ABO study guide – Flashcards
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Th basic curve upon which many ophthalmic lenses are shaped is:
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Sphere
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The curve of a cut off section of a sphere or ball:
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Convex curve (plus lens)
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An "impression" of the sphere and its shape
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Concave curve (minus lens)
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The distance from the geometric center of the circle to its periphery line.
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Radius of curvature
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As the radius of curvature becomes longer the circle:
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Gets larger
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To determine the total power of the lens:
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add the front side to the back side of the lens
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Biconvex lens
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Both sides of lens plus
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plano convex
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One surface lens flat, and the other surface plus
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plus meniscus
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One surface of lens plus (front), and one surface of lens minus (back)
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Minus meniscus
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One surface concave (front), and one surface convex (back)
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plano concave
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One surface flat (front), one surface minus (back)
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Biconcave
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both surfaces are minus
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As lenses increase in plus power the ocular surface
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The ocular surface generally gets flatter
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Minus lenses are shaped with _____ back curves.
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steeper
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Base curve is always found on which side?
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bifocal segment side (from surface)
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As the powers increase in plus the curves gradually:
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gets steeper
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Base curves in minus lenses gradually become ______ as power increases:
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flatter
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Measures power in diopters, and curve of spherical lenses
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lens clock
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Base curve of lens power +12.25 and over
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+16.00
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base curve of lens power +10.75 to +12.25
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+14.00
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base curve of lens power +9.00 to +10.50
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+12.00
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base curve of lens power +5.50 to +8.75
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+10.00
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base curve of lens power +2.25 to +5.25
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+8.00
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base curve of lens power plano to +2.00 and plano to -1.75
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+6.00
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base curve of lens power -2.00 to -4.50
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+4.00
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base curve of lens power -4.75 to -8.00
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+2.00
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base curve of lens power -8.00 to -9.00
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+1.25 or +.50
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base curve of lens power -9.00 and over
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plano or minus
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Used for correction of an astigmatism
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cylinder
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A cylinder with no curve and contains no power
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cylinder axis
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In transposing an RX change the cylinder axis by how many degrees:
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90 (add if axis 90 or below, and minus is 90 and above)
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The distance from the front of the cornea to the ocular (back) surface of the lens
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vertex distance
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percent of cylinder power in effect 30 degrees from axis
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25%
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percent of cylinder power in effect 45 degrees from axis
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50%
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percent of cylinder power in effect 60 degrees from axis
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75%
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percent of cylinder power in effect 90 degrees from axis
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100%
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What is the power of -2.00 -.75 X180 in axis of 90:
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-2.75
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To find the power in 180 or 90:
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add sphere and cylinder
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If a plus lens is moved away from the cornea the power:
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Increases by .50
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To determine the power of a lens in air, it is necessary to consider:
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The front and back surface power, and Index of refraction
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A +1.75 diopter lens fabricated with a +6.00 back curve will require a back curve of:
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-4.25
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Toric transposition is used to determine:
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lens power in the principal meridians
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Which lens material will yield the greatest front surface power
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The lens with the higher index
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Corrected curve lenses are designed to:
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Increase the useful area of the lens
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As the radius of curvature circle gets larger, its curve gets
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flatter
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As the radius of curvature circle gets flatter on a transparent material (lens), the power in diopters of that surface becomes:
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less
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Consider vertex distance in spectacle lenses when the power exceeds:
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+/- 7.00
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When moving a high powered minus lens closer the compensated power must be:
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decreased
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When a plus lens is moved closer to the eye its effective power:
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decreases
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Compound lenses consist of:
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both cylindrical and spherical lenses
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A characteristic of the common lens aberration known as chromatic aberration:
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dispersion
