Lecture 6 Flashcards
What is Helmoltz Treatise on Physiological optics?
Physiological optics is the science of visual perception by the sense of sight and Helmoltz divided it into 3 areas:
- The theory of the path of light in the eye concerning with low light is bent within the eye
- The theory of sensation of the nervous mechanism of vision concerning sensation without taking into visual consciousness into account
- The theory of interpretation of visual sensations concerning with the impression of which we use to form visual interpretation of objects around us
How much focussing power does the cornea have?
There are two main focussing structures of the eye:
the cornea holds 2/3rd of the focussing power of the light rays; the crystalline lens does the fine focussing of the light rays
The cornea is about 0.5mm thick centrally and is composed of 5 layers
Effect of age on cornea’s refractive power?
Explain the filtering property of the cornea?
Filtering property: sunlight consists of UV (295-400 nm wavelengths), visible (400-800 nm), and infrared (IR; 800-1200 nm) light. The cornea absorbs most of the harmful UV light.
The crystalline lens
The cyrstalline lens:
- is suspended by inelastic fibres, zonules which inserted into the lens capsule
- Crystalline protein that give the lens high refractive index power
- Curvature is shaped by the lens capsule to focus light onto the retina
Unaccommodated lens
An emmetropic eye (without refractive error) should be able to focus light from a distant object onto the retina when the lens is at rest in the lens capsule
Accommodated lens
When the eye focuses at near object or during accommodation the ciliary muscle contracts and the zonule is slackened and the lens curvatures changes causing light to focus on the retina.
Accommodation amplitude declines with age due to:
- The loss of lens fibres elasticity
- loss of ciliary muscle contraction power
- Changes of zonule support
- Changes of lens capsule thickness
Role of pupil
- The pupil acts as an aperture
- The pupil regulates and diffracts light
- As scattered light eliminated the image remains sharply focused for a range of viewing distance. Therefore, as the pupil constricts it increases the depth of field.
The optical system of the eye
Refracting surfaces to be considered:
- 2 surfaces of the cornea and the lens
- Refractive index of air, cornea, aq. humour, lens and vitreous humour
Dioptric parameters
Discuss the cardinal, principle and focal points of spherical surfaces?
Spherical refracting surfaces produce certain points along the principle axis and they are known as cardinal points:
There are 3 pairs,
The 2 focal points, 2 principle points, and 2 nodal points
What happens for a thin lens?
When it is a thin lens in air then the two nodal points are the same as the principle points
Gullstrand eye model in mm
What is aberration?
Image blur can be caused by defocussed rays of light or by dspersion of light. Image blue caused by dispersion of light is called aberration.
Aberration: failure of a mirror, refracting surface, or lens to produce exact point-to-point correspondence between an object and its image
Longitudinal and lateral aberrations
Longitudinal and lateral chromatic aberration are caused by different wavelengths of light being refracted differently. Short wavelength (blue) of light is refracted more than long wavelength (red) of light
Spherical aberrations
For an axial point source of monochromatic light, wavefronts diverge from the source. The wavefront reaching the paraxial region of the lens has different dioptric power comparing to the wavefront reaching the peripheral region of the lens.
Presbyopia
Hypermetropia
It is a form of refractive error in which light from distance comes to ofcus behind the retina when the eye is at rest
Myopia
Myopia is a form of refractive error in which parallel rays of light come to focus in front of the retina when the eye is at rest
Astigmatism
Astigmatism is a refractive error condition in which the point of light can not be focussed on the retina or it can be thought of as an unequal refraction of light in different meridians.
More aberration
More aberration
