Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

752 CHAPTER 14
In Table 14-9, we find that the MPE for the eye for 0.25-second viewing time (the
aversion response time) of a He–Ne beam is 2.5 × 10 −3 W/cm 2 . The minimum
required OD is, from Eq. (14.19):
OD = log
−2 W
E
7.8 × 10
= log
cm
MPE 2
W Example 14.14
W
2.5 × 10−3 cm2 = 1.5.
A ruby laser, λ = 694 nm, is pulsed at a rate of 2 pulses per minute. Each pulse
is 20 nanoseconds wide and contains 10 J of energy. The 1/e diameter of the exit
beam is 18 mm, and the beam divergence is 7 milliradians. What is the minimum
OD required to comply with the recommendations in ANSI Z136.1, assuming a
worst-case exposure?
Solution
The radiant exposure per pulse is
H =
π
4
10 J
J
= 3.93 .
(1.8 cm)2 cm2 From Table 14-8, we find that the MPE value for a pulse 2 × 10 −8 second wide is
H (MPE) = 5.0 × 10 −7 J/cm 2 ,
and since the time between pulses is more than 0.25 second, they are considered
singly. The minimum required OD, therefore, is:
OD = log H
2 3.93 J/cm
= log
MPE 5 × 10−7 J/cm2
= 6.9 at 694 nm.
The optimum eyewear provides maximum attenuation of the laser light while
transmitting the maximum amount of ambient light. When specifying laser safety
eyewear, therefore, the luminous transmission must be considered along with the
OD. The optimum goggle is the one that provides maximum attenuation of the
laser light while transmitting the maximum amount of ambient light. Luminous
transmission is given as the percent transmission of light from a standard source
whose spectral distribution is equivalent to that of a blackbody at 6500 ˚A. This source