i need help understanding prism directions in bifocal lenses. I dont understand what base up and base down prism mean. Can someone please explain this to me in kindergarden terms.
i need help understanding prism directions in bifocal lenses. I dont understand what base up and base down prism mean. Can someone please explain this to me in kindergarden terms.
If you are a dispensing optician as your occupation states, and live and work in the state of New Jersey, you passed an exam to get your license and you should know this.
In a general basic sense, when you are looking through a lens with prism you perceive the image/object as having moved up/down/in/out. For example, a base up lens would make the world to appear moved up. Sometimes people feel like the floor is uphill when wearing BU. Now the optics of prism is more involved and calculating how much, what direction, from each particular lens/RX is more than we can properly explain with a optiboard post. Also, what type of conditions benefit from prism is a whole other topic. THere are lots of good free sources on the web about this or at book sellers if you want to buy a book.
no one can explain this to me?
There really is no prism in the distance on this diagram. Where the bases meet together is the optical center. This is assuming that eye is looking directly through this point. The base up prism is introduced as the eye gazes downward a total of 13mm. Using prentice rule .. Prism=Decentration(cm) X power(diopter). In this case the power in the 90 degree meridian is 4.50. That's where the 5.85 prism comes from. The round seg. bifocal produces base up. The two amounts of prism BD and BU cancel out leaving an overall difference of 3.35 BASE UP. MISPRINT on the illustration!
A FT bifocal would induce Base up prism.
Plus powers have base down above the optical center and base down below it. A plus power lens is basically two prisms placed together base to base. Thats why they are thicker in the center and thin on the edges. WHereas, minus lenses have base up above the optical center and base down below. A minus lens is two prisms placed apex to apex, thin in the center thick at the edges. Look at the cross sections of plus and minus lenses and you will see this.
plus
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/ \
\ /
\/
minus
\ /
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/\
/ \
When you are talking about bifocals then there is a litttel plus lens (seg) fused to the main lens. The seg induces prism are you gaze across the lens and the amount of prism varies depending on the seg type. The center of the seg (similar to the optical center of the main lens) to the outer edge of the seg is called the pole position. Have you ever noticed that FT28 isn't really a circular seg exactly cut in half? Where a round seg is a complete circle. Or a trifocal is almost a complete circular seg but with just a bit lopped off the top?
These segs are plus lenses in themselves and create base down prism as you gaze downwards to reading and you suddenly catch the upper portion of the seg. This along with magnification causes image jump for wearers. A progressive has a zero mm pole position and so does an executive bifocal (exec is lopped of exactly across the middle). FT 22-28 have between 4-5mm pole position
I really depends on reading level... how many mm did you look down to read from straight ahead gaze. In distance, no prism if looking through optical center of distance lens, but you must look down to see through the segment add lens. So this downward gaze produces a Base Up effect in the plus distance RX.
However, the segment reading lens produces its own prismatic effect which combines with the distance lens prismatic effect at that particular reading level. Assume distance optical center is 4 mm above round top seg line, and 22mm round seg has its optical center 11mm below seg line... distance optical certer to reading optical center is total of15mm. But looking 13mm down to read from straight ahead would leave the gaze through seg lens at only 2mm above seg optical center, leaving 2 x +2.50/10 segment prismatic effect of 0.50^ base down.
The 'Net combined prismatic effect' at reading level of 13mm would be 5.85 ^BU (from distance lens) + 0.50^BD (from seg add power of +2.50)... and base up distance prism combined with base down seg prism effectively cancel out, leaving you with total 'net prismatic effect' of 5.35^BU.
Your illustration is grossly incorrect for a given reading level of 13mm.
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