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Use a label sensor when the task is detecting the physical boundary of a label — the gap between labels, the label edge, or label presence — especially on transparent labels or films. Use a color-mark (registration-mark) sensor when the task is detecting a printed mark by its contrast against the background, to trigger cutting, positioning or correction. The two answer different questions: a label sensor finds where the label is; a color-mark sensor finds where the print says the web should be cut, positioned or corrected.
Different targets, different physics. Label sensors detect the label/backing difference (thickness, opacity, or acoustic impedance for ultrasonic types); color-mark sensors detect reflected-light contrast between a printed mark and its background.
Transparent labels are the label-sensor case. Where optical label forks struggle with clear-on-clear labels, ultrasonic label sensors detect the material boundary regardless of transparency.
Contrast decides color-mark feasibility. The mark must differ from its background in the light the sensor emits; low-contrast or same-color mark/background pairs are the classic failure — test the real print.
Web speed binds both. Response time must beat the gap or mark passage time at maximum web speed; verify with samples at speed.
