Laser safety

Section C : Nominal Ocular Hazard Distance (N.O.H.D.)

Basics : The N.O.H.D. is the distance from the source at which the intensities and energies fall under the appropriate M.P.E.

The axial intensity (in W/m²) at a distance z from a laser source is given by the following formula :

C.1 Info

This formula assumes a Gaussian beam with a power P0 , an 1/e² intensity radius w (in m) at its waist, a divergence  θ (in rad), and an absorption rate µ due to the atmosphere – commonly neglected. This is a general formula , where z depicts the distance from the source to the operator.

In order to evaluate the N.O.H.D., one replaces in the formula above z by the N.O.H.D. and I by the M.P.E.

Then, when neglecting the absorption by the atmosphere, one obtains the following expression :

C.2 Info

This formula is valid if and only if the beam propagates freely. If any optical instrument is used in order to observe the laser radiation, then one must consider a higher N.O.H.D. in order to take into account the higher energy density, as the radiation may be collimated or concentrated by the optical instrument.

The increase of intensity due to the optical instrument can be evaluated according to the following parameter K :

C.3 Info

where the pupil diameter of the eye is of order 7 mm.

Question

Exercise 3.1 : The considered laser source emits continuously at the central wavelength 400 nm. Its mean optical power is 4 W, the divergence of the beam is 0.7 mrad, and the beam diameter at the output port of the source is 1 mm.

  1. The exposure duration is 2 hours. What is the appropriated M.P.E. ?

  2. The beam propagates freely through the air. What is the N.O.H.D. of this source? The atmospheric absorption can be neglected.

Solution

Question

Exercise 3.2 : In order to reduce the divergence of the beam, an afocal lens pair is placed in the beam, increasing its diameter and collimating it. After this optical system, the beam divergence is reduced down to 0.1 mrad, while the beam diameter is 7 mm. What is the new N.O.H.D. ?

Solution

Question

Exercise 3.3 : A topography laser (He-Ne medium, emitting at 632 nm) emits a continuous radiation with a mean optical power of 3 mW. The beam leaves the cavity with a diameter of 13 mm and diverges along its free space propagation. Fifty meters away from the source, the beam diameter is 18 mm.

  1. 65 meters away from the source, how long can one directly look at the beam without risking eye injuries?

  2. For an exposure duration of 3 min, what is the minimum distance at which one can safely look at the beam ?

Solution

Question

Exercise 3.4 : A telemetry laser using a Neodymium doped gain medium emits laser pulses at 1060 nm. Its peak optical power is 1.5 MW and each pulse carries 45 mJ. The repetition rate is 12 pulses per minute. The beam diameter is 10 mm at the output port of the source, and its divergence is 1 mrad.

  1. What is the N.O.H.D for the naked eye exposed to this radiation ?

  2. What is the N.O.H.D for the eye looking through a 60 mm diameter telescope ?

Solution
AccueilOutils transversesNouvelle pageInformations sur le cours (ouvrir dans une nouvelle fenêtre)Section B : M.P.E. evaluation in the case or repeated laser pulsesSection D : How to choose safety goggles ?