lecture 30 - scattered light in the eye

Question 1

what is criteria for assessing discomfort glare?

Answer 1

• complete loss of spatial resolution accompanied by perception of expanded edges and boundaries
• glow within the glare source which wipes spatial detail
• once “glimpsed” the subject pre-attentively avoids looking directly at the source, this is because long exposure to bright light is likely to cause retinal damage
• the source is too bright with respect to surrounding objects and background

Question 2

what is disability glare ( i.e. loss of contrast ) caused by ?

Answer 2

• caused by forward light scatter in the eye

Question 3

what does forward light scatter enhance?

Answer 3

• forward light scatter enhances significantly the perception of discomfort glare

Question 4

how does amount of scattered light change with age?

Answer 4

scattered light spreads further out and is significantly larger in older subject

Question 5

do you need to have bright source to experience disability glare?

Answer 5

• in order to experience disability glare, you don’t really need to have bright sources of light in the visual field because every object in the scene produces scattered light

Question 6

what is the reason for perceived blurring of the image?

Answer 6

• as you add more scattered light, as result of each point scattering light in all directions , you can see that the sharpness of the edges disappears , and can no longer fine detail which is due to low pass spatial filtering so the only thing that remains in the image are low spatial frequency and that is the reason for the perceived blurring of the image

Question 7

how can the effect of scattered light on the quality of an image be described?

Answer 7

-the effect of scattered light on the quality of an image can be predicted using a point spread function

Question 8

what is point spread function ?

Answer 8

• the image of a point source
• the normal spread function extend over plus/minus 6 and +/-10 min of arc
• whereas when you add the scattered light to the point spread function you can go up to 11 degrees where you get significant amount of scattered light

Question 9

what is convolution and what do you do to calculate it ?

Answer 9

the process of achieving a representation of what the image looks like is called convolution

• so what you do is you have to convolve the light distribution in the object with point spread function of the eye- this means effectively shifting the peak of the point spread function point by point over the whole image
• for each point you sum up the light on the envelope and you plot it
• it predicts accurately what the image would look like with this type of PSF

Question 10

what is forward light scatter?

Answer 10

• light spreads out much further out than expected on the basis of pure aberration of image and this is what one can describe as forward light scatter in the eye

Question 11

what happens when we add scattered light to high contrast ?

Answer 11

• with high contrast, then the addition of increased levels of scattered light wipes out fine edges and contours the sharpness of these letters but can still read letter with some difficulty even when they are significantly blurred

Question 12

what happens when you add scattered light to low contrast?

Answer 12

• with lower contrast then 100% (e.g. 15% contrast ),then you add scattered light as a result of structural changes in the cornea and the lens
• you get to the point where all the spatial detail is wiped out

Question 13

how do we measure forward light scatter in the eye?

Answer 13

• ideally measure the PSF with scatter but it is difficult , this is because you need a point source and then you need to measure how much light is scattered at a certain eccentricity away from the point source
• in order to get large amount of scattered light at eccentricities as large as 10 deg you need excessive retinal illuminance which could cause retinal damage so this is not the preferred technique
• using an annuli, the advantage of the annulus is that it consists of many points which are all on a circle centred at the point of interest
• each point contributes scattered light and each point is the same eccentricity with respect to centre, so it is much easier to experience and to measure the effects of scattered light
• we can put a detector at the back of the eye, we use psychophysical technique which allows us to measure the amount of light scattered form the annulus that ends up at centre

Question 14

what happens if we modify the luminance of the annulus?

Answer 14

• when you increase the luminance, becomes very bright and then we take it to 0 and then becomes bright again and then back to 0 and so on
• when it is bright it produces a certain amount of scattered light over dark disk
• when it’s 0 it produces no scattered light
• this means that subject will experience very faint flicker depending on the amount of scattered light in their own eyes over the dark disc

Question 15

what can we modulate sinusoidally at the same time the scatter source is modulated sinusoidally?

Answer 15

• at the same time the scatter source is modulated sinusoidally, we can also modulate sinusoidally the luminance of the screen over the black disc
• if its not completely zero, there is a small luminance which we can modulate
• when the scattered light is maximum, the screen modulation is zero
• when the scattered light is 0 the screen modulation is maximum

Question 16

how do we achieve direct measure of the amount of scattered light in the eye?

Answer 16

• when the scattered light that comes from the scatter source, which illuminates the dark centre of the annulus is matched by the modulation of the screen, the two add together and there is no modulation
• the subject perceives no flicker
• by altering the luminance of the screen, we can get to a point where there’s no longer any flicker and that is a direct measure of the amount of scattered light in that eye

Question 17

what we do with different annuli sizes ?

Answer 17

• we can employ a number of annuli of different sizes, and for each one of those we can measure the amount of scattered light produced at that eccentricity

Question 18

when is the largest amount of scattered light produced?

Answer 18

the largest amount of scattered light is produced by the smallest annulus because the centre dot is closest to the bright points on the annulus

Question 19

what happens when the annulus is large ?

Answer 19

• theres very little scattered light

Question 20

how do we use the equation
light scatter equation
Ls=k x E x Θ ^-n to obtain the curve?

Answer 20

• the curve is obtained by plotting the measured scattered light as a function of the annulus eccentricity

Question 21

what happens when we log both sides of the

Ls=k x E x Θ ^-n equation?

Answer 21

• we can take the log of both sides of the equation and we end up with
  log Ls = log (E.K) - nlogΘ
• this allows us to plot log Ls against logΘ
• we get a straight line

Question 22

what is the definition of light scatter equation parameters?

Answer 22

• Ls (Θ) = scattered light at angle Θ
• n - the scatter index
  ( determines angular distribution of scattered light )
• k - constant - directly proportional to the amount of scattered light
  k is larger in older subjects compared to younger subjects
• k’ - area under light scatter function integrated from 2 to 90 degrees
• proportional to total amount of scattered light
• measure of total amount of scattered light

Question 23

how does scattered light relate to “discomfort” glare?

Answer 23

• the larger the amount of scattered light in the presence of bright sources, it enhances discomfort glare because it spreads over larger regions of the visual field and wipes out the perception of any spatial detail

Question 24

what are the scatter test parameters for 41 yrs old?

Answer 24

• n:2.06, SD: 0.07- parameter n is relatively steady , with small standard deviation
• k:12.75, SD: 4.15- the value of standard deviation increases
• K’:5.25, SD:0.27- the value of standard deviation decreases

Question 25

how does the scatter index n vary as a function of age?

Answer 25

• n remains relatively constant until one gets about 40 to 45 years when it starts decreasing

Question 26

what happens when the value of n decreases?

Answer 26

• light is scattered further out and can cause more severe loss of visual function

Question 27

how does the parameter k’ vary as a function of age?

Answer 27

• constant until 45 - then increases rapidly

- this suggest the total amount of scattered light increases significantly above 50 years

Question 28

what is the relationship between age and Ls ( scattered light) ?

Answer 28

• as we increase the age , the scatter function moves up

Question 29

how is contrast sensitivity measured?

Answer 29

• contrast sensitivity is measured with sinusoidal gratings
• we vary spatial frequency
• for each spatial frequency we measure the smallest contrast subjects needs to resolve vertical bright and dark bars

Question 30

when is the highest contrast sensitivity obtained ?

Answer 30

• the highest contrast sensitivity is obtained for middle range spatial frequency

Question 31

what happens when we measure the same functional contrast sensitivity in a subject with increased demands of scattered light as result of keratoconus?

Answer 31

• keratoconus causes the bulging out of the cornea and corneal scarring
• there is a high loss of high spatial frequencies with relatively unaffected low frequencies
• right eye has better cs than left

Question 32

what do we get when measuring subjects scatter function?

Answer 32

• when we measure the subjects scatter function we plot log-log scale we get straight line
• both eyes exhibit larger amounts of scattered light
• the right eye shows less scatter

Question 33

• what is contrast sensitivity and scatter function for 6/6 patients?

Answer 33

• left eye scatter function spreads further out and smaller levels of scattered light
• severe loss of sensitivity for higher spatial frequency
• the subject is unable to see any spatial detail even at 100% contrast in these grating for spatial frequencies that approach 20 cycles per degree

Question 34

how does forward light scatter affect retinal sensitivity ( as a result of adding veiling glare ) over the retina?

Answer 34

• when turn glare source on , a vail of glare is added over the letters , so the contrast of the nearest letter to the glare source will be decreased most, with less effect on subjects further away

Question 35

• k=15 n =2
• subject view landolt C ( 3 min arc gap) , contrast of ring is 12%, ring is darker than background so contrast must be -0.12
• luminance of background is 10cd/m^2
• subject requires 8% contrast to see the gap for this ring size
• glare source is added 5deg away from the ring and generates pupil plane illuminance of 16lm/m^2
• calculate the veiling luminance over the ring and hence compute the expected change in the luminance contrast of ring
• is the gap still detectable by the average young eye?

Answer 35

• you light scatter equation
  Ls = K x E x θ ^-n
• use θ=5

1. calculate luminance of letter by using contrast equation
   c= Ll/Lb -1

Ll = 8.8cd/m^2

2. use θ= 5, 10 and 15

Ls = 15 x 16 x 5^-2=9.6 cd/m^2

3.calculate luminance of letter with glare source on
L’l = 8.8+9.6=18.4

4. calculate luminance of background
   Lb’ = 10+9.6 = 19.6
5. hence C’ = -6.12%
   -0.06
   . too small to detect position of gap or to name letter correctly