5.2 - Blurred image height Flashcards
What is h’
Image size at the optical image plane
hb’
Basic blurred image height on retina - less clear
What happens in myopic ray diagram including blurred image size and image size at optical plane
- Sharp image focused infront of retina and has height which we can calculate
- By the time that image height reaches retina, not sure what it is because in diagram central ray reaches retina at one speicfc height but rays from top and bottom of pupil reach retina at other heights = point at top of object is imaged as a blur circle on retina
What happens at the retina for myopia and hypermetropia
Point at top of object is imaged as a blur circle on retina
What is Basic blurred image height
Height of the principal (chief) ray at the retina
What is chief ray
The ray that passes through the centre of the entrance pupil ie. The central ray – don’t want extrme rays, this is good ray – average
Where is hb’ located
On retina
Where is h’ located in myopic eye
Sharp image height ( h’) formed in front of the retina
What is equation of basic blurred image height for distance vision
h’b = h’ ( Fe / K + Fe )
What is equation for image height h’ for distance object
h’ = -n tan x / Fe
What is the correction factor in the equation of basic blurred image height for distance vision
Fe / K + Fe
What do you need to do 1st to determine basic blurred image height
Need to work out optical image height h’
What does letter recognition depend on
The size of the blur circle
What is blur ratio
- Blur-circle diameter / Basic blurred image height on
retina - It is a comparison between blur circle and basic blurred image
Height on retina
Why is it good to workout basic blurred image height on retina
As optical image height not available to eye because its not on retina
Why is blur ratio useful
Can see how actual visual performance behaves when using blur ratio - what level of visual blur can out system tolerate and still recognise letters
What is vision when over 1D of ametropia
Vision falls to 6/24 level
What is vision when 2D of ametropia
Up to 6/60 level
What letters blurred ratio can we recognise for modest levels of spherical ametropi
1
What is blur circle size same as
As letter size ( not limb/gap width ) = some info still there in blurred letter that allows us to recognize it
What is blur ratio equation for distant object
BR = -pK / tan x
- p = pupil size
- K = ocular refraction
- x = angular subtend of letter height NOT gap or limb width e.g. 6/6 – 5’,
What does blur ratio depend on
Ocular refraction and pupil diameter
these are at back of eye – inaccessible to us
What is blur ratio on projected blur circle on 6/60 letter
1
Blur circle diamtert is equal to height of letter
What is projected blur
Project the retinal blur circle back on to the object in object space
What is the actual blur and where is it located
Blur circle diameter
On back of eye
What happens in projection of blur circle
- Refraction happens at vertex of reduced eye ( angle theta become theta’ smaller value as its refracted – both rays refracrted )
- Image forms on retina at distance of k’ but it’s a blur circle because image out of focus on retina
- You project that blur circle diameter back on object at which point it becomes projected blur pb
What is equation for projected blur
pb = DpK
Why is n theta = n’ theta’
BECAUSE DEALING
WITH PARAXIAL APPROXIMATION SO EVEN THOUGH
BLURRED IMAGERY, ITS STILL IDEAL PARAXIAL IMAGERY FORMATION)
Where does refraction occur and what does this mean
- At surface of reduced eye
- So link between blurred circle diameter on retina and projected blur on object space at distance D
Why is projected blur interest to us
- Because get people to put markers on that blur to get them to perceive how big the blur is – present blur to them
- Can be used for comparison with an experiment of perceived blur – how big they perceive blur
What will object with projected blur circles superimposed be
The object with projected blur circles superimposed will only be a scaled version of the image – BLUR CIRCLES ON TOP OF IMAGE – link between 2
Why might actual perceived blur be different from our values (geometrically calculated )
Geometric approaches are simplistic: edges of real blurred objects are unclear so to assess blur on object is difficult as it becomes difficult to know where edge of that object is and the objects often appear to have structure possibly from diffraction in the eye – lens itself – its 1st apporximation
What letters with blur ratios can be recognised
Letters can be recognised with blur ratios as high as 1 provided level of ametropia is low to moderate
What does Blur ratio and projected blur provide
A first approximation to understanding letter recognition with blur present
