Optical Quality of the Eye Flashcards
what is spurious resolution
image phase shift at high spatial frequencies
In a diffraction limited system, what does defocus cause in the MTF
ringing or spurious resolution
when is the magnitude of spurious resoltion (amt of modulation beyond the first 0 in the MTF) larger when the type of SA is…..
when the type of spherical abberation is opposite in sign to the defocus lens
what is monocular dipolopia
when SA and defocus are of opposite sign
subj sees double images of lines
marginal and paraxial rays cross each other at the retinal plane => 2 hotspots in the LSF on the retina
how are pupil size and spatial frequency related to the best focus of the eye
small pupil=best focus indiependent of spatial freq
large pupil=best focus varies w/ spatial freq
low freq require more ______correction for optimal contrast sensitivity
minus correction
what is the measurement of wavefront aberration based on
measurement of ray deviation
-the angle btwn the actual ray path and the path expected if the optical system were perfect
what are the 3 methods to measure wavefront aberration of the human eye
what is the most common
- hartmann-shack: image a point on the retina and use a micro-lens array to sample the wavefront reflected from the eye (MOST COMMON)
- ray tracing: determine ray aberration for diff beam entry locatios in the pupil measured sequentially
- aberroscope aka tscherning method: send light in through matrix of pinholes in pupil plan and record ray positions on retina
how are the hartmann-shack method and the aberroscope method different
hartmann shack images the outgoing beam
aberroscope divide up the ingoing beam
what is wavefront error
difference btwn the eye’s actual wavefront and the reference wavefront
-represented as an elevation map over the pupil area
when is wavefront error considered positive
if the wavefronts are converging when leaving the eye
=the rim of the wavefront is toward the recordering camera
what kind of wavefront defocus error would a hyperopic eye have
negative bc the rays outside the eye would be digerging from the virtual far point of the eye
-wavefront rim facing the virtual far point, away from the recording camera
what is root mean square error
square root of the sum of the squares of all the indiv aberration coefficients
why are wavefront MTFs higher than double pass and interferometric meausrements
do not take into account retinal scatter, which affects both the double-pass and interferometric meausrements
what is the overall wavefront shape commonly specified in terms of
zernike polynomials
-which are a set of functions that are useful in describing intensity variation over a circular aperature
how to calculate zernike coefficient?
root mean square wave error that that particular term
total RMS=2nd order and higher
higher order RMS=3rd order and higher
-higher the order is smaller RMS values
what are the main aberrations of the eye according to zernike’s
lower-order aberrations
who may benefit from correction of higher order aberrations
pts w/ abnormal amounts of higher order ab
-pts w/ keratoconus and post refractive surgery
what are the 2 types of measurements on the human eye that lower optical quality
- retinal image quality
2. wavefront aberration
what is the point spread function
the intensity of distribution of the image for a point object
how is LSF and PSF related
LSF essentially a series of PSFs centered on each point on the line
what is a diffraction limited system
system free of aberrations
-blur only due to diffraction
what controls the size of the airy disc
pupil diameter
when may the airy disk not be ciruclar
aberratiosn such as coma or oblique astig present
