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Session 9: Wednesday, November 04, 2015 Optics and Low Vision Devices Any questions from last week’s lecture? Quizes!! ◦ Complete your “Syndromes Quiz” by November 8! ◦ Next quiz is on material from this lecture only ◦ Due Sunday, November 15, 2015 @ 11:55PM AST Optics Low Vision Devices Light and Theories Refraction/Reflection Vergence of Light Lenses Refractive Errors Sun is a natural source for all of the energy that moves by electromagnetic radiation ◦ Is classified according to its specific wavelength Light energy is the tiny portion of the spectrum that is visible to the human eye Of the visible spectrum: ◦ Red has the longest wavelength Light is composed of electric and magnetic charges that leave from a source in waves to radiate in all directions The waves form concentric wave fronts, which vibrate up and down perpendicular to the line of travel Light is composed of invisible particles called photons with characteristic frequencies As electrons in an atom absorb photons they get to an excited state with increased vibrations, and gain energy from the photon ◦ This absorbed energy may be released as light Each theory is useful Short waves behave like particles Long waves behave more like waves Light has characteristics of both, and behaves as either, or both, depending on the circumstance When light is absorbed (i.e. Fluorescence) or amplified (i.e. lasers) it behaves according to particle theory When light is refracted, reflected or polarized, it behaves according to wave theory Polarization ◦ The process of restricting the vibration directions of the electromagnetic wave to only one direction Reflection ◦ The turning back of light waves from the boundary of a medium ◦ angle of incidence = angle of reflection Refraction ◦ The bending of light rays as they pass obliquely from one medium to another of a different optical density ◦ angle of incidence ≠ angle of refraction Snell’s Law of Refraction Determines the speed of light through a substance by comparing it with the speed of light in air This comparison is the index of refraction The index of refraction of air is 1.00 Index of refraction = Speed of light in air Speed of light in substance Light rays travelling obliquely from a lesser optical density to a greater optical density are refracted toward the normal Light rays travelling obliquely from a greater optical density to a lesser optical density are refracted away from the normal Wedge-shaped, transparent medium that bends light rays toward it’s base A prism can be used to break light up into its constituent spectral colors Measured in units called Prism Diopters (∆) 1 prism diopter deviates light 1cm at a distance of 1 meter Diopter is the measure of the vergence of light ◦ D: dioptric power ◦ m: distance light has travelled in meters D = 1/m Divergence ◦ Spreading apart of light rays as they leave an object ◦ Minus lenses Convergence Plano vergence ◦ Light rays coming together at a point of focus ◦ Does not occur naturally (needs optical device) ◦ Plus lenses ◦ Light rays are travelling parallel A spherical lens is an optical device that bends light rays with the same power in all directions The optical centre of the lens has no prismatic power Lenses can be thought of as a combination of prisms Convex ◦ ◦ ◦ ◦ ◦ Two prisms base to base Converges light Plus power Thicker in the middle and thinner at the edge Objects appear larger than true size Concave ◦ ◦ ◦ ◦ ◦ Two prisms apex to apex Diverges light Minus power Thinner in the middle and thicker at the edge Objects appear smaller than true size The focal length of a lens is the distance from the lens to the point of focus of that lens The dioptric power (D) of the lens is reciprocal to its focal length in meters (m) D = 1/m What is the focal length of a +2.00D lens? D = 1/m 2 = 1/m 2m = 1 m = 0.5 The focal length is 0.5m or 50cm from the lens Lenses have two focal points ◦ One in front of the lens ◦ One behind the lens The focal points have the same focal distance The primary focal point of a plus lens is in front of the lens The primary focal point of a minus lens is behind the lens A light source emanating from the primary focal point emerges parallel from the lens Parallel light rays striking a lens come to a focus point at the secondary focal point Complicated!!! A cylindrical lens has a flat and a curved surface It has a refracting power in one direction The other direction is 90⁰ away and has zero power is called the cylinder axis Our optical system refracts light What two structures can do this????? Emmetropia Ammetropia ◦ Light rays from distant objects refract to a clear focused retinal image without accommodation ◦ The eye has no refractive error ◦ Eyes with refractive errors ◦ Need to use accommodation or corrective lenses to correct defect in optical system Hyperopia (farsighted) ◦ Eye is too short ◦ Without accommodation, parallel light rays come to a focus behind the retina ◦ An uncorrected hyperope has an optical system with too little plus power ◦ Can use correction or accommodate Patient pulls the image onto the fovea by adding plus from their own lens Myopia (nearsighted) ◦ Eye is too long ◦ The optical system will refract parallel rays of light from infinity to a focus in front of the retina ◦ An uncorrected myope has an optical system with too much plus power ◦ There is no option to reduce the power in this eye except by wearing refractive correction or refractive surgery Astigmatism ◦ A refractive error caused by the nonspherical surface of the cornea or lens ◦ Like a football, it is more curved in one direction than in the direction 90⁰ away Hyperopic Correction: ◦ +4.00D Myopic Correction: ◦ -6.00D Astigmatic Correction: ◦ +2.25-1.25x090 ◦ -7.50+2.75x075 If a patient is not looking through the optical center of their lens, they will be experiencing induced prism ◦ Can be done on purpose (to put prism into an Rx) ◦ Sometimes done in error (can be problematic) Patient is looking through base-out prism, which induces an exotropia!!! Low Vision Assessments Optical Aids Electronic Aids Non-Optical Aids An assessment will include: ◦ Thorough history ◦ Information about eye condition ◦ Determination of goals of assessment What do they want to do? Do they need referrals to other services/agencies? ◦ ◦ ◦ ◦ Discussion about lighting Discussion about glare Current low vision aids? Help in selection of low vision aids To maximize their remaining vision!!!! Optical Electronic Non-Optical Many different shapes and sizes May be illuminated or non-illuminated Can be used for distance or near viewing The larger the lens, the weaker the magnification and the farther away the lens can be held away from the material The stronger the lens, the less the depth of field and the more critical it becomes to hold the lens at the correct distance to be seen clearly The strong the magnification, the smaller the area the user can see through at any given time. The closer the lens is held to the eye, the greater the area the user will see through (regardless of magnification) Plastic is lighter than glass, but scratches Glass has better optics, but it’s heavy American System European System M = D/4 or D=4M M = D/4 +1 or D = 4M-4 Example: Example: ◦ A 12D lens is what magnification? 12 = 4M M=3 Answer: 3X magnification ◦ A 12D lens is what magnification? M = 12/4 + 1 M=4 Answer: 4X magnification Advantages Variable work distance Normal reading range Conventional aid Disadvantages Decreased field of view Need to use two hands Spot-reading tasks Readily available in low powers Difficult to maintain focus Difficult for patients with hand tremors/arthritis Advantages Disadvantages Lens is mounted Decreased field of view Predictable focus Requires coordination See details better because is always illuminated Good for patients with arthritis/tremors Not extremely portable (some) Bulky Have to sit at desk/table Strong microscopic glasses High hyperopic correction with base-in prism Range in strength from +4.00D to +12.00D Advantages Hands free Disadvantages Fixed reading distance The stronger the glasses, the closer things need to be held Wide field Prolonged reading Poor posture Monocular or binocular Illumination obstructed Advantages Good for distance viewing Monocular or binocular Disadvantages Restricted visual field Reduced focal range May have to focus Appearance Not popular with elderly people!! CCTV Portable electronic magnifiers Computer programs (e.g. Zoomtext) Synthesized speech (e.g. EyePal) Lighting ◦ Better light, better sight ◦ Position ◦ Intensity of light Reading stands Filters Felt-tip pens Large print calendars/address books Signature/writing guides Talking clocks/talking watches Large print cards/games Large button phones Liquid level indicators CNIB Library ◦ Reading club over the summer Low Vision Service O&M Training Independent Living Skills Employment Assistive Technology Counselling/Peer Support Programs November 18th Guest Speaker: Peter Parsons (APSEA O&M Instructor) Please come prepared to discuss, and have at least one question ready to ask Peter!!!
