114115AA90Z

• light spot: 0,9 mm x 3,8 mm
• Jitter: 5 µs - good for high quality print
• teach-in: 2-point or dynamic
• threshold: 50 % between mark and background
• ON: half covering the register mark - entry
• OFF: half covering the register mark - exit

Result: The switching period is matching with the width of register mark - no problem for higher web speed.

Conclusion

CSS High Speed is optimized for color recognition and not for the precise recognition of registration marks. This may be easier if different colored marks are simply taught in, but it does not solve the requirement for precise register mark recognition.

Therefore, the use of KTS sensors for such pressure applications is strongly recommended. There are many different models available, with the “High Precision” models being best suited for the most demanding requirements in terms of high print image quality.