![1. F1 = +6.00
F2 = -7.00
t = 2 mm [in meter] =2 X 10-3 M
n = 1.523
Fv = +6.00 -7.00 + (6.00)2 x 2x 10-3
1.523
Fv = -1.00 + 36.00 X 2 X 10-3
1.523
Fv = -1.00 + 72 X 10-3
1.523
Fv = -1.00 + 0.072
1.523
Fv = -1 + 0.047
Fv = -0.95D](https://image.slidesharecdn.com/powerspecification-220408081729/85/POWER-SPECIFICATION-ppt-17-320.jpg)
POWER SPECIFICATION.ppt
- 1. Sushil Bhatt PhD Research scholar, PGDST, M.optom
- 2. Thin Lens A lens is considered to be thin if its thickness t is much less than the radii of curvature of both surfaces.
- 3. In the thin-lens approximation, the thickness t of the lens is much, much less than the radii R1 and R2 of curvature of the surfaces of the lens. Light rays are considered to bend at the center of the lens, such as light ray 1. Light ray 2 passes through the center of the lens and is undeviated in the thin-lens approximation
- 5. Equation..
- 11. Refractive Power It is defined as the change in version that occurs when light passes through a lens
- 12. Approximate Power FA = F1+F2. FA= Approximate Power. F1= Front Surface Power. F2= Back Surface Power.
- 13. •Front Surface Back Surface
- 14. Back Vertex Power The reciprocal of the reduced distance from the back pole of the lens to the 2nd focal point. Reduce distance = Actual Distance Refractive Index B.V.P. = 1 / Reduce Distance
- 15. Back pole of Lens Back pole of lens to 2nd focal point = Actual Distance Primary Principal Plane to 2nd focal Point = Reduced Distance F2
- 16. Fv = F1 +F2 + F12 x tn F1 = Front Surface Power F2 = Back Surface Power t = Thickness Of Lens n = Refractive Index
- 17. 1. F1 = +6.00 F2 = -7.00 t = 2 mm [in meter] =2 X 10-3 M n = 1.523 Fv = +6.00 -7.00 + (6.00)2 x 2x 10-3 1.523 Fv = -1.00 + 36.00 X 2 X 10-3 1.523 Fv = -1.00 + 72 X 10-3 1.523 Fv = -1.00 + 0.072 1.523 Fv = -1 + 0.047 Fv = -0.95D
- 18. Front Vertex Power It is called as neutralising power It is defined as negative reciprocal of reduced distance between front pole to primary focal point. F.V.P. is denoted by FN FN = 1 Reduced Distance
- 19. FN = F1 + F2 + F22 X t/n FN = -1 + 49 x 2 x 10-3 1.523 FN = -1.00 + 0.098 1.523 FN = -0.93D
- 20. Equivalent Power (True Power): It is defined as the reciprocal of equivalent focal length Power = 1 / Equivalent Focal Length
- 21. Equivalent Focal Length: It is defined as the distance between principal plane to focal point It can be primary focal length & secondary focal length.
- 22. Effective Power : The ability of a lens to focus parallel rays of a light at a given plane.
- 24. Positive Lens: If a ophthalmic lens is moved towards its screen the effectivity of lens is decreases so we have to increase effective power of lens.
- 25. If a ophthalmic lens is moved away from its screen the effectivity of lens is increases so we have to decreases effective power of lens.
- 27. Thankyou!