A second simulation, that more closely addresses the pulsed nature of the exposure by an SLO with a galvanometer scanner was also considered.
^{ 42 }Let the frame rate be
F Hz, and the number of raster lines used for imaging be
R. In essence, it consists of the exposure of a segment of a raster line with length α
_{S} = 2α
FRt _{min} (distance traveled during the confinement duration
t _{min} for a galvanometer driven SLO) and width the minimum size associated with the Standard (α
_{min}).
^{ 5 }This segment is traversed by
m = α
_{min} R/α raster lines during each frame (assuming the line spacing of the SLO is smaller than α
_{min}, as it usually is), and this occurs
n times (
n = FT) during the total exposure (in this study
T = 900 seconds). If the return beam blanking of the galvanometer scanner is removed, then
m must be increased by a factor 2. The ANSI Standard recommends the use of three rules to assess the safety of repetitive pulsed exposure. The rule predicting the lowest level is the limiting level. Rule 1 checks that a single pulse in the simulation is below the MPE for
t =
t _{min}. Rule 2 checks that the average power for the group of pulses is no higher than a single pulse over the entire size and duration. This rule is equivalent to the thermal and photochemical limits described by
equations A1and
A2 . Rule 3 (thermal) limits the exposure to a peak power that is (total number of pulses)
^{−0.25} times lower than the limiting peak power for a single pulse (
t =
t _{min}). For an SLO, rule 3 will always be lower than rule 1. The limiting peak power for Rule 3 in this simulation (or beam power with either no blanking or blanking depending on m) is then given by
equation A3 :
\[MPE_{\mathrm{peak,\ rule}\ 3}{=}\ \frac{1}{(nm)^{0.25}}\ 6.93{\times}10^{{-}4}\frac{C_{\mathrm{A}}C^{{\ast}}_{\mathrm{E}}}{Pt_{\mathrm{min}}^{0.25}}\ ,\]
where
C _{A} is as above,
t _{min} = 18 μs,
P = 1 (
t < 0.07 seconds).
C ^{*} _{E} is a scaling factor for the rectangular line segment and is
C _{E} (for a circular field) multiplied by the ratio of areas of the rectangular field to a circular field.
^{ 42 }Thus,
C ^{*} _{E} = 8α
_{S}/[π(α
_{S} + α
_{min})]. After substitution and rearrangement, one obtains the MPE for a galvanometer SLO with no blanking of the back scan:
\[MPE_{\mathrm{peak,rule}\ 3}(W){=}4.94{\times}10^{{-}7}R^{0.75}\frac{C_{\mathrm{A}}{\alpha}^{1.25}F^{0.75}T^{{-}0.25}}{(1{+}2.4{\times}10^{{-}5}{\times}R{\alpha}F)}\ .\]
This is the limit for the peak power in the simulation or the laser power at the pupil. Results for the MPE average power are calculated and given in the
Table A1for the AOSLO with
R = 512 raster lines and
F = 27 Hz.