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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?
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What is the advantage of bifocal lenses over single-vision lenses for presbyopic clients?
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What are the two powers required by a presbyopic client?
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What is the segment diameter?
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What is the difference between segment size and segment diameter?
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How is the segment height measured?
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What is the common misconception about the segment drop?
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How is the segment top position measured?
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What is the segment inset, and what is not considered in its measurement?
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What is the base curve of the multifocal or progressive lens referred to in the text?
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What is the formula used to calculate the surface powers in the text?
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What is the power of the near portion of the lens?
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What is the value of F1near in the near portion of the lens?
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What is the formula used to calculate the surface radii in the text?
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What is the value of 'n' in the calculation of the back surface radius?
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Why is it crucial to control the furnace temperature during the manufacturing process of fused bifocals?
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What steps must be taken to prevent air bubbles from forming during the manufacturing process?
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How are the different types of fused bifocals (and trifocals) produced?
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What is the purpose of grinding the additional piece of glass with a convex curve?
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What happens to the extra piece of glass on the top of the button when the fusing process is complete?
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How can different shapes of segment be produced in fused D-segment bifocals?
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What is the power of the near zone in the given example?
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What is the power of the intermediate zone in the given example?
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What is the formula to calculate the back vertex power of a solid trifocal lens?
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What is the process of manufacturing solid trifocal lenses?
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What is the main difference in the manufacturing process of fused trifocal lenses compared to fused bifocal lenses?
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What are the popular designs of fused trifocal lenses available in the market?
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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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Description
Understand how to determine the spherical equivalent for lens powers with a cylinder component, including calculating the location of the circle of least confusion. Learn how to apply this concept in optics prescriptions. Explore the relationship between spherical equivalent and astigmatic line foci.
