edge clearance


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edge clearance

A small, peripheral gap between the edge of a rigid contact lens and the cornea. It is important as it allows tear exchange and eases lens removal. The absence of edge clearance in rigid contact lenses may lead to superficial corneal damage.
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b) Vertical pooling with a small edge clearance, and horizontal touch with a large edge clearance
c) Vertical pooling with a large edge clearance, and horizontal touch with a small edge clearance
d) Vertical touch with a large edge clearance, and horizontal pooling with a small edge clearance
(9) Inferior edge clearance exceeding 200[micro]/m would allow ingress of air and probability of lens 'stand-off', subsequent displacement (and likely loss) after two or three blinks.
Keep the holes outside the 3 mm edge clearance area and keep a free space of 8 mm diameter around these holes.
* 3 and 9 o'clock staining (see Figure 2) will often be evident in patients having both 'with' and ' against the rule' astigmatism, due to a lack of lens movement caused by reduced edge clearance
* Choosing a lens with smaller total diameter will help to minimise the exaggeration between the two different meridians on the corneal surface, which in turn will help to avoid the excessive edge clearance in the steeper meridian.
The ideal fit exhibits good centration, having minimal clearance or light touch at the area of the cone, combined with good alignment over the cornea, with a band of edge clearance 0.50mm to 0.80mm wide.
* Review clearances both in terms of component clearance relative to the rest of the unit and edge clearances on the PCB assembly relative to automated handling.
Tooling and registration holes, edge clearances, component lead-hole sizes, physical component size and assembly angles were all standardized to optimize component packaging, board design and insertion throughputs.