blurred image height 5.2 Flashcards
where is the height of optical image in an eye with spherical ametropia ?
. height of optical image isn’t on the retina
. the central ray reaches the retina at a particular point
. rays from top and bottom of pupil reach retina at other image height
this tell us that the point at the top of object is imaged as a blur circle on retina
what is the basic blurred image height hb’ ?
. defined as height of the principle ( chief ) ray at the retina
. the chief ray is the ray that passes through the centre of the entrance of pupil
what is the difference between blurred image height hb’ and sharp image height h’ in a myopic eye ?
hb’ formed on retina
h’ formed in front of the retina in a myopic eye
what is the expression for basic blurred image height
hb’ for similar triangles ?
from similar triangles hb’/h’ = k’/l’ = L’/k’
what is the expression for basic blurred image height hb’ for distance vision ?
for distance vision hb’= h’ ( F/k +F)
this expression tells us that basic blurred image height is effectively the optical image height h’ x correction factor
what is letter recognition depend on ?
letter recognition depends on the size of blur circle
what is blur ration ?
blur circle diameter / basic blurred image height
it also tells us what levels of blur can our visual system tolerate and still be able to recognise letters
what do the results for blur ratio show ?
as the lines get more shallow you get a higher blur ratio
- less shallow is less blur ratio
this shows that we are able to recognise letters for blur ratio of 1 for modest levels of spherical ametropia , this means that blur circle size is the same as letter size, much bigger than gap and limb width , this shows that there is some information still there within this blurred letter that allows us to be able to recognise it and therefore call it correctly
how to calculate blur ratio for distant object
e.g. test chart ?
BR = -PK/tan Ω
. can work out BR for different letters e.g. for 6m letter use 5 min of arc and for 18m letter use 15 min of arc
. depends on pupil diameter and ocular refraction
what is projected blur pb ?
it is possible to project the retinal blur circle back on the object
how to calculate projected blur pb ?
-It is possible project the retinal blur circle back on to the object
from snells law nΘ=n’Θ’
for small angles Θ=b/k’ and Θ=pb/d
pb = D x p x K
what must you know about projected blur?
. the object with projected blur circles will only be a scaled version of the image
. can be used for comparison with an experiment of perceived blur
. actual perceived blur may be different from our values
what do blur ration and projected blur provide ?
provide a first approximation to understanding letter recognition with blur present
what blur ratio must be present to recognise letter ?
letters can be recognised with blur ratios as high 1
why do we use geometric approaches ?
geometric approaches are simplistic as edges of real blurred objects are unclear and the objects often appear to have structure possibly from diffraction in the eye
