Physiological Optics 9th Lecture PDF

PHYSIOLOGICAL OPTICS TH 9 LECTURE  OPTICAL PRESCRIPTIONS, SPECTACLE LENSES WHEN PRESCRIBING A SPECTACLE LENS, THE PROPERTIES OF THE LENS REQUIRED ARE SPECIFIED IN THE FOLLOWING WAY. A SPHERICAL LENS ALONE IS WRITTEN AS, FOR EXAMPLE, +2.00 DS (DIOPTRE SPHERE) OR –3.25 DS.  OP...

PHYSIOLOGICAL OPTICS TH 9 LECTURE  OPTICAL PRESCRIPTIONS, SPECTACLE LENSES WHEN PRESCRIBING A SPECTACLE LENS, THE PROPERTIES OF THE LENS REQUIRED ARE SPECIFIED IN THE FOLLOWING WAY. A SPHERICAL LENS ALONE IS WRITTEN AS, FOR EXAMPLE, +2.00 DS (DIOPTRE SPHERE) OR –3.25 DS.  OPTICAL PRESCRIPTIONS, SPECTACLE LENSES IN THE CASE OF A CYLINDRICAL LENS ALONE, BOTH THE DIOPTRIC POWER AND THE ORIENTATION OF THE AXIS MUST BE SPECIFIED. THE AXIS OF THE CYLINDER IS MARKED ON EACH TRIAL LENS BY A LINE, AND TRIAL FRAMES ARE MARKED ACCORDING TO A STANDARD INTERNATIONAL CONVENTION CONVENTIONAL ORIENTATION FOR CYLINDRICAL LENSES. CONVENTIONAL ORIENTATION FOR CYLINDRICAL LENSES. THUS, A CYLINDER OF –2.0 DIOPTER POWER, PLACED WITH ITS AXIS (OF NO POWER) VERTICAL IS WRITTEN AS RT: PLANO/ –2.0 DC AXIS 90° (DC = DIOPTER CYLINDER). OFTEN THE CORRECTION OF A REFRACTIVE ERROR ENTAILS THE PRESCRIPTION OF BOTH A SPHERICAL AND A CYLINDRICAL COMPONENT, I.E. A TORIC ASTIGMATIC CORRECTION. IN SUCH A CASE, AT THE END OF REFRACTION THE TRIAL FRAME CONTAINS A SPHERICAL LENS (E.G. +2.0 DS) AND A CYLINDRICAL LENS (E.G. +1.0 DC AXIS 90°). CONVENTIONAL ORIENTATION FOR CYLINDRICAL LENSES. THE CYLINDRICAL LENS IS USUALLY PLACED IN FRONT OF THE SPHERICAL LENS TO ALLOW THE AXIS LINE TO BE SEEN. THE PRESCRIPTION IS WRITTEN AS +2.00 DS/ +1.00 DC AXIS 90°, AND THIS MAY BE ABBREVIATED TO +2.00/ +1.00 X 90°  TRANSPOSITION OF LENSES WHEN A LENS PRESCRIPTION IS CHANGED FROM ONE LENS FORM TO ANOTHER OPTICALLY EQUIVALENT FORM, THE PROCESS IS CALLED TRANSPOSITION OF THE LENS. SIMPLE TRANSPOSITION OF SPHERES THIS APPLIES TO THE ALTERATION OF THE LENS FORM OF SPHERICAL LENSES. THE LENS POWER IS GIVEN BY THE ALGEBRAIC SUM OF THE SURFACE POWERS SIMPLE TRANSPOSITION OF CYLINDERS SIMPLE TRANSPOSITION OF THE CYLINDER IS OFTEN NECESSARY WHEN THE EXAMINER WISHES TO COMPARE THE PRESENT REFRACTION WITH A PREVIOUS PRESCRIPTION. EXAMPLE SIMPLE TRANSPOSITION OF CYLINDERS THE LENS DEPICTED IN FIG CAN BE DESCRIBED IN TWO WAYS.  LET THE CYLINDRICAL ELEMENT BE AT AXIS 90°: THE LENS IS NOW +2.0 DS/+1.0 DC AXIS 90°. LET THE CYLINDRICAL ELEMENT BE OF OPPOSITE POWER AND AT AXIS 180°: THE LENS IS NOW +3.0 DS/–1.0 DC AXIS 180°. SIMPLE TRANSPOSITION OF CYLINDERS THIS CHANGE IN THE DESCRIPTION OF THE LENS MAY BE EASILY ACCOMPLISHED FOR ANY LENS BY PERFORMING THE FOLLOWING STEPS.  SUM. ALGEBRAIC ADDITION OF SPHERE AND CYLINDER GIVES NEW POWER OF SPHERE.  SIGN. CHANGE SIGN OF CYLINDER, RETAINING NUMERICAL POWER.  AXIS. ROTATE AXIS OF CYLINDER THROUGH 90°. (ADD 90° IF THE ORIGINAL AXIS IS AT OR LESS THAN 90°. SUBTRACT 90° FROM ANY AXIS FIGURE GREATER THAN 90°.) EXAMPLES