Eye safety analysis for non-uniform retinal scanning laser trajectories
2016
Scanning the retinae of the human eyes with a laser beam is an approved diagnosis method in ophthalmology; moreover
the retinal blood vessels form a biometric modality for identifying persons. Medical applied Scanning Laser
Ophthalmoscopes (SLOs) usually contain galvanometric mirror systems to move the laser spot with a defined speed
across the retina. Hence, the load of laser radiation is uniformly distributed and eye safety requirements can be easily
complied. Micro machined mirrors also known as Micro Electro Mechanical Systems (MEMS) are interesting
alternatives for designing retina scanning systems. In particular double-resonant MEMS are well suited for mass
fabrication at low cost. However, their Lissajous-shaped scanning figure requires a particular analysis and specific
measures to meet the requirements for a Class 1 laser device, i.e. eye-safe operation.
The scanning laser spot causes a non-uniform pulsing radiation load hitting the retinal elements within the field of view
(FoV). The relevant laser safety standards define a smallest considerable element for eye-related impacts to be a point
source that is visible with an angle of maximum 1.5 mrad. For non-uniform pulsing expositions onto retinal elements the
standard requires to consider all particular impacts, i.e. single pulses, pulse sequences in certain time intervals and
cumulated laser radiation loads. As it may be expected, a Lissajous scanning figure causes the most critical radiation
loads at its edges and borders. Depending on the applied power the laser has to be switched off here to avoid any retinal
injury.
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