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3.5
Sources of error
Images in the ocular
Wrong fluorescein ring . . . . . . . . . . . . . 13 - 14
Wrong distance to the patient. . . . . . . . 15 - 16
Position too much at right / left . . . . . . . 17 - 20
Position too high / low. . . . . . . . . . . . . . 22 - 25
Inaccurate pressure . . . . . . . . . . . . . . . 27 - 29
The fluorescein ring is too wide (13)
The measuring prism was not dried after cleaning, or the
eyelids came into contact with the measuring prism whilst
measuring.
The slit lamp should be withdrawn and the measuring
prism dried with a cotton wool swab.
The fluorescein ring is too narrow (14)
The lacrimal fluid has dried.
Let the patient close the eyes once or twice. Then re-
peat the measuring procedure.
No semi circular rings appear, only the centre line (15)
The measuring prism does not touch the cornea! Should the
patient withdraw the head slightly, then irregular pulsations will
occur and the measuring prism only contacts the eye intermit-
tently. Should the patient withdraw the head further, then the
fluorescein rings will disappear altogether.
Possibly employ a headband
on the patient.
Both of the too large semi circular rings appear partly (16)
Should the slit lamp be moved forward towards the patient,
or should the patient move towards the slit lamp whilst the
measurement is being taken, then the feeler arm will come
into contact with a sprung stop piece. The applanation sur-
face is then too large.
The image will not change when turning the measuring
drum. Withdraw the slit lamp until the regular pulsations
of a corresponding smaller applanation surface indicate
the correct measuring position and pressure changes
lead to immediate applanation surface changes.
13
14
15
16
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