Optical System Of Eye
- 1. BY- NAMAN JAIN BSC OPTOMETRY OPTICAL SYSTEM OF THE EYE
- 2. INTRODUCTION •Eye is a compound optical system. •It is an adaptive optical system. •Various elements in the path of light.
- 4. COMPONENTS 1. The Cornea 2. The Anterior Chamber 3. The Iris and Pupil 4. The Crystalline Lens 5. The Retina
- 5. The Cornea •Reasons of refraction. •Curvature. •Significant difference in refractive indices of air and cornea. •Cornea is a highly transparent structure.
- 6. The Corneal Shape •Meniscus lens. •Vertical diameter slightly less than horizontal. •Front apical radius 7.7mm = 48.83 D •Back apical radius 6.8mm = -5.88 D •Actual refractive index of cornea = 1.376 •Power of Cornea = +43 D •Not optically homogeneous.
- 7. The Anterior Chamber •Cavity between cornea and iris. •Filled with aqueous humour, water content 98%. •Depth of A.C. about 2.5 – 4.0mm. •Refractive index of aqueous humour – 1.336
- 8. The Iris and Pupil •Regulate amount of light entering of eye. •Effect on quality of images. •At 2.4mm pupil size, best retinal image obtained as aberration and diffraction are balanced. •Size can be effected by external or secondary agencies – Drugs, Emotions, etc.
- 9. Pupil Size and Effects • Pinhole aperture if placed, size of blur circle reduce. - Uncorrected myopia individual squint to obtain a pinhole effect. • Pinhole improvement- Possibility of corneal/lenticular light scattering, irregular astigmation. • Average size: 2.4mm • Small pupil : Depth of focus increases. Concept used as pinhole test in refraction. • Large pupil : Retinal image quality improves. Size of blur circle increases.
- 10. The Crystalline Lens • Thickness Birth 3.5 - 4mm. Adult 4.75 – 5mm. • Radius of curvature Anterior surface 10mm. Posterior surface 6mm. • Refractive index of lens Nucleus 1.41 Pole 1.385 Equator 1.375 • Total power. 15 – 18 D • Accomodative power At Birth 14 – 16 D At Adult 7 – 8 D At 50 years 1 – 2 D
- 11. Accomodation • Provide a mechanisms of focusing at different distance. Optical changes in cataractous lens • Visual changes acuity reduction. • Myopic shift. • Monocular diplopia. • Glare. • Colour shift. • Visual field loss. Vitreous • Refractive index same as a aqueous.
- 12. The Retina •Maximum resolving power of fovea. •A concave spherical surface with r = -12mm •Advantage of curvature of retina over plane image forming surface of camera as and optical instruments. •The curved images formed by the optical system is brought in the right order. •A much wider field of view is covered by the steeply curved retina.
- 18. Eye as a Camera • Eye lids Shutter • Lens Focusing system • Cornea Focusing system • Iris Diaphragm • Choroid Dark chamber • Retina Light sensitive
- 20. Transparency • During early stages – Opaque. • Later on becomes transparent. • Avascular. • Absences of chromophores. • Presence of highly organised structure. • Perfect packing of crystalline cause minimal light scatter. • Newly formed cortical fibers do not scatter light. • Regular arrangement of lens fibers with minimal intracellular space. • Combined refractive index of epithelial layer and the capsule same as aqueous.
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