Box System of Measurements
- 2 sets of number used
- horizontal lens size + bridge size = box centre
- bridge size is the distance between both lens frames
Frame Markings
1) horizontal lens size x distance bet lenses (mm) eg 50x18
2) Total length of side (mm) eg 135
3) Manufacturer/design/suppliers name or trademark
4) Frame model
5) Frame colour
Effective Lens diameter
Distance from centre of lens to widest part of lens multiplied by 2
Field of View
=total area visible through the lens when the eye is in its primary position
- positive lens means FOV is less
- negative lens increases FOV
Apparent FOV
Angle subtended by the empty frame aperture at the eyes centre of rotation
Real FOV
FOV obtained when a spectacle lens is glazed into the frame
FOV is influenced by:
- Frame and therefore lens shape
- vertex distance
- power of the lens (real FOV only)
Near Vision Effectivity Error
-mainly effects people who wears positive lens
- back vertex power of lens - vergence which leaves the lens when light originates from a distant object
near vision - light not arriving from the lens at near also depends on thickness and form of lens
-vergence leaving the lens at near also depends on thickness and form of lens
- lenses with same back vertex power but of diff forms are not interchangeable
NVRE = diff bet the image vergence produced by a thick lens at near and the expected vergence from a thin lens
Lens Mountings
can be:
- full rim
- rimless
- supra
Lens edge
will be different depending on the frame mounting type, to allow the lens to remain securely in the frame:
- bevel
- supra
- flat
- mini bevel
Frame Bridge
= the bridge forms the main connection bet the right and left rims. It may or may not have pads
- diff designs for plastic and metal frame s
Frame Pads
=extensions or attachments to the bridge which form the bearing surface of the frame for the nose
-pads for a fixed pad bridge frame are moulded into the rim and are the same material as the frame itself
Bridge types
Fixed pad bridge - plastic frames only, moulded pad shapes
Regular bridge - plastics frames only, no moulded pad shapes
In Metal frames, pads are:
pads are on arms
- pads on arms can be screw in or push in
- made from silicone (flexible lightweight, soft) or vinyl (lightweight, rigid, cheaper)
- can be saddle/comfort/strap bridge with same material and attachment
Sides
can be drop end, loop end, curl sides
