What must be detected
Detection capability, commonly called resolution, describes the size the device can reliably recognise. It must suit the exposed body part and the actual configuration. Selecting emitter and receiver lengths that span an opening is not sufficient.
The distance between the units, optical interference, reflective surfaces and environmental conditions also matter. Selection belongs within the design of Machinery safety functions and performance level, rather than being an isolated sensor purchase.
Why distance matters
Where the curtain initiates a stop, the hazard must cease before a person can reach it. Assessment includes sensor response, control response and machine stopping under unfavourable conditions. Measuring the sensor response alone leaves out a critical part.
The installation must prevent undetected access around, over or beneath the field. If someone can pass through and remain inside, the design must also address continued presence and prevention of unexpected restart.
What it does not replace
A light curtain does not contain swarf, ejected workpieces or hot liquids. Those hazards may require Machine guards or other measures. Nor does a curtain provide energy isolation and securing for maintenance interventions.
Clearing an interrupted field must not be treated as universal permission to start. Reset and start logic are application-specific and must prevent hazardous activation while people remain inside.
A review example
In a hypothetical cell, an enclosure is replaced with a curtain to ease loading. Review identifies possible workpiece ejection and a space behind the field where someone could stand. Moving the curtain closer does not solve these issues: both hazards require controls and the combined response must be validated.
Acceptance records retain the configuration, test results and criteria for subsequent checks. Changes to speed, braking, tooling or workstation layout can alter protection and require technical review. The cited INSST notes are guidance published in 2018, not certification of an installation; OSHA material reflects the US context.
Practical review should include loading and unloading: new trolleys, boxes or materials may create an access route around the field that did not previously exist. Define who authorises these changes and how protection is checked before production resumes.
