Optics - REFRACTION Flashcards
What are the 2 ways Pinhole acuity works?
- It reduces the size of the blur circle
2. Increases depth of focus of eye
How many diopters can pinhole correct for?
3D
What does a Pinhole of <1.2mm do?
- Increase diffraction
2. Decrease amount of light entering the eye.
Legal blindness is defined as what?
- BCVA in better ye is 20/200 or worse
2. VF in better eye is 20 degrees or less in diameter.
Where is the far point for hyperopia and is it real or virtual?
Behind the eye and virtual.
What is the difference between far point and focal point?
Far point can extend all the way to infinity.
Focal point never more than few mm away from retina.
Total internal reflection occurs only from what to what type of index?
From Higher to Lower index.
What is an example of Chromatic aberration ?
Duochrome Test
In the duochrome test where is the green light focused?
In front of the eye. When Green is brighter, more plus is needed to move the green light to the focus closer to the retina.
What is the duochrome test pneumonic?
RAM - GAP
Up to what vision can the duochrome test be used?
20/30. Remember only 0.5D seperates red and green on visual axis spectrum.
What is COMA? What produces COMA?
Coma is an off axis spherical aberration. Peripheral rays produce coma. The image is a series of circles that form a comet shape. This is a primary problem for large aperture optical systems an can be ignored in spectacles b/c of limited affect of the pupil.
When the aperture is closer to the lens, great coma occurs.
What is the REFRACTIVE power of the Cornea?
45D
On the ETDRS chart a 3 line gain or loss indicate what of the visual angle?
Doubling or halving
What is stenopeic slit?
An elongated pinhole that may be useful as a guide to determine subjective astigmatic refractive error.
Vertex distance is measured from where to where?
From the back of the spectacle lens to the surface of the CLOSED eyelid.
Calculating the vertex distance for a myopic lens do you add or subtract the vertex distance to calculate the new lens power?
SUBTRACT
What does pantoscopic tilt do?
Changes BOTH sphere and cylinder of the same sign of the original lens.
How does Radial Keratotomy work?
Radial incisions that steepen the periphery (with compensatory flattening in the center)
When does Krypton have an advantage over Argon?
When there is VH or mature len b/c it is poorly absorbed by heme and xanthophyll.
Why does Diode laser work through sclera?
Minimal diode laser is absorbed by the sclera and is SOLELY absorbed by MELANIN in RPE layer with thermal damage to surrounding tissues.
What is the order of cycloplegics from most to least?
Atropine, scolpolamine, homatropine, cyclopentolate, tropicamide
Submerging a persons head in water without googles does what to their refraction and why?
Refractive hyperopia. An increase in the index of refraction of water induces a decrease in power of the cornea.
What is 2 examples of spherical aberration?
- Night myopia (low light causes a larger pupil allowing more peripheral rays to enter the eye and the focus shifts anteriorly)
- Fluctuating vision following keratorefractive surgery
The bifocal segment is usually how far down from the optical axis?
2mm
When people read how far down and in do they look?
8mm, 2mm
According to Knapp’s law, the anterior focal point is how anterior to the cornea?
15.7mm (people like to work at 12mm and this causes 2% change in magnification or minification of the image)
During duochrome test if the patient see red better what does that mean?
too hyperopic - add minus (RAM)
During duochrome testing if the patient sees green better what does that mean?
too myopic - add plus (GAP)
Thick lenses with multiple nodal points does what to the dioptric power of a lens?
REDUCES!!
In retinoscopy the orientation of the streak equals?
AXIS (horizontal or vertical)
In retinoscopy the direction of movement equals?
Meridian of POWER (up or down, left or right)
