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Light Scattering in the Human Eye

Wavefront congress Athens, February 11, 2005

Tom van den Berg Netherlands Ophthalmic Res. Inst. Glare A typical situation in traffic: blinding by headlights of oncoming cars. Eye is imperfect scattering leads to retinal straylight CIE (Commission Internationale d’Eclairage): Disability Glare / Straylight PSF and the straylight parameter s

Retinal straylight is the outer skirt of the Point Spread Function (_ > 1 degree)

Point Spread Function (PSF)

no stray light

Point stray light source q (deg)

Leq = equivalent luminance PSF(θ)=Leq/Ebl Ebl = illuminance of glare source at the eye

Stiles-Holladay: PSF(_) µ 1/__ [1°<_<30°] Straylight parameter: s(_)=PSF(_)·__

- s is not very dependent of angle. - s can be used to specify the outer skirt of the PSF with a single value - normally the 10log of s is given Visual function disturbance by retinal straylight A typical situation in traffic: blinding by headlights of oncoming cars.

Corneal haze or early cataract, Young and healthy, log(s)=1.0 log(s)=1.6 1 1927-1984 Stiles, Holladay, Crawford, Vos a.o. Glare explained on the basis of an optical phenomenon: entoptic light scattering Ë retinal straylight Ë retinal sensitivity loss Ë blinding.

2 Light scattering by what? Small assumed. Strong dependence of small scattering ( of the sky); Rayleigh 1/_4 or Rayleigh-Gans-Debeye 1/ _n scattering.

3 till 1987 (Wooten&Geri review) No wavelength dependence found for retinal straylight. Large scattering elements assumed.

4 1990ies Compensation method for retinal straylight assessment. Different sources isolated: A red source of light scattering in the pigmented structures of the eye. Cornea light scattering is Rayleigh type. Lens light scattering is Rayleigh/Rayleigh-Gans type.

5 2004 Wavelength dependence measured, including darkly pigmented subjects. They show the Rayleigh type strong wavelength dependence.

6 Conclusion. Retinal straylight originates from small particle scattering in cornea and lens. Added is scattering from pigmented structures, and from aging effects (in the lens). 1/_4

Wooten and Geri 1987

“ ….., we conclude that there is no appreciable Rayleigh scatter in the human eye.

…(the scattered light) .., must be produced by particles or cellular structures substantially larger than the wavelength of light.”

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Direct compensation method for retinal straylight assessment

Ring on: straylight on fovea (fixed)

On L

t

Ring off: compensation light on fovea (variable) Off

No compensation Stimulus ring (straylight source)

Retinal modulation Direct Test field with Compensation counterphase 0 modulation in test field modulation s Suitable for routine use of retinal straylight assessment

0.5

0.45 2-alternative forced choice approach 0.4 of compensation comparison 0.35 in two half fields 0.3

0.25

0.2 Repeated measures s.d. standard deviation 0.15 in 1500 subjects

0.1

0.05

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 fraction

Market instrument for retinal straylight assessment

Compensation method implemented by Oculus in 2004 in the “C-Quant” 1 1927-1984 Stiles, Holladay, Crawford, Vos a.o. Glare explained on the basis of an optical phenomenon: entoptic light scattering Ë retinal straylight Ë retinal contrast loss Ë blinding.

2 Light scattering by what? Small particles assumed. Strong wavelength dependence of small particle scattering (blue of the sky); Rayleigh 1/_4 or Rayleigh-Gans-Debeye 1/ _n scattering.

3 till 1987 (Wooten&Geri review) No wavelength dependence found for retinal straylight. Large scattering elements assumed.

4 1990ies Compensation method for retinal straylight assessment. Different sources isolated: A red source of light scattering in the pigmented structures of the eye. Cornea light scattering is Rayleigh type. Lens light scattering is Rayleigh/Rayleigh-Gans type, depending on age/cataract.

5 2004 Wavelength dependence measured, including darkly pigmented subjects. They show the Rayleigh type strong wavelength dependence.

6 Conclusion. Retinal straylight originates from small particle scattering in cornea and lens. Added is scattering from pigmented structures, and from aging effects (in the lens). Pigmentation dependence of retinal straylight

van den Berg et al. 1991 Cornea transmittance points to Rayleigh type scattering

log(transmittance) = -0.016 – c/_4 (_ = wavelength in nm)

Triangles: average on 10 aphakic subjects.

No age effect see individual functions (arbitrary vertical position) for the oldest subject (squares, 75 years) and the youngest subject (stars, 14 years). Light scattering by isolated donor lenses (here 50 year old donor)

10° 20° 30° 50° 90° 165° forward scattering backward ~0.7 m particles : (to retina) (to slit lamp) Rayleigh-Gans Small particles Rayleigh

van den Berg et al. 1999 • Age related increase in straylight

• Population = 109 Caucasian subjects, V.A.>=1.0

• Age related increase starts early in life. This may be of particular relevance in darkly pigmented eyes, having lower than (caucasian) average straylight values. Straylight in (early) cataract

67±8-year olds with early cataract (Visual Acuity 0.54±0.20) show much increased straylight. Glare in driving often is the first complaint in early cataract. c=cortical, n=nuclear, p=post. subc. cataract parameter Log(s) straylight Log Additive behavior: retinal straylight is the sum from 4 different sources, and 2 dependencies, age and pigmentation.

1 Cornea 2 Eye-Wall 3 Lens (constant) (pigmentation) (age) 4 Fundus (pigmentation)

2 1 4 3 Physical characteristics of the 4 sources of retinal straylight

1 Cornea: Rayleigh type 4 Fundus re- scattering 1/_4 flectance adds red component

3 Lens: Rayleigh(- Gans) type scatte- ring 1/_n 2 Iris/sclera transmit red Age/cataract light diffusely dependent 1 1927-1984 Stiles, Holladay, Crawford, Vos a.o. Glare explained on the basis of an optical phenomenon: entoptic light scattering Ë retinal straylight Ë retinal contrast loss Ë blinding.

2 Light scattering by what? Small particles assumed. Strong wavelength dependence of small particle scattering (blue of the sky); Rayleigh 1/_4 or Rayleigh-Gans-Debeye 1/ _n scattering.

3 till 1987 (Wooten&Geri review) No wavelength dependence found for retinal straylight. Large scattering elements assumed.

4 1990ies Compensation method for retinal straylight assessment. Different sources isolated: A red source of light scattering in the pigmented structures of the eye. Cornea light scattering is Rayleigh type. Lens light scattering is Rayleigh/Rayleigh-Gans type.

5 2004 Wavelength dependence of retinal straylight remeasured, including darkly pigmented subjects. They show the Rayleigh type strong wavelength dependence.

6 Conclusion. Retinal straylight originates from small particle scattering in cornea and lens. Added is scattering from pigmented structures, and from aging effects (in the lens). Wavelength dependence of straylight remeasured

Pigmentation (and age) differences included, especially darkly pigmented eyes, so as to better isolate scattering in the optical media.

Data on 38 subjects. Differences found, depending on pigmentation group.

The following results are from papers in the process of publication.

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Sorry for the inconvenience Resume

1 The basic human eye (young, properly pigmented) only suffers from Rayleigh type scattering (scattering by small particles; 1/_4), located in cornea and lens.

2 Less darkly pigmented eyes (caucasian) suffer from added straylight predominantly at the red side of the spectrum, depending on their state of pigmentation, originating from fundus reflectance and eye-wall translucence.

3 With aging straylight is added of weaker wavelength dependence for particles of the order of wavelength size, located in the lens, in combination with effects of loss of pigmentation.

4 Problems in the optical media like cataract and corneal disturbance may add strong, but less wavelenght dependent components. Thank you

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