Perceptual Optics for Prisms and Lenses (for vision therapists)

Transcription

1 Perceptual Optics for Prisms and Lenses (for vision therapists) Goal of lecture: Understand optical characteristics in order to improve the uses of lenses and prisms in optometric vision therapy. Kellye Knueppel, OD, FCOVD Great Lakes Congress Light Transformers Lenses and prisms are light transformers. While you can calculate how they transform light, you can not predict human awareness or behavioral changes because of the transformation. What do lenses and prisms do? It is more appropriate to ask, What do patients do in response to lenses? John Streff, OD, DOS, FCOVD, FAAO 3 4 Lens transformations in two directions Towards the person Away from the person Simplified Way to Think About Optometric Vision Therapy The patient is looking in the wrong place in space. Optometric Vision Therapy helps them learn how to look in the right place. Visual Motor is a key to making the change. 5 1

2 Where is the image? Concave Lens Thin Lens Equation: 1/d i + 1/d o = 1/f d i = distance from lens to image d o = distance from lens to object f = focal distance of lens 7 8 Concave Lens (MINUS) Optics: Real Object anywhere in space always creates a Virtual Image closer to the lens and smaller. (Small In) We can calculate the location of the virtual image if we know the lens power and the distance the object is from the lens. MINUS Lenses in VT Accommodation Accommodative Rock (Monocular & Binocular) Red Rock (Biocular) Mental Minus Space Perception Window Rock Minus Lens for Localization and SILO Aniso Lenses 9 10 Convex Lenses (PLUS): Two Situations PLUS (Convex) Lens 1 (Object between Focal Point and Lens)

3 We need to know the Focal Distance of the Lens Equation: 1/Lens Power = Focal Distance (in meters) Examples: 1.00 D Lens 1/1D = 1 meter = 100 centimeters 2.00 D Lens 1/2D = 0.5 meters = 50 centimeters 3.00 D Lens 1/3D = 0.33 meters = 33 centimeters 4.00 D Lens 1/4D = 0.25 meters = 25 centimeters Go back to MINUS Lenses Where is the image? Looking at an object across the room the image will be approximately at the focal distance of the lens. Examples: D Lens: 1/4 =.25 meters = 25 centimeters D Lens: 1/6 =.17 meters = 17 centimeters Therapy Procedures Minus Lens for Localization and SILO Mental Minus Accommodative Rock PLUS (Convex) Lenses: Two Situations PLUS (Convex) Lens 1 (Object between Focal Point and Lens) PLUS (Convex) Lens (Object between Focal Point and Lens) PLUS (Convex) Lens 2 (Object Beyond Focal Distance of Lens) Optics: Real Object between the focal point and the lens creates a Virtual Image farther away and larger. (Large Out)

4 PLUS (Convex) Lens 2 (Object Beyond Focal Distance of Lens) Optics: Real Object beyond the focal distance of the lens creates a Real Image on the opposite side of the lens. In this case the calculated location in space of the real image ends up in the person s eye/head or often quite a distance behind the person. Looks blurry! And it s upside down. Keep in mind that if the lens is held directly in front of the eye (like glasses), the optics of the eye would also have an effect on the final calculated distance of the image. Hyperopes (+ lens prescriptions) may still be able to see the image. PLUS (Convex) Lenses: Two Situations PLUS Lens Situation 1: Object is between the focal distance and the person. Image is farther away from them and bigger. PLUS Lens Situation 2: Object is farther away than the focal distance. Image is behind the person and upside down PLUS (Convex) Lenses in VT Accommodation Accommodative Rock (Monocular & Binocular) Red Rock (Biocular) Space Perception Window Rock Aniso Lenses Web Sites with Interactive Lens Models This is fun to play with and maybe can help you: Interactives/Refraction-and-Lenses/Optics- Bench/Optics-Bench-Refraction-Interactive Quick Review of Thin Lens Optics Concave Lenses (Minus) Image always CLOSER and SMALLER Convex Lenses (Plus) Object between eye and Focal Distance Image always FURTHER and LARGER Object beyond Focal Distance Image BEHIND patient Optics of Prism Image displacement 1 prism diopter = 1 cm at 1 meter Three dimensional perceptual effects Predictable based on optics See next slide

5 Prisms cause: Shift the main optical effect of prisms Slant extended surfaces may appear rotated. Curvature extended surfaces may appear curved R. Jones, OD, PhD How far does the image shift? Looking at an object 1 meter away, the image shifts 1 centimeter for each prism diopter 1 Prism Diopter (PD) = 1 cm 2 Prism Diopters = 2 cm 10 Prism Diopters = 10 cm The image will move farther if the object is farther away. Multiply by the distance. 1 PD at 5 meters = 1 cm x 5 = 5 cm 10 PD at 5 meters = 10 cm x 5 = 50 cm Is the shift toward the apex side or the base side? Prism VT Activity Prism Jumps How does this apply to vergences and vectograms? Prism Image Formation

6 Optics: 3D spatial perception (Slant!) The ophthalmic optics of prisms is characterized by having a non-uniform deviation of light rays. We cannot fully understand the optics of prisms if we limit our concept of changes to a flat plane. Prisms produce a gradient of simultaneous magnification and minification. Ron Jones, OD, PhD Spatial Distortions of Prisms: Slant 1. Asymmetric magnification Effect is equivalent to an induced slant Only apparent with an extended surface. R. Jones, OD, PhD Linear perspective Based on the relationship between angular size and distance. 33 Optics: 3D spatial perception (Slant!) Prisms change the direction of light AND rotate spatial aspects of the visual field BASE: As if things moved FARTHER away 2D grid lines appear closer together Sometimes described as minification. APEX: As if things moved CLOSER 2D grid lines appear farther apart Sometimes described as magnification. 34 Curvature with Prisms Reminder: Optics of Prism Curvature of lines perpendicular to base-apex. R Jones, OD, PhD image object L R Base-in convex R L Base-out concave Shift Slant Prism Base: As if things moved FARTHER away Prism Apex: As if things moved CLOSER Curvature of field

7 Primary Effects of Prism Prescriptions YOKED PRISMS (Bases up, down, left, right) Cause shift and slant OPPOSING BASE PRISMS (Base Out, Base In) Cause distortions of space, distance, size Prisms in VT Prism Jumps Doubling Activities Vergence Activities Yoked Prism Activities Question: In each of these activities, are we primarily concerned with Shift or Slant or both? Take Home Message: Lenses MINUS Lenses Image always CLOSER and SMALLER PLUS Lenses Situation 1 : Object between eye and Focal Distance Image always FURTHER and LARGER Situation 2: Object beyond Focal Distance Image BEHIND patient and upside down Prisms Cause shift in direction of apex Cause slant expands away on base side. Looks smaller because it s farther away Curvature of field 39 7