Welch Allyn 18200 Manual Del Usuario página 10

All manuals and user guides at all-guides.com
2. Astigmatism can be measured in one of two ways:
a) Neutralize one principal meridian first. Then add the
appropriate plus or minus cylindrical lens until the
other principal meridian is neutralized.
b) Neutralization may also be done by continuing to add
spherical lenses until the second principal meridian
is neutralized. Then the astigmatic error is equal to
the difference in strength of lenses necessary to
neutralize the two meridians.
5. Special Considerations
A) Axis of astigmatism: Extreme care must be used in setting the
axis of the cylinder. If the correcting cylinder is of the proper
power, a 10° error in axis will produce a new astigmatism
of approximately one third of the strength of the original
astigmatism with its principal meridian at approximately 45°
to those of the original astigmatism. The technique for setting
the axis is referred to as "straddling". When you have an
approximate correction of the refractive error and wish to
refine the axis setting, the following technique will be helpful.
Move up closer to the eye so that the edges of the reflex can
be seen, and compare the widths of the two reflexes as you
rotate the streak 45° to either side of the correcting cylinder
axis. Recede slowly while doing this. Compare the widths of
the two reflexes. If there is an axis error, the reflex will be
different widths in the two positions. If you are using plus
cylinders, rotate the axis toward the narrow band until the
reflex widths are equal. With minus cylinders, move the axis
away from the narrow band. When the reflex widths are
equal, the proper axis has been determined. It is important
that the spherical and cylindrical strength be checked again
after completion of this maneuver.
8