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Image formation and anomalies of refraction
 

Image formation and anomalies of refraction

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Image formation and anomalies of refraction Image formation and anomalies of refraction Presentation Transcript

  • Image formation and Anomalies of refraction Gauri S. Shrestha, M.Optom, Fellow of International Association of Contact lens educators (Australia)Lecturer and Optometrist at Institute of Medicine, Maharajgunj, KTM Gauri S Shrestha, M.Optom, FIACLE
  • Image formation Image forms in retina in inverted position. The rays of light scattered in all direction from a point source that pass through the lens to converge in macula where the image forms. Since the cornea has an fixed curvature the curved image forms in the curved retina so that the final perception is straight and the image is erected due to mental perception. In lens opacities at periphery of lens, sharp image forms but brightness can be reduced. In opacities on the center, the vision may decrease. Gauri S Shrestha, M.Optom, FIACLE
  • Image formation In myopia image forms in front of retina that is corrected by placing the negative lenses. For hypermetropia, positive lens is placed in the eyes to make the image fall on the retina. For astigmatism, plano-cylinder or sphero-cylinder lenses are placed to coincide the image on retina Gauri S Shrestha, M.Optom, FIACLE
  • Main classification of ametropia Emmetropia  A parallel pencils of rays from distance are sharply focused on the retina when accommodation is at rest Deviation from this definition is called as ametropia and is optical Category of ametropia  Spherical  Astigmatism AKA: Anomalies of refraction Gauri S Shrestha, M.Optom, FIACLE
  • Anomalies of refraction: The variations from perfect coincidence of the principal focus of the eye with the retina. Gauri S Shrestha, M.Optom, FIACLE
  • Myopia If sharp image is formed in front of retina the resulting error of refraction is called myopia.  Having an optical system too powerful for its axial length  Light must reach to retina in the state of divergence  Object must be at some finite distance from the eye The point conjugate with the fovea of the un- accommodated eye is called the FAR POINT (MR) MR l (-ve) l’ Gauri S Shrestha, M.Optom, FIACLE
  • Myopia Myope can focus objects with in the far point only The situation becomes worsen by accommodation Myope can focus object at shorter distance than usual (craftmanship) Gauri S Shrestha, M.Optom, FIACLE
  • Hypermetropia If the pencils within the eye are intercepted by the retina before reaching their focus  Too weak to suit its axial length  Light must reach to the retina in the state of convergence  Far point is behind the eye Object can be focused into the retina by virtue of accommodation provided sufficient eye’s dioptric power l’ l (+ve) Gauri S Shrestha, M.Optom, FIACLE
  • Ocular refraction If a myopes far point= 200mm  l= -200mm then L= -5.00D l’= image distance (axial length of reduced eye)  L’= image vergence= n’/l’ For the reduced eye  L’=L+Fe  L=L’-Fe Ocular refraction = eye’s dioptric length - its power Refractive error= K’-Fe Eye’s dioptric length= index by true axial length Gauri S Shrestha, M.Optom, FIACLE
  • Example A reduced eye has an axial length of 21mm and a power of +62.00D. What is the ocular refraction and where is the far point? K’= n’/l’ = 4/3x1/21x1000 = +63.49D Fe = +62.0D L= K’-Fe = +1.49D l= 1/L = +671mm Gauri S Shrestha, M.Optom, FIACLE
  • Axial and refractive ametropia In axial ametropia  Eye assumes a standard power of +60.0D  Defect is attributed to error in axial length In refractive ametropia  Axial length of reduced eye = 22.22  Defect is attributed to the error in the power Low grade of ametropia, no firm relationship establishes b/w axial length and ametropia Gauri S Shrestha, M.Optom, FIACLE
  • Axial and refractive ametropia Change in ∆l’ in the value of L’ would produce an identical change ∆l in refractive error if Fe is constant dL’ dn’l’-1 -n’L’2 -L’2 = = -n’l’-2 = = dl’ dl’ n’ n’2 -L’2 ∆L’ = ∆l’ n’ Gauri S Shrestha, M.Optom, FIACLE
  • Axial and refractive ametropia If +60D is taken as a mean value for L’, this expression becomes∆L’ = -3600/n’ X ∆l’ = -3.6/n’ X ∆l’ (l’ in mm) = -2.7 X ∆l’ An increase of 1mm in the axial length would produce a change in ametropia of -2.7D Gauri S Shrestha, M.Optom, FIACLE
  • Correcting lens The un-accommodated eye is in focus for objects in the far point A lens second focus for distant object is equivalent to eye’s far point MR l (-ve) l’ l’ l (+ve) Gauri S Shrestha, M.Optom, FIACLE
  • Spectacle refraction Back vertex power: the distance in meters from the back vertex of the lens to its second principal focus d F’ MR f’sp l’ l (-ve)l= fsp- d 1 1 FspL= l = = fsp- d 1-dFsp Gauri S Shrestha, M.Optom, FIACLE
  • Spectacle refraction l’ MR F’ d l (+ve) f’sp f’sp= d+l 1 LFsp= d+l = 1+dL Gauri S Shrestha, M.Optom, FIACLE
  • Example An eye with an ocular refraction of +5.00D is to be corrected by a spectacle lens placed at a vertex distance of 13mm. What should be its power? An eye is corrected for distance vision by a lens of power -15.00D placed 14mm from its pricipal point. What is the ocular refraction? A prescription reads -8.00at 16. What lens power would be needed if the vertex distance were reduced to 13mm? Gauri S Shrestha, M.Optom, FIACLE
  • Hypermetropia and accommodation emmetrope: accoomodate eye for only viewing near Total hypermetropia object Latent Manifest Myopes accommodate eye only for object Facultative Absolute viewing nearer the far point A person has +3.0D on Hypermetrope: exert retinoscopy, accept up to +2.0D accommodation to view without blurring of vision. On object at any distance cycloplegic refraction it is with great effort +5.0D. Describe the components of hypermetropia? Gauri S Shrestha, M.Optom, FIACLE
  • Retinal image in corrected ametropia Two stages  Lens forms a real or virtual image  Image is real if formed in front of eye  Virtual if formed behind the eye  Image becomes a object for the eye Gauri S Shrestha, M.Optom, FIACLE
  • f’spQ l d l’ u u0 u’ Q’2 h’2 h1’= h2 Q’1 Hypermetropia Gauri S Shrestha, M.Optom, FIACLE
  • Q Myopia Q’1 l’h1’= h2 d u0 u u’ h’2 Q’2 f’sp l Gauri S Shrestha, M.Optom, FIACLE
  • Retinal image in corrected ametropiah1’= h2 =-u0 f’sp (Radian)h1’= h2 =-u0 f’sp/100 (prism diopter)h’2= h2 L2/L’2 f’sp= d+l l’= axial length of eye h2 = h’1= image formed by spectacle lens= -u0 f’sp Gauri S Shrestha, M.Optom, FIACLE
  • An eye of axial length 24.80mm is corrected for distance by a -5.00D lens placed 12mm from its principal point. Find thesize of the retinal image of a distant object subtended an angle of 15pd (n’=4/3) f’sp= -200mm h’1= h2= -u0 f’sp= -15 X -200mm/100= +30mm l= fsp-d= -200-12= -212mm L= -4.72D L’= n’/l’= +53.76D h’2= h2 L/L’= 30 X -4.72/53.76 = -2.63mm Gauri S Shrestha, M.Optom, FIACLE
  • Blurred retinal image l k’ l’ j= l’-k’ j=g (K’-L’/K’)= g (K-L/K’)g l’ Gauri S Shrestha, M.Optom, FIACLE
  • An unaccommodated eye which has a power of +62.00D, anocular refraction of -6.00D, and a pupil diameter of 4mm viewsa point object at a distance of 250mm. Find the diamter of the retinal blur circle. L= 1000/-250= -4.00D L’= L+Fe= -4.0D+62.0D= +58.00D K’= K+Fe= -6.0D+62.00D= 56.00D J= g(K’-L’/K’)= 4 (56-58)= -0.14mm 56.00 L= vergence coming to eye K= ocular refraction = refractive error Gauri S Shrestha, M.Optom, FIACLE
  • Vision in spherical ametropia Based on the finding of simulated myopia study Log D (in feet) = 1.360 log S + 1.817  Hirsch (1945)- up to -14.0DS  Crawford et al (1945)  Rubin et al (1951) Gauri S Shrestha, M.Optom, FIACLE
  • Astigmatism Astigmatism is defined as a refractive condition in which a variation of power exists in the different meridians of the eye. BB-base curve B CC cross curve Plano/+2.00DC C C B +1.50DS/+2.00DC Gauri S Shrestha, M.Optom, FIACLE
  • Ocular astigmatism Corneal astigmatism  With the rule  Against the rule  10% of front surface astigmatism is neutralized by back surface corneal astigmatism Crystalline lens (lenticular astigmatism)  Astigmatisc surface  decentration Gauri S Shrestha, M.Optom, FIACLE
  • Axis notation Standard axis notation (Tabo, Axint)  As adopted in 1950 by the International Federation of Ophthalmological Societies 90 180 0 135 45 135 45 180 0 90 Gauri S Shrestha, M.Optom, FIACLE
  • Sturms Conoid CIRCLE OF LEAST CONFUSION F1lens INTERVAL OF STURM Rays of light entering cannot F2 converge to a point focus but forms a focal lines Gauri S Shrestha, M.Optom, FIACLE
  • Image formation in the astigmatic eye B’β α B’α F1 B’zlens Second focal lineβ First focal line F2 Gauri S Shrestha, M.Optom, FIACLE
  • Given an object at dioptric distace L, the respective imagevergence after refraction by the eye areL’α= L+F αL’β= L+F β Gauri S Shrestha, M.Optom, FIACLE
  • B’z H B’β B’α αg b β a z β J l’α α l’z l’β Length of first focal line l’β - l’α L’α –L’β a= g g = = l’β L’ α Gauri S Shrestha, M.Optom, FIACLE
  • Length of Second focal line l’β - l’α L’α –L’βb= g =g = l’ α L’ β L’z=?Diameter of circle of least confusion l’β - l’z L’z –L’α L’α –L’βz= g =g = =g = l’ β L’ α L’α +L’β Gauri S Shrestha, M.Optom, FIACLE
  • An astigmatic reduced eye (n’=4/3) with a pupil diameter of 5mmhas a power of +62.0D in the 30 meridian and +64.00D in the 120 meridian. Determine the main feature of the image of an axial object point at a distance of one meter from the eye’s principal point. 120 30 L -1.00D -1.00D Fe +64.00D +62.00D L’ +63.00D +61.00D l’ (n’/L’) +21.16mm +21.86mm Gauri S Shrestha, M.Optom, FIACLE
  • Length of focal lines L’α –L’β 63 –61 a== g = =5 = = 0.159mm L’ α 63 L’α –L’β 63 - 61 b== g = =5 =0.164mm L’ β 61 L’z= 1/2 L’α +L’β =+62.00D l’z= n’/L’z= +21.50mm L’α –L’β 63 –61 z =g =5 =0.081mm = 63 +61 L’α +L’β Gauri S Shrestha, M.Optom, FIACLE
  • Clinical classification: Astigmatism A. Compound myopic astigmatism B. Simple myopic astigmatism C. Mixed astigmatism D. Simple hyperopic astigmatism E. Compound hyperopic astigmatism Gauri S Shrestha, M.Optom, FIACLE
  • Aphakia: It means the absence of crystalline lens from the eye. It produce high degree of hyperopia. Parallel rays of light are brought a the focus 31mm behind the cornea Cause: a) congenital absence of the lens. b) surgical aphakia. c) aphakia due to absorption of the lens matter after trauma in children. d) traumatic extrusion. e) Posterior dislocation of the lens in the vitreous. Gauri S Shrestha, M.Optom, FIACLE
  • Aphakia Optics of the aphakic eye.• eye becomes highly hyperopic.• Power of the eye reduces to 44D.• There occurs total loss of accommodation.• The posterior focal point is about 7mm behind the eye ball or anterior principle focus -17.05mm in front of the cornea• The nodal point of the eye moved forward. Gauri S Shrestha, M.Optom, FIACLE
  • To study aphakia, let’s consider G-E schematic eye r= 7.8mm, n= 4/3, l’ = 23.89mm L’= n’/l’= 55.81D Fe= n’-n/r= +42.73D n’= 4/3 L= L’-Fe= 55.81-42.73= +13.08D l= 76.45mm l’= 23.89mm f’sp= l+d= 76.45+12= +88.45mm F’sp= 1/88.45= +11.31D Gauri S Shrestha, M.Optom, FIACLE
  • Presbyopia The condition with physiological diminution of amplitude of accommodation with increasing age to the point where clear or comfortable vision at the near is not acceptable. When accommodation is minimum or absent called absolute presbyopia. Corrected hyperopes develop presbyopia earlier than myopes or emmetropes Symptoms:  Blurred near vision, symptom of uncorrected hyperopia, asthenopia or headache, pupil greatly constrict, drowsy or falling sleepy while reading . Correction : Bifocal or (PALs) Multifocal contact lenses in gas permeable or soft lens materials Monovision Surgery Gauri S Shrestha, M.Optom, FIACLE
  • Gauri S Shrestha, M.Optom, FIACLE