D: Spectacle Frame Specification - Week 2 Flashcards
What is the key to the datum system?
The key is establishing a ‘datum line’ – a horizontal line that exists halfway b/w the top of the frame and the bottom
Define ‘datum centre’
The point half way b/w the edges of the lens (on datum)
What are ‘eye size’ and ‘bridge size’ in the datum system?
Eye size = the width of the lens measured at the datum line
Bridge size = the separation of the lenses at datum line (i.e. width across bridge)
How does the boxing system work? How is the bridge size defined here? What about eye size?
2 boxes formed by a tangent and parallel line split into quadrants cover each lens, the bridge size is the distance between the boxes. Eye size is the width of box (+1mm)
Where is the geometric centre in the boxing system? Why would this not necessarily equal the centre of the datum system?
At the centre of the bisecting lines. Likely different to datum because boxing system considers asymetrical shapes more
In what scenario would both the datum and boxing systems have the exact same geometric centre?
When the frame is uniform shape and equidistant
Why must you add 1mm to all our measurements on the eyewire in both boxing and datum systems?
To account for the bevels (0.5mm each side)
What are “effective diameter” and “frame size”?
Effective diameter: is twice the longest distance from the geometric centre to farthest rim (peak of bevel that is furthest away)
Frame size: (or “frame pd”) is eyesize + bridge size
How do we measure bridge size (a.k.a “distance between lenses”)? (how does this apply for each system?)
is measured as the distance between the INSIDE grooves in the frame rim (eyewire)
(datum: distance along datum line. boxing: smallest separation distance found)
What symbol do we typically use to notate the use of the boxing system? Give an example of boxing data and compare to datum
a box symbol (o pretend this ‘o’ is a square)
e.g. 52o20 140 MW Saturn Brown – boxing system
52/18 140 MW Atlas Grey – datum system
(note: 52 = eye size, 20 = bridge size, 140 = temple length)
Why is (horizontal) centration important?
- to prevent distortion of visual scene (with advanced lens designs)
- to avoid unwanted prismatic effect from a mismatching of centration distances and pupil centres (particularly at higher Rx powers)
(p=cF)
Name 3 apparatus for measuring PD
- PD ruler (monoc/binoc)
- Pupillometer
- pen torch/ophthalmoscope
Name 3 landmarks we can use for measuring PD
- pupil centre
- lateral limbus (outer to inner limbus)
- corneal reflex
What kind of errors can occur when taking PD with a PD ruler? (3-5)
(Parallax errors):
- optom doesn’t close non-observing eye
- optom + px not directly in line
- not moving pen torch to be in line with eye
(Ruler errors):
- not straight or close enough to spectacle plane
- movement of zero position when swap eyes
What kind of errors can occur when taking PD with a pupillometer? (1-2)
- pupillometer not steady or tilted or positioned properly on px nose
