Base curve

BASE CURVE

1. BASE CURVE &FORM OF THE LENS Presented by- Kaustav Gogoi Roll No-6 B.Optom 1 st Year Ridley College of Optometry

2. LAYOUTS…… Definition Characteristics Importance Selection of Base Curve Form of the lens Best form of the lens

3. DEFINITION The base curve of a lens is the surface curve that serves as the basis or starting point from which the remaining curve will be calculated.

4. CHARACTERISTICS The selection of base curve is made when the lens is the semi finished state, in which case the base curve is always on the finished side of the lens. In toric surfaces, the flatter of the two curvatures is considered the base curve

5. IMPORTANCE Base curve will determine the thickness of the lens The choice for base curve will control the aberrations associated with the lens If the dispenser combines proper base curve along with good fitting and perfect optical center the result is good cosmetics and great optics

6. WOLLASTON FORM OF LENSES William Wollaston in 1804 studied image formation experimentally by using different form of lenses. According to Wollaston , best image for oblique rays is produced by the use of very steep surface powers. Wollaston form not only eliminates oblique astigmatism , but also minimizes distortion. Wollaston form is more expensive , difficult to manufacture and cosmetically is not good due to it’s bulbous apperance .

7. OSWALT FORM OF LENSES F. Ostwalt ( 1898 ) of France presented calculation for lenses that were free of oblique astigmatism- a shallow form & the deeper form The Ostwalt form is used as a basis of the design of modern ophthalmic lenses

8. TSCHERNING’S ELLIPSE Within the range of lens powers over which oblique astigmatism is zero,both the Oswalt and Wollaston forms are plotted and an ellipse is generated.This ellipse is known as Tscherning’s ellipse. This is the point between Wollaston and Oswalt form)

9. TSCHERNING’S ELLIPSE

10. BASE CURVE SELECTION There are two formulae for selecting base curve- (a) Vogel’s formula (b) Manufacturer’s table

11. VOGEL’S FORMULA This formula helps in determining the approximately what base curve might be expected for a given lens power. For plus lenses- Base Curve=spherical equivalent+6.00D For minus lenses- Base Curve=spherical equivalent/2 + 6.00D

12. Eg . +3.00/-4.00×180̊ Spherical equivalent= sphere power +cylinder power/2 = +3.00+1/2 (-4.00) = +6.00-4.00 2 = +2.00 2 = +1.00D Since, the spherical equivalent is positive BC= SE+6.00D = +1.00D+6.00=+7.00D

13. MANUFACTURER’S TABLE Manufacturers typically produce a series of semi-finished lens blanks,each with it’s own base curve. This base curve series is a system of lens blanks that increases incrementallay in surface power (eg.+0.50D,+2.00D and so on) The more base curves available in the series,the broader the prescription range of the lens.

14. MANUFACTURER’S TABLE

15. FORM OF THE LENS The term form of the lens refers to the relationship between the front and back surface curvature of a lens. For a lens of given power,an infinite number of forms are possible. The simplest lens forms- PLANO-CONVEX in which one surface is flat and other surface is convex PLANO-CONCAVE in which one surface is flat and other surface is concave

16. Another simple lens form is the BI-CONVEX or BI-CONCAVE having half the power on the front surface and half on the back surface. Another type of lens is MENISCUS lens,which have a -6.00D back surface power(for plus lens) or a front surface power of +6.00D(for minus lens)

17. BEST FORM OF LENSES A lens is said to be in best form when it follows following criterias - (1) Mechanical criteria (2) Optical criteria

18. MECHANICAL CRITERIA Flatter lens forms are slightly  thinner  than steeper lens forms, and vice versa.  Since the lenses are thinner, they also have less mass making them lighter in weight as well.  Due to variation in the lens thickness,the lens form will also produce significant differences in the  plate height , or overall bulge, between lenses of the same power. Essentially, plate height is the height of a lens as measured from a flat plane

19. This table represents a range of +4.00 D lenses in hard resin plastic, edged to a 70-mm diameter and a 1-mm minimum edge thickness. Base curve Center Plate Weight 10.00D 6.9mm 15.3mm 21.7g 8.00D 6.3mm 11.7mm 19.5g 6.00D 6.0 mm 8.7mm 18.3g 4.OOD 5.9 mm 6.0mm 17.7g

20. The table, below, represents a range of -4.00 D lenses in hard resin, edged to a 70-mm diameter and a 2-mm minimum center thickness Base Curve Edge Plate Weight 6.00D 8.7mm 16.4mm 25.4g 4.00D 7.8mm 12.8mm 24.0g 2.00D 7.3mm 9.7mm 23.2g 0.00D 7.0mm 7.0mm 22.8g

21. Flatter (less "bulge") Plus  lenses with flatter plate heights do not fall out of frames as easily, which is especially important with large or exotic frame shapes. In addition, flatter plate heights are also more cosmetically pleasing than steeper, bulbous ones—particularly in plus powers.

22. Less magnification ( or minification ) A reduction in plate height will also provide a significant reduction in the  magnification associated with  plus  lenses. Since a flatter plate height brings the back surface closer to the eye, the  minification  associated with  minus  lenses is also reduced slightly. This gives the wearer's eyes a more natural appearance through the lenses

23. Thinner center thickness (plus) or edge thickness (minus) Lighter in weight Better frame retention (in plus powers)

24. OPTICAL CRITERIA Lens aberrations act as errors in power from the desired prescription, and can degrade the image quality produced by the lens as the wearer gazes away from—or obliquely to—its optical axis. There are six different lens aberrations that can affect the quality of peripheral vision through a spectacle lens: Oblique Astigmatism Power Error Spherical Aberration Coma Distortion Chromatic Aberration

25. Chromatic aberration is caused by the material from which the lens is made and therefore it can be controlled to some extent by the judicious selection of the material. Monochromatic aberration on the other hand caused by factors such as size of lens aperture and position of the lens in relation to the eye. These are controlled by varrying the front and back surface powers and the thickness of the lens and also by varrying vertex distance

26. REFERENCE- Clinical Optics System for Practical Dispensing www.Opticampus.com

27. THANK YOU…