Optics: Subjective And Objective Refraction Flashcards
On
The patient makes judgement on correct focus
Objective techniques:
Instrument makes a judgement of the correct focus
What is a simple optometer?
A device that controls the focus of the retinal image for the purpose of measuring the power of the eye
Advantage of a simple optometer?
Allows for both positive and negative vergence
Disadvantages to simple optometer:
Target vergence and angular size varies inversely with the target distance
What is a badal optometer?
Where an object can be moved toward or away from the eye without changing the retinal image’s angular subtense, but still allowing change in optical vergence
What are the principles of the badal optometer?
- an auxiliary lens placed between target and badal lens
- eye is placed at the back focal point of the badal lens
- the target vergence and angular size of the perceived object remains independent of the target distance allowing for positive and negative vergence
Four advantages to badal optometer:
-linear power scale
-uniform sensitivity
-irradiance of the eye
-constant focus (accommodation)
How is the badal optical system accomplished?
By placing a convex lens in front of the eye so that the secondary focal point of lens coincides with the primary focal point of the eyes optical system
What is the nagel optometer?
Based on a similar concept as badal, but there is a plus lens whose posterior focal plane is coincident with the nodal point of the eye
The nagal optometer changes ___________ and keep ____________ constant
Object distance, image size
What is stigmatoscopy?
Accommodative repose to the VA stimulus
When are optometer used?
For refractive error and accommodative response
What is a laser speckle optometer?
A laser light is reflected from a matte surface or transmitted through a diffuser, causes interference pattern at the retina that is perceived by the observer as a speckle pattern
How is the apparent velocity of the principle speckle optometer determined?
By the distance between the plane of the stationary and the plane of focus of the eye
Ina myopic eye, the speckle pattern appears to move in _________________as the moving screen?
The same direction
In a hyperopic eye, the speckle pattern appears to move in the ________________
Opposite direction of motion observed
How to know when you reached the point of being emmetropic?
Point source is axially adjusted until its conjugate with the retinal plane and the speckle will not appear to be moving, but appear to “boil” irrespective of the diffuser speed
Applications of laser speckle optometer?
-measure of subjective refractive error in dim conditions
-investigate aspects of accommodation, like determining resting place of accommodation
What is the resting place of accommodation?
The distance your eyes focus when there is nothing to focus on
What is a chromatic optometer?
A small white light viewed through a piece of cobalt glass with blue light focused in front of a red light inside the eye
What does an emmetrope see in a chromatic optometer?
A purple disc because the red and blue image are approx. equal in size
What does a myope see in a chromatic optometer?
A central reed spot surrounded by a blue stimulus
What will a hyperopic eye see in a chromatic optometer?
A blue central spot surrounded by a red annulus
When to know you have the right prescription using the chromatic optometer?
When the most positive lens placed to the eye that reduced the colored fringes to a single purple disc
What is the duochrome test?
A test that uses the eyes chromatic aberration in the determination of the near end of the refraction
What is photorefraction?
The refractive state of the eye is assessed by photographing the pattern of light returned from the pupil of the eye
What does the size and location of the pupil reflex in photorefraction indicate?
The degree and direction of refractive error?
Advantages to photorefraction:
-refracts both eyes simultaneously
-very quick -> good for kids
Applications of photorefraction:
-refraction of infants/children
-study of anisometropia
-diagnosis of astigmatism
Photorefraction of hyperopic eye:
-red rays will appear in the periphery and blue in the central point spread
Photo reflection in the myopic eye:
Blue rays appear in the periphery and red in the central of the point spread
What is considered to be the gold standard?
Subjective phoropter
What are the disadvantages to phoropters?
-poor repeatability
-not comprehensive
What is the shack-Hartmann aberrometer?
It gives the comprehensive description of the eyes optical aberrations, from which the retinal image can be computed using the theory of physical or geometrical optics
Where will the reflected plane wave be focused in a PERFECT eye?
A perfect lattice of point images with each falling on the optical axis of the corresponding lenslet
What does an aberration eye reflect?
A distorted wavefront
When the wavefront is distorted, what happens to the lens lets and the focus?
The wavelength is now different for each lenslet which causes the wavelengths to be focused into a distorted collection of spot images
How can you deduce the slope of the aberration wavefront?
By measuring the displacement of each spot from its corresponding lenslet axis
What can differ between objective and subjective refractions?
-chromatic aberrations
-stiles-Crawford effect
-instrument myopia
-maximum plus technique
-measurement planes
How to avoid confounding factors:
-compare the VA and wavefront when both measured with same wavelength, same pupil size, and without accommodation
Does the refractive error for subjective methods change when the pupil dilates?
Not significantly
How accurate is the a prediction of subjective refraction?
About 2/3 of the given objective metrics were able to accurately predict subjective refraction within 1/8th of the diopter THATS GOOD
