Physiological Optics

1. Dr. Mohammad Shehadeh 2nd lecture Physiological Optics

2. Clinical Testing of Colour Vision • Clinical tests of colour vision are designed to be performed in illumination equivalent to afternoon daylight in the northern hemisphere.

3. Farnsworth-Munsell (FM) hue 100 • test is the most comprehensive method. It comprises 84 coloured discs, numbered in sequence on the undersurface and divided into four groups of 21 • The colours of each group occupy a portion of the colour spectrum.

4. The colours differ only in hue and have equivalent brightness and saturation. • Each group must be arranged in a row with the reference colours at each end and the intervening discs in order of closest colour match. • The order of placement indicates the nature of the colour defect.

6. The D-15 test • The D-15 test uses colours from all parts of the spectrum which must be arranged in order from a single reference colour. The test does not distinguish mild colour defects, but for most purposes those passing the test are unlikely to have problems with hue discrimination.

7. Go and check ur self • http://www.colblindor.com/color-arrangement-test/

9. Ishihara pseudoisochromatic test plates • specifically test for congenital red-green defects, the most common abnormality of colour vision. • The test plates consist of random spots of varying isochromatic density. Numbers or wavy lines (for illiterates) are represented by spots of different colours. • A patient who is colour blind will see only a random pattern of spots or incorrect numbers. • The figures can only be distinguished from their background by their colour and not by a difference in contrast.

10. http://colorvisiontesting.com/ishihara.htm#plate with 10 answer

11. Ultraviolet Light • The retinal photoreceptors are also sensitive to wavelengths between 400 nm and 350 nm in the near ultraviolet (UV-A). • These wavelengths are normally absorbed by the lens of the eye. In aphakic eyes or pseudophakic eyes with intraocular implants without UV filter, such UV radiation gives rise to the sensation of blue or violet colours. • Newly aphakic patients frequently remark that 'everything looks bluer than before the operation'.

12. The bright illumination employed in modern ophthalmic instruments may also cause retinal damage under some circumstances. • Prolonged exposure to high intensity indirect ophthalmoscope illumination, intraocular light pipe illumination and operating microscope light is potentially damaging to the retina, which may in many instances already be unhealthy. • Some instruments have yellow filters built into them to reduce exposure to the most damaging wavelengths.

13. Fluorescence • Fluorescence is the property of a molecule to spontaneously emit light of a longer wavelength when stimulated by light of a shorter wavelength. • For example, the orange dye fluorescein sodium when excited by blue light (465-490 nm) emits yellow-green light (520-530 nm)

15. Fluorescine angiography principles • Fluorescein angiography allows the state of retinal and choroidal circulation to be studied by photographing the passage of fluorescein through the vasculature after it has been administered systemically. • White light from the flash unit of a fluorescein camera passes through a blue 'excitation' filter to illuminate the fundus with blue light . • The wavelengths transmitted by the excitation filter approximate to the absorption spectrum of fluorescein.

16. Most of the light is absorbed, some is reflected unchanged, and some is changed to yellow-green light by fluorescence. • The blue reflected light and yellow-green fluorescent light leaving the eye are separated by a yellow-green 'barrier' filter in the camera. • This blocks blue light and exposes the camera film only to yellow-green light from the fluorescein, thereby delineating vascular structures and leakage of dye

18. Wave Theory of Light • The path of light through an optical medium, e.g. glass, is always straight if no obstacle or interface between optical media is encountered. • Diagrammatically light is represented as a straight arrowed line or ray. • some experimental observations of the behaviour of light are not fully explained by the simple concept of light as rays, and it is now understood that light really travels as waves although its path is often represented as a 'ray'.

19. Figure 1.5 illustrates the different ways of depicting the progress of light away from a point source. • Figure 1.5a shows the light as rays; • Fig. 1.5b shows the wave motion of each ray, • Fig. 1.5c illustrates the wave front set up by the combined effect of many rays, the concentric circles being drawn through the crests of the waves.

20. Wave motion consists of a disturbance, or energy, passing through a medium. The medium itself does not move, but its constituent particles vibrate at right angles to the direc-tion of travel of the wave (Fig. 1.6). • (Imagine a ribbon tied to a rope along which a wave is 'thrown'. The crest of the wave moves along the length of the rope, but the ribbon moves up and down at one point on the rope.)

21. The wavelength, λ, is defined as the distance between two symmetrical parts of the wave motion. • a cycle :One complete oscillation, and occupies one wavelength. • The amplitude, A, is the maximum displacement of an imaginary particle on the wave from the base line. • Any portion of a cycle is called a phase. • If two waves of equal wavelength (but not necessarily of equal amplitude) are travelling in the same direction but are 'out of step' with each other, the fraction of a cycle or wavelength by which one leads the other is known as the phase difference

23. Light waves that are out of phase are called incoherent, • while light composed of waves exactly in phase is termed coherent.

24. Interference • When two waves of light travel along the same path, the effect produced depends upon whether or not the waves are in phase with one another. • If they are in phase, the resultant wave will be a summation of the two, and this is called constructive interference • If the two waves of equal amplitude are out of phase by half a cycle , they will cancel each other out: destructive interference. • The final effect in each case is as if the waves were superimposed and added (in the algebraic sense) to each other. • Phase differences of less than half a cycle thus result in a wave of intermediate amplitude

26. Destructive interference occurs within the stroma of the cornea. The collagen bundles of the stroma are so spaced that any light deviated by them is eliminated by destructive interference. • Interference phenomena are also utilised in optical instruments. One example is low reflection coatings which are applied to lens surfaces. The coating consists of a thin layer of transparent material of appropriate thickness. • Light reflected from the superficial surface of the layer and light reflected from the deep surface of the layer eliminate each other by destructive interference