Laser Interlock Timing
Interlock stopping time against the time to reach the beam (ISO 13855 approach speeds) and the time the beam takes to exceed the eye limits.
Educational only — not compliance-grade
These pages implement the published exposure limits for the cases each one states, but they are educational tools: no laser safety officer has signed them off.
- Do not use these results alone for PPE selection, NHZ / NOHD approval, controlled-area sign-off, enclosure/interlock approval, or formal classification.
- Always verify against ANSI Z136.1, IEC 60825-1, your local procedures, and the responsible laser safety officer / CLSO.
Interlock timing
Fast enough: the emission stops before anyone reaches the beam
A hand or eye reaches the beam after 250 ms; the emission stops after 50.0 ms.
Limit reached after (beam held on the eye)
157 ns
Retina, thermal
Time to reach the beam (S/K)
250 ms
50.0 cm at 2 m/s
Longest stopping time
250 ms
S/K + t_max; yours is 50.0 ms
Shortest distance for your stopping time
10.0 cm
K × (T − t_max), ISO 13855 without an intrusion allowance
An interlocked guard has to stop the emission before anyone who opens it can put an eye in the beam for longer than the eye limits allow. From the moment the guard starts to open, a person moving at the approach speed K reaches a beam S behind the opening after S/K; the emission stops after the stopping time T (sensor, logic and shutter or laser shut-down). The eye is then in the beam for T − S/K at most, which must not exceed t_max, the time the beam takes to exceed the first ICNIRP 2013 eye limit when held on the eye (retina over 7 mm, cornea over 1–3.5 mm; a round Gaussian beam centred on the pupil, the worst case).
ISO 13855 uses K = 2.0 m/s for hands and arms and 1.6 m/s for the body, and adds an intrusion allowance C for sensing devices such as light curtains (S = K·T + C). In the retinal hazard region (400–1400 nm) t_max is nanoseconds to microseconds, so the shutter has to close before the beam can be reached: T ≤ S/K. In the mid and far infrared t_max can be milliseconds to seconds. Not modelled: reflections inside the enclosure, the skin, the interlock's reliability (ISO 13849 performance level) and IEC 60825-1's rule that a Class 3B or 4 product's interlocks prevent access to radiation above the Class 1 limits.
Each eye limit as an irradiance, H(t)/t, and the beam irradiance over that limit's aperture: the beam is within the limits up to the first crossing (t_max). The diamond marks the time an eye spends in the beam before the emission stops.
Model and references
Textbook approximation. A standard approximation from the literature (for example paraxial, plane-wave or Gaussian-beam), valid within the range the page states. About the tiers
Eye in the beam for T − S/K (ISO 13855 approach model, no intrusion allowance), held against the time the stationary beam takes to exceed the ICNIRP 2013 eye limits. Not modelled: reflections, skin, reliability.
- ICNIRP (2013). Guidelines on limits of exposure to laser radiation of wavelengths between 180 nm and 1,000 µm. Health Phys. 105(3), 271–295. Tables 5 and 8. doi:10.1097/HP.0b013e3182983fd4
- ISO 13855:2010. Safety of machinery – Positioning of safeguards with respect to the approach speeds of parts of the human body. Minimum distance S = K·T + C.