Optics: Spherical Equivalent for Lens Powers

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Questions and Answers

What is the purpose of using the spherical equivalent in discussions related to selecting base curves for a given prescription?

To find the average power of the two principal meridians (FSPH and FCYL) and locate the circle of least confusion.

How can the spherical equivalent be calculated?

By adding half of the cylinder power to the sphere power of the prescription, or by using the equation FMEAN = Fsph + FCYL/2.

What is the location of the circle of least confusion in relation to astigmatic line foci?

It is the dioptric mid-point of the astigmatic line foci.

What is the difficulty faced by presbyopic clients when using a single pair of lenses for close work?

<p>Reduced range of vision and difficulty in accommodating for both distance and near work.</p> Signup and view all the answers

What is the advantage of bifocal lenses over single-vision lenses for presbyopic clients?

<p>Bifocal lenses offer multiple powers in a single lens, providing a convenient solution for presbyopic clients with varying visual needs.</p> Signup and view all the answers

What are the two powers required by a presbyopic client?

<p>One for distance (either plano or with a prescription) and one for near work.</p> Signup and view all the answers

What is the segment diameter?

<p>The widest horizontal measurement of the segment.</p> Signup and view all the answers

What is the difference between segment size and segment diameter?

<p>Segment size is the segment diameter or, in the case of a shaped bifocal, its diameter and its depth.</p> Signup and view all the answers

How is the segment height measured?

<p>The vertical distance from the segment top to a horizontal line tangent to the lens periphery at its lowest point.</p> Signup and view all the answers

What is the common misconception about the segment drop?

<p>Many interpret the segment drop as the distance from the horizontal centreline to the segment top, but this is more correctly known as the segment top position.</p> Signup and view all the answers

How is the segment top position measured?

<p>The vertical distance from the horizontal centreline to the segment top.</p> Signup and view all the answers

What is the segment inset, and what is not considered in its measurement?

<p>The horizontal distance between the distance and segment optical centres, not considering prescribed prism.</p> Signup and view all the answers

What is the base curve of the multifocal or progressive lens referred to in the text?

<p>+4.50 D</p> Signup and view all the answers

What is the formula used to calculate the surface powers in the text?

<p>F = F1 + F2</p> Signup and view all the answers

What is the power of the near portion of the lens?

<p>+2.75 D</p> Signup and view all the answers

What is the value of F1near in the near portion of the lens?

<p>+6.50 D</p> Signup and view all the answers

What is the formula used to calculate the surface radii in the text?

<p>No specific formula is given, but it is mentioned that the surface power formulae are used</p> Signup and view all the answers

What is the value of 'n' in the calculation of the back surface radius?

<p>1.498</p> Signup and view all the answers

Why is it crucial to control the furnace temperature during the manufacturing process of fused bifocals?

<p>To ensure that the fusing process is successful and the main lens does not warp.</p> Signup and view all the answers

What steps must be taken to prevent air bubbles from forming during the manufacturing process?

<p>Steps must be taken to ensure that there are no air-bubbles trapped between the main lens and the segment.</p> Signup and view all the answers

How are the different types of fused bifocals (and trifocals) produced?

<p>By a slight modification in the process.</p> Signup and view all the answers

What is the purpose of grinding the additional piece of glass with a convex curve?

<p>To fit the ground piece of glass to the main lens.</p> Signup and view all the answers

What happens to the extra piece of glass on the top of the button when the fusing process is complete?

<p>It fuses in with no dividing lines and becomes invisible.</p> Signup and view all the answers

How can different shapes of segment be produced in fused D-segment bifocals?

<p>By varying the amount of flint glass ground away from the button or adding crown glass at the bottom of the button.</p> Signup and view all the answers

What is the power of the near zone in the given example?

<p>+7.00 D</p> Signup and view all the answers

What is the power of the intermediate zone in the given example?

<p>+5.75 D</p> Signup and view all the answers

What is the formula to calculate the back vertex power of a solid trifocal lens?

<p>F = F1 + F2</p> Signup and view all the answers

What is the process of manufacturing solid trifocal lenses?

<p>Ground or moulded, depending on whether they are glass or plastic</p> Signup and view all the answers

What is the main difference in the manufacturing process of fused trifocal lenses compared to fused bifocal lenses?

<p>Trifocals require two different glasses to be fused to the main lens to produce the intermediate and the near adds</p> Signup and view all the answers

What are the popular designs of fused trifocal lenses available in the market?

<p>D-segment or flat-top, and curved-top</p> Signup and view all the answers

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Study Notes

Spherical Equivalent

  • Defined as the mean or average power of the two principal meridians (FSPH and FCYL)
  • Can be calculated by adding half of the cylinder power to the sphere power of the prescription or using the equation FMEAN = Fsph + 𝐹𝑐𝑦𝑙/2
  • Example: FMEAN = +2.50 + (-0.50)/2 = +2.25 D

Bifocals and Trifocals

  • For presbyopes, bifocals offer advantages over single-vision lenses, providing two powers of lenses for distance and near work
  • Segment diameter: widest horizontal measurement of the segment
  • Segment size: segment diameter or diameter and depth for shaped bifocals
  • Segment height: vertical distance from segment top to horizontal line tangent to lens periphery at lowest point
  • Segment drop: vertical distance from segment top to distance optical centre
  • Segment depth: vertical distance from segment top to horizontal line tangent to segment bottom
  • Segment top position: vertical distance from horizontal centreline to segment top
  • Segment inset: horizontal distance between distance and segment optical centres (not considering prescribed prism)

Calculations for Bifocals

  • Surface powers calculated using the thin lens formula: F = F1 + F2
  • Example: +0.75 = +4.50 + F2 => F2 = -3.75 D
  • For near portion: Fnear = +0.75 + 2.00 = +2.75 D => F1near = +6.50 D
  • Surface radii calculated using surface power formulae: = +110.67 mm, = +76.62 mm

Trifocals

  • Like bifocals, trifocals are worn by people who require added power for near vision and a proportion of this added power for intermediate vision
  • Intermediate and near adds derived solely from front surface curves
  • Powers for zones calculated as: near zone: +4.50 + 2.50 = +7.00 D, intermediate zone: +4.50 + 2.50/2 = +5.75 D
  • Solid trifocals either ground or moulded, supplied to prescription lab as semi-finished blank
  • Fused trifocals made only in glass, basic manufacturing process similar to fused glass bifocals, using two different glasses fused to main lens to produce intermediate and near adds

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