Optics of normal eye Dr Cynthia Arunachalam Professor

Optics of normal eye Dr Cynthia Arunachalam Professor and Head Department of Ophthalmology Yenepoya Medical College Yenepoya University

Properties of light • Part of electromagnetic radiation • Non ionising radiation • Range of visible light 400700 nm • Visible light – VIBGYOR (passing through prisms) • Violet- longer wavelength, red – short wavelength Wavelength In phase Out of phase Amplitude frequency

Properties of light Interference • superposition of individual waves when they cross paths. • Constructive interference is when the amplitude of the resultant wave is greater than the individual waves • destructive interference is when the amplitude of the resultant wave is less than the individual waves Coherent monochromatic

Properties of light Diffraction • phenomena exhibited by light when it interacts with barriers and obstacles. • Secondary waveforms produced which are out of phase with the primary wave form – diffraction patterns

Properties of light Scattering • Deflection of a ray of light from a straight path by any irregularities in the propagating medium • Wavelength dependant • Frequency dependant

Properties of light Polarization • Only those wave forms are transmitted through a polarizing medium which are perpendicular to the direction of the polarizing substance in the medium (angle of incidence = to the polarizing angle of the medium)

Optical radiation

Absorption spectra of the eye <270 nm <315 nm&>1400 nm <350 nm ALL

Gullstrand’s schematic eye 2 principal points 2 principal foci 2 nodal points Eye is a refracting ideal spherical surface, separating two media of refracting indices of 1. 00 and 1. 33. • Ant corneal surface – spherical with ROC 8 mm • Optical axis connects the COC of cornea and lens to the fovea • •

Listing’s reduced eye • Thick lens system • One lens system with an optical centre ( nodal point) • Single ideal refracting surface with ROC 5. 73 mm • Power – 58. 6 D • AFD- 15. 7 mm • PFD- 24 mm • Principal axis – COC of the ant & post surface of the lens to fovea

Refractive media of the eye

Reduced eye • Assumes power of 60 D at the corneal surface • Anterior focal point at approx. 17 mm • Length of the eye 22. 6 mm • Nodal point 5. 6 mm behind the cornea

Reduced eye • Retinal image size may be determined easily using the reduced eye, because the nodal point is at the centre of curvature of the single anterior refracting surface

OPTICAL ABERRATIONS • Imperfections or lapses in the optical system • Though they normally exist to a small degree, functionally they are immaterial. • Affect mainly the peripheral rays

Diffraction of light • The actual pattern of a diffracted image point produced by the pupil is a series of concentric dark and bright rings. • At its centre is a bright spot known as the Airy Disc. • Diffraction blur increases with the small size of the pupil

Spherical aberrations • Spherical lens refracts peripheral rays more strongly than paraxial rays • As a result the incoming rays do not come to a point focus

Spherical aberrations Factors contributing to the diminution of spherical aberrations in human eye are 1. Lens is flatter at the periphery than the centre. 2. Central portion of the lens has greater density and curvature than the periphery. 3. Iris blocks the peripheral rays and allows only paraxial rays

Chromatic aberrations Occurs due to the fact that the ref of light through any transparent medium varies with the wavelength of the incident light. emmetropic eyehypermetropic for red and myopic for blue & green This forms the basis of the duochrome test in subjective refraction.

Chromatic aberrations in the eye is minimized by • Yellow rays form most sharply defined images on the retina. • Fovea lacks blue cones. • Narrow spectral sensitivity band of long & medium wavelength cones.

coma • This is an off-axis aberration. The light rays entering the optical system away from the middle at an angle are focused at different points than those entering the optical system on or near the optical axis. This results in a comet-like image being formed away from the middle of the image.

Sturms conoid • A lens with cylinder power produces an astigmatic focus. • A vertical focal line, corresponding to the focus of the horizontal principal meridian, • A horizontal focal line, corresponding to the focus of the vertical principal meridian.

Sturms conoid

Sturms conoid • The region between these two lines is known as the conoid of Sturm or Sturm's interval. • At the dioptric mid-point between these two focal lines, the astigmatic focus forms a circular patch known as the circle of least confusion

Sturms conoid • The location of the circle of least confusion is equal to the spherical equivalent of the prescription:

Sturms conoid • An image that falls on the retina can be thought of as being made up of many dots, just like a photo in the newspaper. • If an ametropic eye , is not optically corrected, then the image will consist of many blur circles instead of sharp dots. The more out of focus the image is, the larger the circles are. If an astigmatic eye is not optically corrected, the blur circles will be distorted into ellipses

Sturms conoid • example, if an emmetropic person is looking at a cross, it may be represented as an image with sharp dots, like this—

Sturms conoid • A minus 2 D myope, without correction, may see the image like this—

Sturms conoid • Sometimes we do not supply the patient with his full astigmatic correction: such as patients with 1. 00 D or less cylinder correction when performing a visual field exam, or perhaps the soft contact lens patient with 1. 00 D or less astigmatism in one eye. In these situations we use the spherical equivalent. • These patients will not see the sharply focused dot image. The spherical equivalent, representing the "Circle" of Least Confusion, provides a blur circle instead of a blur ellipse. The basic idea is that if the image is going to be a little blurry, it is better that it not be distorted also.