Schematic eye

SAMAY SHAH

1. Schematic eye - Samay Shah 3 nd year , B.OPTOM BMCO, Surat

2. Presentation Layout General overview History Schematic eye Vs. Real eye Cardinal data Gullstrand schematic eye Reduced schematic eye

3. SCHEMATIC EYE IN GENRAL A schematic eye is a mathematical or physical model that represents the basic optical features of the real eye. Assume that all ocular surfaces are perfectly centred Stranded emmetropic schematic eyes are derived from the average constant of large numbers of real emmetropic eye Schematic eye have many applications, particular as teaching aids in optics , optometry , ophthalmology , psychology(vision & visual perception) and visual ergonomics .

4. Optics of the eye

5. History The first physical model of the eye – “ real eye” Christian Huygens (1629-1695) Smith in1738, described Huygens eye 2 hemisphere , cornea & retina Le Grand, Moser in 1844 was the first to construct a theoretical schematic eye First accurate schematic eye- Listing, 1851 Helmholtz developed a modified version of Listing’s eye . Tschering publish a more complex eye that contained the posterior corneal surface , he claimed to measure it first .

6. Con…. Allvar Gullstrand developed a more improved schematic eye with four surface lens & extra lens complexicity . Gullstrand's Noble prize wining work – schematic eyes bear his name . Invented slit-lamp & improved Helmholtz’s ophthalmoscope

7. Schematic Vs. Real Eyes Schematic eye models are approximates to real eye : Use only spherical surfaces , Lenses constant refractive index, Known as paraxial models . Real eyes have aspheric surfaces & a lens with a gradient index Aspherising one or more spherical surfaces, Using a gradient refractive index lens .

8. Cardinal points of the eye

9. For ideal system the basic imaging properties such as image size, location, and orientation are completely determined by the location of the cardinal points Cardinal points provide a way to analytically simplify a system with many components , allowing the imaging characteristics of system to be approximately determined with simple calculations. Mains cardinal points : principal focus principal points nodal points

10. P & P’ anterior and posterior principal points F & F’ anterior and posterior focal points N & N’ anterior and posterior nodal points

11. Gulstrand schematic eye Six refracting surfaces, 4 different refractive indices , seprates anterior and posterior corneal surfaces , seprates crystalline lens cortex and nucleus

12. #1 :Exact Eye – Thick Lens Ant. Cornea Pos. Cornea Ant. Lens Ant. Nucleus Pos. Nucleus Pos. Lens

13. Exact Eye (Thickn lens perameter)

14. #2: Simplified Schematic Eye •three refracting surfaces, 2 different indices , single corneal surfaces , single homogeneous crystalline lens medium .

15. Simplified Schematic Eye (Thickn lens perameter ) Cornea r 1 = +7.80mm position 0.0mm Anterior lens r 2 = +10.00mm position 3.60mm Posterior lens r 3 =-6.00 mm position 7.20mmn Refractive indice :- Aqueous. n 1 = 1.336 Lens. n 2 = 1.413 Vetreous. n 3 = 1.336

16. Types according to inventers Types of this eyes 1.Listing reduced eye 2.Emsely reduced eye 3.Donder’s reduced eye 4.Bennet & Rabbet’s reduced eye

17. Types of Schematic Eyes- paraxial schematic eyes Models simplified only useful in paraxial region Aberrations much greater than those of real eye -Refracting surfaces co-axial & spherical -Visual axis coincides with optical axis -Refractive index of lens is constant (usually)

18. Paraxial schematic eyes – Single refracting surface (reduced eye) Listing has simplified the schematic eye Simplest of schematic eye- anatomically inaccurate 1 refracting surface at front of eye Lens is completely neglected Results from P & P’ and N & N’ at same location Shorter length than other schematic eye -Radius of curvature : 5.7mm - an. & pos. focal length : 17.2& 22.63mm - Refractive index :1.333

19. Emsley’s reduced eye - Equivalent power : +60D - Refractive index :1.333 - Radius of curvature : 5.55* - Position of aperture stop: principal points & nodal points

20. Applications Serve as a frame work for studying the Gaussian properties for e.g. Equivalent power & positions of the cardinal point Calculation of retinal image sizes Magnifications Retinal illumination Entrance & exit pupil positions & diameters Surface reflections and some of the cause and effects of refractive errors Paraxial models accurately predict chromatic aberration

21. Limitations Approximation of real eyes that are Constructed with rotationally symmetric sph surfaces Refractive index assumed to be constant Construction parameters mean of many individual values called as average eye Very poor predictors of monocular aberrations

22. Paraxial schematic eye – Three refracting surfaces 1 corneal & 2 lens surfaces Aperture stop placed in correct position Gullstrand’s number 2 “simplified” eye as modified by Emsley - Cardinal points at reasonable locations -Accommodated from by 10.9D: ant. surface moves forward by 0.4mm surface radii decrease Preferred for refractive error & accommodation calculations often little gained by more complex models

23. Paraxial schematic eyes- Four refracting surfaces 2corneal & 2 lens surfaces Le Grand’s full theoretical eye - Relaxed form -Accommodation form by 7.1D: ant. surface moves forward 0.4mm, back surface away 0.1mm , surface radii decrease Adaptive eye developed; equations shows some parameters varying with accommodation/age

24. Paraxial schematic eye- Six refracting surfaces 2cornal & 4 lens surfaces Lens gradient index: inner nucleus & outer cortex Anatomically most accurate of the common paraxial schematic eyes Gullstrand’s data on refractive indices of the components of the optical sys. of the eye are as: Cornea : 1.376 Aqueous : 1.336 Lens cortex : 1.386 Lens core : 1.406 Vitreous : 1.336

25. Applications Framework for calculations of: Retinal image sizes Magnifications Retinal illumination Aberration analysis Light level distribution As a model for the design of visual optical instruments Analysis of IOL’s

26. REFRENCE A.K Khurana optics & refraction DUKE ELDER’S Goggle

27.  THANK YOU  The eyes are the window to your soul