Download measuring and modelling forward light scattering in the human eye PDF

Titlemeasuring and modelling forward light scattering in the human eye
LanguageEnglish
File Size3.8 MB
Total Pages172
Document Text Contents
Page 2

2


THESIS CONTENT



THESIS CONTENT ...................................................................................................................... 2

LIST OF FIGURES ....................................................................................................................... 5

LIST OF TABLES ........................................................................................................................ 10

LIST OF EQUATIONS ............................................................................................................... 11

ABBREVIATIONS LIST ........................................................................................................... 12

ABSTRACT ................................................................................................................................... 13

DECLARATION .......................................................................................................................... 14

THESIS FORMAT ....................................................................................................................... 14

COPYRIGHT STATEMENT .................................................................................................... 15

ACKNOWLEDGEMENTS ....................................................................................................... 16

1. INTRODUCTION ................................................................................................................ 17

1.1 OVERVIEW .......................................................................................................... 17

1.2 WHAT IS INTRAOCULAR LIGHT SCATTER? ................................................ 18

1.3 INTRAOCULAR SCATTER, GLARE AND STRAYLIGHT.............................. 22

1.4 INTRAOCULAR SCATTER AND CONTRAST SENSITIVITY ....................... 23

1.5 SCATTER AND POINT SPREAD FUNCTION .................................................. 24

1.6 SOURCES OF SCATTER ..................................................................................... 26

1.6.1 CORNEA ....................................................................................................... 27

1.6.2 CRYSTALLINE LENS .................................................................................. 29

1.6.3 IRIS, SCLERA AND UVEAL TRACT ......................................................... 32

1.6.4 RETINA ......................................................................................................... 33

1.7 FACTORS AFFECTING INTRAOCULAR LIGHT SCATTER .......................... 33

1.7.1 PHYSIOLOGICAL ........................................................................................ 34

1.7.2 PATHOLOGICAL .......................................................................................... 34

1.7.3 OPTICAL ....................................................................................................... 39

1.8 METHODS TO MEASURE INTRAOCULAR LIGHT SCATTER .................... 40

1.8.1 METHODS BASED ON THE MEASUREMENT OF CONTRAST

SENTITIVITY .............................................................................................................. 40

1.8.2 METHODS BASED ON THE EQUIVALENT LUMINANCE TECHNIQUE

……………………………………………………………………………...42

1.8.3 ESTIMATIONS OF FORWARD LIGHT SCATTER FROM BACKWARD

LIGHT SCATTER ........................................................................................................ 49

Page 86

86


difficult to analyse, as information around the centre of the PSFlet is lost by

clipping. To avoid this problem, images where image histogram showed a third

peak were considered overexposed and removed from calculations (Donnelly Iii

and Applegate, 2005). In addition, some of the PSFlet may present local

saturation. It was considered that PSFlets with more than 2 saturated pixels i.e.

pixel value of 255 (the maximum value possible to be recorded on the HS sensor),

do not contribute to the scatter calculations for the whole image.

 Extracting a metric for FLS from the Hartmanngrams: Hartmanngrams from the HS

are converted into an intensity image. To reduce noise (fluctuation in the signal

produced by all electronic instruments) the lowest intensity value of the image is

subtracted to all pixel values of the hartmanngram. Subsequently, the intensity

image is thresholded to find the location of the centroids of each PSFlet and to

index them. Thresholding of the images was done empirically in order to assess

the lowest intensity pixels of the neighbourhoods (most information on FLS within

the neighbourhood is stored near the borders of the neighbourhood, where

intensity pixels are lower) after removing the noise of the image. A 13x13 pixel

neighbourhood is defined around each centroid. PSFlets with very low intensity

values with respect to the threshold or those with more than two saturated points

are not considered for calculations. Finally, the standard deviation of the pixel

intensities from each PSFlet of the Hartmanngram is calculated and the maximum

one is used as a measure of scatter. This is the metric suggested by Donnelly et al.

(Donnelly Iii et al., 2004) and used in Cerviño et al. (Cerviño et al., 2008).

Page 87

87


EXPERIMENT 1: MEASUREMENT OF FLS WITH CUSTOMIZED CONTACT LENSES

In order to validate this program, measurements were obtained with a model eye

(RME - http://www.optomshop.co.uk/Retinoscopy_Model_Eye.htm, January 2013). To

simulate different levels of scatter, different concentrations of aerosol containing

aluminium chlorohydrate (refractive index = 1.51) were sprayed over six RPG contact

lenses (Figure 3.3). The lenses were sprayed for approximately 0 (none), 0.5, 1 and 1.5

seconds at a distance of 40 centimetres. The concentration of the aerosol particles was

then measured by obtaining an image of the central zone of each lens (representing 60%

of the whole lens area) and processing it with Matlab to estimate the droplets’ density.

Images of the lenses 1 hour after spraying were also taken to assess that the density and

opacity of the droplets over the lenses remained stable.

To measure the magnitude of light scatter, the model eye was collocated in front

of the aberrometer with clamps. The end of the model eye was covered with a black

rough surface to reduce the amount of light reflected and to avoid overexposing the HS

sensor. The recording of the hartmanngrams was executed in scotopic light (less than 0.5

lux), changing only the lens holder with the contact lens in front of the model eye and

leaving the rest of the set-up steady. The HS images were obtained for the non-sprayed

lenses first and then the lenses were changed in incremental order with the most sprayed

lenses being tested in the end. Three hartmanngrams were recorded, processed and

averaged for each contact lens per level of scatter. Therefore, there were 12 images in

total, obtained with the HS sensor for each lens. The whole process was repeated for all

the six lenses.



http://www.optomshop.co.uk/Retinoscopy_Model_Eye.htm

Similer Documents