Visual acuities .

what is acuity

means shaprt or acute

sharpness of vision: VA

the capacity for seeing distinctly the details of an object

what is visual acuity

it is a measure of the abi;ity to discriminate some particular detail in the image of a test object 

could inolve presenting targets or letters ect

concept of visual angle

from persepctive of photo the cows look the same

near images look bigger when closer tha far 

bigger objects have larger reitnal image sizes than smaller ones 

but viewing distance also comes into it 

here a larger distant object and a smaller but near object have the same retinal size 

what is retinal image size determined by

determined by both object size and viewing distance 

how far away and how big the letters are

eg if small letters are far cant see but if close its visible

together these are captured by the concept of visual angle

visual angle 

the smaller the angle of VA, then px wont be able to read 

size of the target is changed , viewing distance fixed until the patient is able to read it 

when just recognisable, any smaller and they wont be able to read it 

reflects distance and letter size; bigger the angle then recognised, if smallest then is not 

different approach to VA

can also do it where the angle remains the same, but the viewing distance changes

target size is fixed 

what does visual acuity measurements involve

involves finding the size of the visual angle of the target when the task can JUST be performed

if any smaller the letter cant be recognised, any bigger then easily recognised

the letter size and viewing diatance determine the angle the letter will make at the eye

small angles mean good VA, and large angles means poor VA

what is MAR 

it is the minimum angle of resolution : measure of visual auity 

VA is specified as 1/minimum angle ( angle when just recognisable)

minimum angle given the symbol w

how the angle w is specified / measured depends on the nature of the test target, for letter targets the w is the visual angle made by the whole target

detection acuity

done until just recognisable, any thinner for that px and they wont be able to see

determined by line thickness and viewing distance

under ideal conditions w can be as low as 1 second of arc ( 1/360th degree)

VA detection can be as high as 60

VA detection of 10 means that you can detect 0.4mm dark line against a white background from a distance of around 14m

detection acuity:what is happening on the retina

the delta I : the change in intensity correspomding to the fact tht the line is dark compared to the white background

I is the intensity due to the white background

if the deta I is too small then the line is not detectable

when delta I gets bigger then the line becomes detectable : bigger dip when the line is thicker

the value of delta I depends on I, so that delta I/ I is constant : this means if I is bigger, then deltaI needs to be bigger

hyperacuities 

involves judging the location of one target element relative to the rest of the target ( localisation acuity)

vernier acuity

2 lines present: asking whether the line on the top is to the left or right and same question for the bottom

resolution acuity is the order of 0.5 to 1 minute of arc 

hyperacuity is typically <10 in visually normal individuals 

the lower the number the better the acuity since the angle is smalle

measures ability to detect extremely small differences in the position or alignment of objects 

vernier acuity

the variation in intensity in the light ditribution in the reitnal image above and below the break in the lines

as with detection acuity, when deltaI is just large enough the break in the line will be identified

when the break in the line i sjust detectable the angular separation between the lines is a measure of the venier acuity

angular because it takes physical separation and viewing distance of observer into account

resolution acuity 

targets for a resoltuin task can be dark objects on a white background or vice versa eg stars in the night sky 

eg if angle small can see 2 stars if big then will be seeing one star 

simplest target consists of 2 points of light 

they are moved closer together or further apart until the 2 points are just visible 

if any closer, only one point is seen, if any further then easy to see that there are 2 points

resolution acuity: what is the limit of resolution

defined as the minimum separation of 2 images so they are seen as separate when viewing through an optical system

each point of light forms an image on the retina. The profile of the image a point is called: the point spread function

between the points is where there is a drop in intensity : deltaI

point spread function PSF

describes how a single point of light is blurred by an optical system

is is the image that an optical system produces when it tries to image a perfect point source

depends on the shape and size of the pupil, on diffraction, aberrations and scatter

if the points are sufficiently close, then the PSF overlap on the retina

optical limits on resolution 

states that 2 points will just be resolvable by an otical system if theyre separated by not less than:

(1.22 x wavelength of the light) / pupil 

eg for a wavelength of 578nm and a pupil diameter of 3mm, the optical resoltuin is arund 50 seconds of arc 

the optical system of the eye will be able to resolve the points as seperate provided the visual angle is not smaller than 50 

if pupil is smaller, the resoltuion increases : eg like the pinhole accluder

2 point resolution task

to distinguish the points as separate, the reina mst be able to register the light level at 3 points; at the two peaks and at the trough inbetween (arrows)

one arrow represents a cone recieving the light from one point/star

middle is reciving the dip in change of intensity

3rd is recieving the light from the other point/star

if the retina is to resolve the points as separate based on the image formed on the retina, the foveal cones need to be close emough to one another so that the 3 cones correspond to locations of the peak, trough and peak above

spacing of the foveal cones

at the centre of the fovea, the centre to centre cone spacing is around 25 seconds of arc

at the resolution limit, the 3 cones that recieve light from the 2 pointd are separated by 1 lesser stimulated cone

hence the stimulated cones are 50 seconds of arc apart ( neural limit)

other resolution acuity task: landolt ring

px is asked to resolve the gap by stating its position; up down left or right

usually done if the px cannot speak 

other resoltion tasks : grating acuity 

target lines can be horizontal or vertical. lines get progressively thinner, resolution limit is reached when the lines are thinned to the extent that the orientation can just be identified 

resolution acuity: checkerboard pattern

block sizes made smaller until pattern is indistinguishable from a uniform grey pattern

cycles per degree

resolution acuity that is measured using gratings can be specifiied in cylces per degree

one cycle: width of 1 light bar

spatial frequency is a measure of how closely spaced the lines are

low spatial frequency: row 1

high spatial frequency: row 3

why does 6/6 acuity correspond to 30 cycles per degree

use the letter E to demonstrate the snellen principle : the E looks like a grating

measure acuity by measuring the gratings

1 cycle is 2 minutes of arc → as there are 60 min in one degree

so one degree will contain 30 cycles

VA is therefore 30 cycles per degree when visual acuity is 6/6

resolution acuity chart 

these targets are forced choice; landolt C has 8 possible gap orientations tumbling E has 4 

means that px can gues correctly even if they cant resolve

children or px with cognitive difficulties may confuse orientations 

learning effects can occur with repetition 

spme chapes are easier to recognise, and some depend on orientation perception

recognition acuity

test always involve the presentation of letters

recog acuity limit is set by contrast threshold

in landolt ring test there is a ¼ chance of guessing but in a letter acuity chart there is 1/26 chance of guessing

however only letters of roughly equal legibility is used- px dont know this

why we use letters for visual acuity testing

point sources and grating do not represent the type of detail that a patient normally views

it is important that the measure of acuity used is sensitive to retinal image blur: letter acuity is sensitive to blur, but some types of acuity like vernier are not