Do: Rx Analysis Flashcards
What is a myopic rx?
Light is focused before the retina “too strong or too long”
What disadvantage do hypermetropes have?
Reduced fov
What is a hypermetropic rx?
Light focuses after the retina “too short too weak”
What is with the rule astigmatism?
With-the-rule astigmatism – the vertical meridian is steepest (a rugby ball or American football lying on its side)
-cyl axis at 180
What is against the rule astigmatism?
the horizontal meridian is steepest (a rugby ball or American football standing on its end.
-cyl at 90
What is oblique astigmatism?
The steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.[2]
What is irregular astigmatism?
principal meridians are not perpendicular
Describe myopic astigmatism
One focal point is on the retina the other is in front of it
Describe hyperopic astigmatism
One focal point is on the retina the other is behind it
Describe mixed astigmatism
One focal point is in front and one behind the retina
What is compound astigmatism?
Both focal points are behind or In front of the retina
What is symmetrical astigmatism
The axis of both cyl meridians are symmetrical binocularly and approximately add up to 180
What is asymmetrical astigmatism?
Principal meridians do not add up to 180 and are not both WTR or ATR
What is aphakia
Px has had the crystalline lens removed usually due to a cataract. Px will require a lens of around +12.00 with a reading add of +3.00 if not psudophakic
What can cause a cataract?
Old age/excessive uv/congenital/trauma
What is exophoria?
Eye turns outwards when fusion is not maintained (eg other eye is occluded) divergent
What is a phoria?
A tendency for one (or both) eyes to deviate if fusion is not maintained.
What is esophoria?
The eye turns inwards when fusion is not maintained (convergent
What is a tropia?
a constant imbalance pulling the eye in or out
When would you add a prism?
1) Cosmetically eg frame is too large (best with eso hyperopes and exo myopes) aslong as the induced prism is less than the Px’s heterophoria.
2) when Px previous spectacles have been tolerated with an incorrect prism and correct OCs cause diplopia
What is heterophoria?
The Px eyes resting position (eg Px is in completely dark environment and eyes are free to remain aligned or deviate) Ve angle is aligned for a distance of 1metre
At what age does presbyopia begin?
Between 40-45 (35-40cm)
At what age does an int add become required?
50-55 (40-60mm)
What is an average bvd?
10-13mm
What is then abbé value for cr39?
56.8
What is the abbé value for polycarbonate?
30
What is the abbé value for 1.54 resin?
47
What is the abbé value for trivex?
43-45
What is the index of allyl digicol carbonate?
1.498
What is the n of polycarbonate?
1.586
What is the n of mid index plastics?
1.54-1.56 (spectralite1,54/ormex1.56)
What is the n of high index resins?
1.6-1.74. Finalite + thin&light1.6(uv) / seiko mr-10 + stylus1.67 / fusio + hyper1.74 + teslalid + protection)
What is the n of trivex?
1.53
What type of light is used to measure n in the uk?
Helium d line nm= 587.6
What methods are there to tint lenses?
Solid (tint permeates the material)/ dip dye/ bonded equitint/ vacuum
What level of v diff prism is tolerable?
1prism diopter
How do aspheric lenses prevent lens aberration?
The surface astigmatism offsets the oblique astigmatism caused by flattening the lens. If a purely spherical lens was flattened significant optical errors will occur
What rx would you consider putting in a myodisc/lenticular lens?
+8.00
Why are aspherics used at higher powers
They reduce lens aberrations that cannot be corrected with spherical lenses at this range
What to remember with toric transposition
Flip sing on working part and add to original sph
What band is UVA?
315-380
What band is UVB?
280-315
Uvc?
100-280
What is blue lights wavelength?
380-500
What luminous transmission is not acceptable for evening driving?
75%
What bs en iso applies to skiing tints?
12312
What colour tint is good for skeet/ clay pigeon shooting?
Bronze
What colour tint is useful for black and white shooting?
Yellow
Orange for orange targets
Red/pink for black and green are also good on orange and for green perception
Abs should not be too dark as large pupils affect performance (think pinhole)
What would you advise for a cricketer?
Mirror due to long uv tints vary to light levels
What lens material is good for hard ball sports?
Cr39 toughened can be used but trivex or poly are preferred
