NIH Public Access Author Manuscript J Refract Surg. Author manuscript; available in PMC 2007 January 8.
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1 NIH Public Access Author Manuscript Published in final edited form as: J Refract Surg ; 21(5): S547 S551. Influence of Exposure Time and Pupil Size on a Shack-Hartmann Metric of Forward Scatter William J. Donnelly III, MS and Raymond A. Applegate, OD, PhD From Visual Optics Institute, College of Optometry, University of Houston, Houston, Tex. Abstract PURPOSE: To determine the influence of exposure time and pupil size on a Shack-Hartmann (S/ H) derived metric of forward scatter (MAX_SD) using a physical model of nuclear cataract. METHODS: A physical model eye was developed and mounted to a S/H wavefront sensor. The eye model consisted of a lens, variable pupil, simulated cataract, and retina. Located behind the pupil, a cuvette contained one of five polystyrene microsphere solutions simulating five levels of nuclear cataract severity. Cataract severity was described using a S/H derived metric of forward scatter (MAX_SD), which measures aspects of forward scatter contained in the S/H lenslet point spread functions (PSF). To determine the impact of exposure time and pupil size, measurements of MAX_SD were regressed against cataract severity for three different exposure times and three different pupil sizes. RESULTS: MAX_SD was well correlated to cataract severity. Exposure time had the largest influence, and pupil size had the smallest influence on the forward scatter metric. When pupil size and exposure time were allowed to vary and image saturation was allowed to occur, MAX_SD explained 83% of the variance in cataract severity. Excluding images where saturation occurred, holding optimal exposure time constant, and varying pupil size, MAX_SD explained 97% of the variance in cataract severity. CONCLUSIONS: The ability of the forward scatter metric derived from S/H measurements to predict cataract severity for a longitudinal study is optimized by selecting a patient-specific exposure at the initial cataract assessment to avoid saturation and maximize the dynamic range of the system. This patient-specific exposure should be used in all future visits. At the University of Houston College of Optometry Visual Optics Institute, we are developing methods to use Shack-Hartmann (S/H) images to characterize both wavefront error and forward light scatter from the eye in one single measurement. 1 To validate and improve our techniques, we developed a physical model of nuclear cataract with controllable parameters. We simulated nuclear cataract to create forward light scatter and test the effects of pupil size and exposure time on our ability to predict nuclear cataract severity. FORWARD SCATTER VS BACKSCATTER Intraocular light scatter is light that has reflected, refracted, diffracted, or experienced multiple combinations of all three from particles along the optical path of travel. 2 If the scattered light reaches the retina it is called forward scatter. If the scattered light ends up leaving the eye before Correspondence: William J. Donnelly III, MS, Visual Optics Institute, College of Optometry, University of Houston, Houston, TX Tel: ; Fax: ; wdonnelly@uh.edu. Dr Applegate has proprietary interests in metrics of forward scatter. This work was supported in part by NIH grant R01 EY05820 to Dr Applegate and CORE grant P30 EY07551 to the College of Optometry, University of Houston, Houston, Tex.
2 Donnelly and Applegate Page 2 hitting the retina it is called backscatter. Forward scatter degrades image formation. 2 Backscatter does not degrade image formation. Backscatter decreases the available light in the retinal image. Forward light scatter causes contrast loss 3,4 and is reported perceptually as glare, halos, or coronae. 5,6 The cornea, iris, sclera, retina, and lens are all sources of light scatter. 7 Light scatter from these sources can be aggravated by ocular trauma, disease, or surgery. 6 The lens is normally the largest contributor to light scatter. 7,8 With advancing cataract, light scatter from the lens increases while other sources of scatter remain relatively stable. 7,9 As cataract severity increases, forward scatter in the lens dominates over backscatter. 9 Forward scatter has proven difficult to measure objectively. 5,10 Backscatter can be measured noninvasively in an attempt to predict forward scatter. 8,9 Methods to measure backscatter include slit-lamp imaging of cataract and grading (eg, LOCS-III) and Scheimpflug densitometry. 8,11,12 These tools (particularly the slitlamp) are commonly used to assess cataract severity. A major problem with using backscatter to predict forward scatter in cataract is that with Mie scattering, no direct relationship exists when particle size and density varies over a large range. 2,9 Researchers and clinicians are aware that backscatter measures are indirect measures of forward scatter and lacking when used to predict visual performance. 5,13,14 Thibos and Hong 15 suggested measuring forward scatter as a new application of S/H technology and Applegate and Thibos 16 were the first to report observations using a S/H wavefront sensor (SHWS) to measure forward scatter. Applegate and Thibos capitalized on the fact that in S/H wavefront sensing, light from a probe beam forms a point source on the retina. Light from this retinal point source travels in a forward direction towards the pupil. As the wavefront travels through the eye, light is scattered by the ocular media in a forward direction coming out of the eye through the pupil. It is reasoned, in patients with nuclear cataract, that forward scatter outbound over a full pupil originating from a retinal point source is reasonably equivalent (as scattering elements are fairly uniformly distributed in an anterior posterior direction) to forward scatter inbound to the retina from a distant object point source. However, it is acknowledged that it will not be exactly the same. Shack-Hartmann estimates of cortical and posterior sub-capsular cataract will have less reversibility than nuclear cataract due to their anterior and/or posterior localization. Nonetheless, it is fully anticipated that cortical and posterior sub-capsular cataract will have their unique S/H image signatures. Here we focus on nuclear cataract as a first step in a systematic development of the technique. A major goal of the laboratory is to develop a physical model that could be used to test and develop a computer model of forward scatter. A computer model that can replicate different physical eyes will facilitate development of better metrics in a time efficient manner. In developing this physical model, we took the opportunity to demonstrate the importance of proper exposure given the fact that current S/H systems purposely favor overexposure or use image processing to accentuate spot centers. MATERIALS AND METHODS PHYSICAL EYE MODEL To control all parameters during testing, we built a physical eye model (Fig 1). This system was developed in Zemax software (Zemax, San Diego, Calif) and was designed to match other researchers' modulation transfer function data on real eyes. 17,18 The main optic was a 19-mm focal length plano-convex silica lens with a diameter of 12.7 mm, n = Directly behind the lens was an adjustable pupil. To hold solutions of scattering media, an interchangeable glass cuvette followed directly behind the pupil plane. The cuvette, simulating a cataractous
3 Donnelly and Applegate Page 3 MEASUREMENTS ANALYSIS crystalline lens, was filled with various concentrations of 1-μm diameter microspheres in distilled water. 19 A cuvette path length of 5 mm was chosen to approximate a lens thickness equal to that of an older adult aged lens. 20 The retina of the system was a piece of white paper mounted to a locking micrometer drive allowing focus adjustment. Based on the work of Cox et al, 19 five levels of simulated nuclear cataract were chosen to span the range encountered in a clinical population of nuclear cataract patients. One cuvette contained only distilled water as a non-cataract configuration. Four cuvettes contained 1-μm microsphere solutions of increasing microsphere density to produce the same scatter profile function through a path of one half the length of the cuvette used by Cox et al. 19 This yielded microsphere solutions of 0.00%, 0.08%, 0.16%, 0.32%, and 0.48%. The physical eye model was mounted to the front of the SHWS, 21 and S/H images were acquired for the five cataract levels over 5-, 7-, and 9-mm pupils. Three exposure times were used: 100, 200, and 300 ms. The input source for the retinal guide star was held constant. Lenslet pitch was 400 μm and the lenslet focal length was 24 mm. 21 Cuvettes and pupil size could be changed without disturbing the positioning of the physical eye model. To reduce instrument noise, as in real eye measurements, background images were subtracted from the S/H images. Each lenslet point spread function (PSF) was individually analyzed and statistics were calculated on all lenslet data in the S/H image to extract a metric of forward scatter MAX_SD. To determine MAX_SD, the standard deviation is calculated from pixel values over a defined square area centered on a lenslet PSF peak. The sides of the square are equal to the average distance between adjacent lenslet peaks. MAX_SD is the maximum of all standard deviations from all lenslet PSFs in the image. If desired, greater detail of the calculation of MAX_SD can be found in Donnelly et al. 1 MAX_SD was regressed against cataract severity, represented as % microspheres in solution. To appropriately apply regression, the independent variable needs to be well distributed with respect to the dependent variable. To achieve such a distribution, the independent variable (cataract severity expressed as % microspheres in solution) was transformed by taking the square root of percent of microspheres in solution. OVEREXPOSED IMAGES RESULTS Overexposure caused pixel saturation in the S/H image, causing many pixels to have values near 255 (maximally white), artificially widening the lenslet PSFs (Fig 2C). To identify which S/H images were saturated (overexposed), we used an image histogram. An image histogram is a plot of the number of pixels having a particular value in a S/H image (after background subtraction) as a function of image value. Saturation can be observed in a S/H image histogram as a third peak near value 255, ie, an excessive number of pixels having many values near 255 (Fig 2A). Shack-Hartmann images without saturation (Fig 2D) do not exhibit a third peak (no excessive white pixels) in the histogram near 255 (Fig 2B). Figure 3 plots MAX_SD at three pupil diameters and increasing exposure time including data from saturated images (Figs 3A-3C) and without saturated images (Figs 3D-3F). When the regression analysis included saturated images (see Figs 3A-3C) over all pupils, R 2 is best for the lowest exposure, 100 ms (average R 2 =0.955). R 2 decreased for increasing exposure times
4 Donnelly and Applegate Page 4 DISCUSSION REFERENCES (average 200 ms R 2 =0.925, and average 300 ms R 2 =0.856). If overexposed images are excluded, low to high exposure time caused the data to shift to increased values of MAX_SD for all pupil diameters (see Figs 3D-3F). All coefficients of determination improved and were similar in value once overexposed images were excluded from the analysis (see Figs 3D-3F). The results of this study indicate that the ability of MAX_SD to predict cataract severity or forward scatter in a longitudinal study of nuclear cataract is optimized by avoiding overexposure and maximizing the dynamic range of pixel values in the S/H image at the first visit for each patient. A possible downside of this approach would be if the forward scatter decreased in severity with time, future images will likely be overexposed, for instance, following a therapy for cataract that not only slows down cataract progression but actually causes the cataract to regress. If this is a concern, instead of maximizing the dynamic range at the initial visit, a cushion to prevent overexposure at future visits should be considered in the process to decrease exposure time. In either case, once a particular exposure time is determined for an individual it should be maintained for the duration of the experiment. It is good photographic practice to neither underexpose nor overexpose an image when detailed contrast information is the goal. However, S/H wavefront sensing is relatively insensitive to overexposure and is very sensitive to underexposure. As a result, S/H images are typically overexposed, or if underexposed, image processed to add brightness to the spots. Here we wish to make the point that if good photographic practices are followed (optimize dynamic range without saturation), then scatter detection is optimized at no cost to wavefront analysis. It is useful to note that the rule for optimizing a S/H image for MAX_SD is easy to implement in software such that the user would simply need to take a few (likely one or two) S/H images and let the software adjust exposure time to optimize the image for MAX_SD extraction. The optimization parameters would then be stored in the patient record and on subsequent visits called up for use. In a SHWS, the limiting aperture is the aperture of each lenslet, not the eye's pupil. Consequently, we did not expect pupil size to have a significant impact on MAX_SD, and as predicted, it did not. Exposure time matters when extracting forward scatter metric MAX_SD from a S/H image. By properly choosing exposure time, simulated nuclear cataract is well predicted by the forward scatter metric MAX_SD. Cataract is only one application of this technique. Forward scatter caused by any of the mechanisms described in the introduction can also be measured using MAX_SD. 1. WJ, Donnelly, III; K, Pesudovs; JD, Marsack; EJ, Sarver; Applegate, RA. Quantifying scatter in Shack- Hartmann images to evaluate nuclear cataract. J Refract Surg 2004;20:S515 S522. [PubMed: ] 2. Bohren, C. Scattering by particles. In: Optical Society of America., editor. Handbook of Optics. 2nd ed.. McGrawHill; New York, NY: p D, Regan; DE, Giaschi; Fresco, BB. Measurement of glare sensitivity in cataract patients using lowcontrast letter charts. Ophthalmic Physiol Opt 1993;13: [PubMed: ] 4. PR, Herse; Bedell, HE. Contrast sensitivity for letter and grating targets under various stimulus conditions. Optom Vis Sci 1989;66: [PubMed: ] 5. van den Berg TJ, IJspeert JK. Clinical assessment of intraocular stray light. Appl Opt 1992;31:1 3.
5 Donnelly and Applegate Page 5 6. van Meeteren A, Vos JJ. Resolution and contrast sensitivity at low luminances. Vision Res 1972;12: [PubMed: ] 7. de Waard PW, JK IJspeert, van den Berg TJ, de Jong PT. Intraocular light scattering in age-related cataracts. Invest Ophthalmol Vis Sci 1992;33: [PubMed: ] 8. LT, Chylack, Jr; JK, Wolfe; DM, Singer; MC, Leske; MA, Bullimore; IL, Bailey; J, Friend; D, McCarthy; Wu, SY. The Lens Opacities Classification System III. The Longitudinal Study of Cataract Study Group. Arch Ophthalmol 1993;111: [PubMed: ] 9. FA, Bettelheim; Ali, S. Light scattering of normal human lens. III. Relationship between forward and back scatter of whole excised lenses. Exp Eye Res 1985;41:1 9. [PubMed: ] 10. DB, Elliott; Bullimore, MA. Assessing the reliability, discriminative ability, and validity of disability glare tests. Invest Ophthalmol Vis Sci 1993;34: [PubMed: ] 11. Brown N. Quantitative slit-image photography of the lens. Trans Ophthalmol Soc U K 1972;92: [PubMed: ] 12. Hockwin O. Cataract classification. Doc Ophthalmol 1994;1995: Elliott DB. Contrast sensitivity decline with ageing: a neural or optical phenomenon? Ophthalmic Physiol Opt 1987;7: [PubMed: ] 14. Whitaker D, Steen R, Elliott DB. Light scatter in the normal young, elderly, and cataractous eye demonstrates little wavelength dependency. Optom Vis Sci 1993;70: [PubMed: ] 15. Thibos LN, Hong X. Clinical applications of the Shack-Hartmann aberrometer. Optom Vis Sci 1999;76: [PubMed: ] 16. Applegate RA, Thibos LN. Localized measurement of scatter due to cataract. Invest Ophthalmol Vis Sci 2000;41:S3.Abstract Arnulf A, Dupuy O. The transmission of contrasts by the optical system of the eye and the retinal thresholds of contrast. Comptes Rendus Hebdomadaires des Seances 1960;250: Campbell FW, Green DG. Optical and retinal factors affecting visual resolution. J Physiol 1965;181: [PubMed: ] 19. Cox MJ, Atchison DA, Scott DH. Scatter and its implications for the measurement of optical image quality in human eyes. Optom Vis Sci 2003;80: [PubMed: ] 20. Glasser A, Campbell MC. Presbyopia and the optical changes in the human crystalline lens with age. Vision Res 1998;38: [PubMed: ] 21. Roorda A, Bobier WR, Campbell MC. An infrared eccentric photo-optometer. Vision Res 1998;38: [PubMed: ]
6 Donnelly and Applegate Page 6 Figure 1. The physical eye model. This assembly picture was used to model a human eye with various levels of nuclear cataract. The model mounts to the front of the SHWS. Left to right is a Lambertian reflector acting as the retina, a cuvette filled with microsphere solution acting as lens cataract, a sliding adjustable pupil, and a plano-convex lens as the main focusing optic.
7 Donnelly and Applegate Page 7 Figure 2. S/H image histograms and respective S/H images. A, B) Pixel intensity histograms are plotted for C, D) two S/H images. The highest quantity in any S/H image will be near 0 or black (1st peak) corresponding to the black background. The next highest quantity is for the pixel value corresponding to the background level within the pupil (2nd peak). 1 With saturation, there is a 3rd peak in quantity of bright pixels near 255 exceeding the 2nd peak (see 2A). Unsaturated S/H images have no 3rd peak (see 2B). The existence or absence of a 3rd peak was used as a criterion to automatically detect lenslet PSF saturation in a S/H image.
8 Donnelly and Applegate Page 8 Figure 3. MAX_SD regressed against cataract level (square root microsphere solution). Pupil diameters were 5, 7, and 9 mm (solid circles, open circles, and solid triangles, respectively) at 100, 200, and 300 ms. A-C) Graphs contain some data points derived from overexposed (saturated) images. D-F) Graphs have data points derived from saturated images removed. The regressions that contain points derived from overexposed images have R 2 that decrease with increasing exposure time. Without saturation, slopes between exposure times (D-F) remain similar, indicating that if MAX_SD is normalized such that the y intercept is 1 then each normalized value would correspond to a given level of cataract severity.
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