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, 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 ◦ Light rays coming together at a point of focus ◦ Does not occur naturally (needs optical device) ◦ Plus lenses  Plano vergence ◦ 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? 2 = 1/m 2m = 1 m = 0.5 The focal length is 0.5m or 50cm from the lens D = 1/m

 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 ◦ Light rays from distant objects refract to a clear focused retinal image without accommodation ◦ The eye has no refractive error  Ammetropia ◦ 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: ◦ x090 ◦ x075

 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 SystemEuropean System  M = D/4 or  D=4M  Example: ◦ A 12D lens is what magnification? 12 = 4M M = 3 Answer: 3X magnification  M = D/4 +1 or  D = 4M-4  Example: ◦ A 12D lens is what magnification? M = 12/4 + 1 M = 4 Answer: 4X magnification

AdvantagesDisadvantages  Variable work distance  Normal reading range  Conventional aid  Spot-reading tasks  Readily available in low powers  Decreased field of view  Need to use two hands  Difficult to maintain focus  Difficult for patients with hand tremors/arthritis

AdvantagesDisadvantages  Lens is mounted  Predictable focus  See details better because is always illuminated  Good for patients with arthritis/tremors  Decreased field of view  Requires coordination  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 D

AdvantagesDisadvantages  Hands free  Wide field  Prolonged reading  Monocular or binocular  Fixed reading distance  The stronger the glasses, the closer things need to be held  Poor posture  Illumination obstructed

AdvantagesDisadvantages  Good for distance viewing  Monocular or binocular  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 18 th Guest Speaker: Peter Parsons (APSEA O&M Instructor) Please come prepared to discuss, and have at least one question ready to ask Peter!!!