Clinical Guide
Smart532
™
A-28 UM-1151440EN, Rev. C, May 2016
having to pass through the gas bubble, it makes the laser treatment
easier.
● A third potential advantage over conventional slit lamp delivery
systems is the fact that the LIO does not employ a contact lens to
direct the laser beam. Because there is no contact with the eye, it may
be possible to treat eyes that are injured and cannot have any force
exerted on them.
● The LIO can also be used with scleral depression in place of, or as an
adjunct to cryotherapy. It has been theorized that the use of
cryotherapy scatters cells from the retinal pigment epithelium, which
may lead to proliferative vitreoretinopathy. There is no evidence that
laser treatment scatters these cells, and so it may be an improved
treatment modality for retinal reattachment procedures.
● The laser spot size can be changed by varying the distance between the
LIO and the condensing lens, and by varying the distance from the
condensing lens to the retina. Both of these distances are limited if a
clear view of the fundus is to be obtained. However, the ability of the
user's eyes to accommodate changing focal length can have an effect
on the laser spot size if the user is not aware of this phenomenon while
adjusting the image of the retina.
● Because this condensing lens is the last optical element that the laser
beam passes through, the power and position of this lens will have an
effect on the laser beam in terms of spot size and the focus of the
image of the laser beam on the retina.
● For most eyes, the image of the retina formed by the hand-held lens is
real, inverted, and located a few centimeters above the hand-held lens.
The magnification, M, of the two-lens system created by the hand-held
lens and the eye is:
=
60
Where 60 diopter is the refractive power of a “standard” eye. The
magnification of an indirect ophthalmoscope using a 20-diopter hand-held
lens is 3X, and the magnification of the system when using a 30-diopter
lens is 2X.
Because the laser beam is a converging beam, the size of the laser beam in
the retinal image plane can be varied by moving the point of focus of the
laser beam out of the laser beam's object plane (retinal image plane).
Figure A-1 shows how altering working distance affects retinal spot size.
Note that because the laser beam divergence angle of small spot sizes is
greater than the laser beam divergence angle of large spot sizes, working
distance has a greater effect on the small spot sizes.
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