Ceilometers must be eye safe and meet Class or Class 1m Laser Safety standard under the international specification IEC 60825-1 or ANSI Z136 in the USA
The phrase “eye-safe” is used below.
Class 1: This class is eye-safe under all operating conditions.
Class 1M: This class is safe for viewing directly with the naked eye, but may be hazardous to view with the aid of optical instruments. In general, the use of magnifying glasses increases the hazard from a widely-diverging beam (eg LEDs and bare laser diodes), and binoculars or telescopes increase the hazard from a wide, collimated beam Radiation in classes 1 and 1M can be visible, invisible or both.
The beam from a ceilometer has a very low divergence, which is mainly determined by the finite size of the laser source and the ceilometer lens/mirror focal length, but can also be effected by spherical aberration and diffraction effects in the optical path in the instrument.