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Progressive Addition Lenses 163

lenses. The latest design from Essilor’s Ipseo takes into account the actual
degree of head and eye rotation that the wearer employs when viewing
through the intermediate and near zones of the lens. Research has shown
that each individual has specific head and eye movement behavior, which
can be measured and the progressive surface can be designed to incorporate
the wearer’s behavior. This principle has been incorporated into the
personalized progressive design from Essilor-the Varilux Ipseo.

Fig. 11.42: Vision print system
A new instrument the vision print system as shown in the Figure 11.42,
has been developed to determine eye/head movement for a specific
individual, the result of which can be incorporated into the progressive
surface. The system consists of three lamps – the central lamp is viewed at
40 cms from the centre of the wearers forehead, with two separate lamps,
40 cms on either side of the central lamp. The wearer is directed to look at
the lamp, which is illuminated and their head movement is recorded by an
ultrasonic signal that is emitted by the system and reflected by a transponder
attached to the special trial frame the wearer is wearing. As far as the wearer
is concerned, the lamps are illuminated at random following a short
15 second cycle and it continuous over a 90 second cycle which enables the
systems to calculate and display the eye/head movement ratio, along with
a consistency factor referred to as the “Stability co-efficient” (ST). ST is
used to adjust the rate of transition between the central and peripheral
zones of the lens. Most people’s eye/head ratio is relatively stable, having
stability co-efficient below 0.15. ST value 0.00 shows the high stability of
head and eyes movement. The indicator A in the instrument indicates head/
eye proportion, the number at B shows the head/eye movement ratio and
the number at point C shows the stability co-efficient. The number 0.31 in
the Figure reflects that the subject is performing 31% of the gaze movement
with their head and 69% with their eyes. The ST of 0.07 indicates high
stability of the head and eye movement behavior.