Lenticular Optics Flashcards
The horizontal visible iris diameter, HVID
The horizontal corneal diameter
Vertical visible iris diameter (VVID)
Vertical corneal diameter
The anterior corneal curvature (ACC) relates to the ______ of the cornea
Shape of the front surface
Crystalline lens proved approximately ______ of the total static refractive power of the eye
1/3
What are the two purposes of the crystalline lens
- combines with cornea to form image on the retina
- mechanism for focusing at different distances
What kind of optical elements is the crystalline lens
Active
Crystalline lens is ______ in form
Biconvex
What is the diameter of the crystalline lens
9mm
What is the thickness of the lens when it is relaxed
3.6mm
How much bigger is the anterior surface radius of the lens than the posterior
1.7 times
Cornea is what kind of optical element
Static
What are the fundamental building block of the lens
Many lens fiber cells arranged in a hexagonal pattern
What do lens fibers cells do
Interdigitate to form sutures that make the lens very strong, and the suture is continuous through entire lens thickness
Lens growth throughout life
Grows constantly with new cells forming at the equator, then elongating as ‘fibers’ that wrap around the peripheral surface of the lens
What happens when the anatomy of the lens changes
The optical properties do to
What happens when the lens fibers are too coarse? *20microns)
Act as a diffraction grating
Close packing of the lens does what
Reduces light scatter at cell boundaries
How much light does the lens absorb
Very little due to no pigment
Lack of nutrients in the lens means what
These cells are metabolically inactive.
Why is the lack of nutrients and the fact that it is metaboclicalyl inactive goof
It is conducive for transmitting light, but cell damage cannot be reversed (cataract)
Anterior surface power of the crystalline lens
+5.1
Posterior surface power of the crystalline lens
+8.5D
What is the total lenticular power and why is it particular
+13.3D, it should be 21.35D and the difference is due to the different refractive indices in the lens
Where is the RI largest in the lens
Canter (1.402)
Where is the RI smallest in the lens
Edge (1.386)
A lens with a varying RI is called
Gradient index lens (GRIN)
What makes up for the +8D of optical power of the lens
The small 1.2% change in RI between 1.402 and 1.386
Manufacturing of GRIN lenses
Very difficult to manufacture and therefore rare in commercial optics, however such lense are common in nature
What would happen if we replace the gradient index of human eye lens with a single lens with the same thickness, radii of curvature and refractive index?
The refractive index of the lens should be higher than the maximum RI of the gradient lens
What is the equibvalent refractive index for lens
1.42, want to choose higher
How does the human crystalline lens generate refraction?
- varying refractive indices
- varying its axial length
Where does refraction occur at the lens
Both the anterior and posterior surfaces of the lens as well as within
The combined refraction of all surfaces of the lens produces _____ (relaxed)
+21..35D
At accommodate state, lens refraction is _____
31.85D
Relaxed axial thickness of lens
3-6mm
Accomodative axial thickness of lens
4mm
The distance between the near and the far points on the lens
Accommodation range
When is the lens focused on the far point
Relaxed
When is the lens focused on the near point
Accommodative
What is a normal near point
2.5cm
The difference between the vergence of the far and near points
Amplitude of accommodation
Calculate the amplitude of accommodation (AA) of an eye with a far point of 1.25m and a near point of 10cm
Since AA is the difference between the vergence of the far and near points
The vergence of the far point is 1/1.25=0.8D
The vergence of the near point is 1/0.10=10D
The difference is 0.8D-10D=9.2D
What is the equation to calculate AA based on age
AA=15-1/4(age)
10 y/o
15-1/4(10)=12.5D
50 y/o
15-1/4(50)=2.5D
______ of both UV and visible wavelengths decrease with increase in age
Transmittance
Scattering of light in aging eye
Happens in both forward and backward direction, increases with age, after 40 y/o
Lens axial thickness and age
Increases all through life at a rate of 13 microns per year. This increase in axial thickness of lens decreases the anterior chamber depth through out life
Anterior chamber depth and aging lens
Decreases because the lens axial thickness is increasing and taking up the room
Anterior radius of curvature (shape) of lens and age
Reduces with increase in age. This change is from 16mm at 8yrs to 8.3mm at 82yrs
Posterior radius of curvature(shape) of lens and age
Reduces slightly with increase age. This change is from 8.3mm at 8yrs to 7.5mm at 82 yrs
Equatorial diameter of the lens and age
Of the unaccommodated lens increases from 8.5mm to 9.5mm between the age 15-85
All possible shape changes in the lens and age
Declines with age, as reflected in the decrease in AA with age
Any disturbance in the optical homogeneity of the lens, i.e., local changes in the refractive index, including fluid filled pools and opaque spots that scatter light
Cataract
Color of lens and age
Yellows
What does the nucleus of lens do with age
Scatters and prefertinally absorbs more light
Eye contains the natural crystalline lens
Phakic
Eye is without the natural crystalline lens
Aphakic
Eye contains an IOL
Pseudophakic
Best solution for phakic people (no lens)
- specs
- CL
- IOL (BEST OPTION)
