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 .
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.
8. Schematic Vs. Real Eyes
• Schematic eye models are approximates to real eye :
a. Use only spherical surfaces ,
b. Lenses constant refractive index,
c. Known as paraxial models .
Real eyes have aspheric surfaces & a lens with a gradient
index
a. Aspherising one or more spherical surfaces,
b. Using a gradient refractive index lens .
10. • 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
11. P & P’ anterior and posterior principal points
F & F’ anterior and posterior focal points
N & N’ anterior and posterior nodal points
12. Gulstrand schematic eye
• Six refracting surfaces, 4 different refractive
indices , seprates anterior and posterior
corneal surfaces , seprates crystalline lens
cortex and nucleus
15. #2: Simplified Schematic Eye
•three refracting surfaces, 2 different indices ,
single corneal surfaces , single homogeneous
crystalline lens medium .
17. 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
18. 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)
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
o Serve as a frame work for studying the Gaussian properties
for e.g. Equivalent power & positions of the cardinal point
o Calculation of retinal image sizes
o Magnifications
o Retinal illumination
o Entrance & exit pupil positions & diameters
o Surface reflections and some of the cause and effects of
refractive errors
o Paraxial models accurately predict chromatic aberration
21. Limitations
o Approximation of real eyes that are Constructed with rotationally
symmetric sph surfaces
o Refractive index assumed to be constant
o Construction parameters mean of many individual values called as
average eye
o Very poor predictors of monocular aberrations
22. Paraxial schematic eye – Three
refracting surfaces
o 1 corneal & 2 lens surfaces
o Aperture stop placed in correct position
o 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
oPreferred for refractive error & accommodation calculations often
little gained by more complex models
23. Paraxial schematic eyes- Four
refracting surfaces
o 2corneal & 2 lens surfaces
o 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
o Adaptive eye developed; equations shows some parameters
varying with accommodation/age
24. Paraxial schematic eye- Six
refracting surfaces
o 2cornal & 4 lens surfaces
o Lens gradient index: inner nucleus & outer cortex
o Anatomically most accurate of the common paraxial
schematic eyes
o 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:
1. Retinal image sizes
2. Magnifications
3. Retinal illumination
4. Aberration analysis
5. Light level distribution
6. As a model for the design of visual optical instruments
7. Analysis of IOL’s
