Sources Of Error - Perkins Mk2 Manuel D'instructions

9.

Sources of Error

1
4.5.1 Fluorescein ring / Distance to the patient
Result
The fluorescein ring
is too wide
The fluorescein ring
is too narrow
No semi circular
rings appear, only
the centre line
Both of the too large
semi circular rings
appear partly
Fig 10
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Image in ocular Cause
The measuring prism was
not dried after cleaning,
or the eyelids came into
contact with the
measuring prism whilst
measuring.
The lacrimal fluid has
dried.
The measuring prism is
not touching the cornea.
Should the patient
withdraw their head
slightly, the irregular
pulsations will occur and
the prism will only
contact the eye
intermittently. Should the
patient withdraw the
head further, then the
fluorescein rings will
disappear altogether.
Should the tonometer be
moved forward towards
the patient, or should the
patient move towards the
tonometer whilst the
measurement is being
taken, then the feeler arm
will come into contact
with a sprung stop piece.
The applanation surface
is then too large.
en
Correction
The tonometer should be
withdrawn and the meas-
uring prism dried with a
cotton wool swab.
Let the patient close the
eyes once or twice. Then
repeat the measuring
procedure.
Steady patient's head.
The image will not
change when turning the
milled thumb wheel.
Withdraw the tonometer
until the regular pulsations
of a corresponding smaller
applanation surface
indicate the correct
measuring position and
pressure changes lead to
immediate applanation
surface changes.
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