Subjective And Objective Refraction Flashcards
The patient makes a judgment of the correct focus
Subjective techniques
An instrument makes a judgement of the correct focus
Objective techniques
The half width of the diffraction limited PSF is 3.23. If the spherical aberration of 0.5 microns is added to this PSF, what would happen to the PSF
Increase
Calculate the strehl ratio, if the maximum light level of the diffraction limited PSF is 65 lumens, and the maximum light level of the aberrated PSF is 40 lumens
40.65=0.6
What gets transferred in a grating
Contrast (modulation)
How do you measure MTF
A’/A
Which of the following aberrations decrease in amplitude and experience transverse shift
Coma
The spatial freqqucny at which MTF goes to 0 is called as
Resolution limit
Devices that control the focus of the retinal image for the purpose of measuring the power of the eye are called
Optometers
What is the root name for the profession of optometry
Optometer
When are optometers subjective
If the patient judges the clarity of the retinal image
When is an optometer objective
If a machine or the examiner does it
What does a simple optometer consist of
Moveable target and a fixed positive power lens placed at its focal distance away from the eye
Advantages of simple optometer
Allows both positive and negative vergences
Disadvantages of simple optometer
Target vergence and angular size varies inversely with the target distance
What are the 3 draw backs of not using instruments in optometry
- increases target vergence
- object looks magnified (increase in angular size)
- no 0 or + vergence, only negative vergence
How did badal optometer overcame the challenges of simple optometer
Moveable auxiliary lens
Badal principle
- Auxiliary lens (minus) placed in between the target and the badal lens
- Eye is placed at the back focal point of the badal lens (plus)
- Then the target vergence and angular size of the perceived object remains independent of the target distance allowing for both positive and negative vergence
Four advantages of the badal optometers
- linear power scale
- uniform sensitivity
- irradiance of the eye
- constant focus: (accommodation)
How is power related to the actual target distance
Power is linearly related to the actual target distance
-power increases as target distance increases
What happens to the angular size with badal optometer
Remains unchanged
Irradiance of the eye in the badal optometer
The irradiance of the source at the eye is constant for any distance of the target
Focus and the badal optometer
Constant focus (accommodation) -the apparent position of the eye will be at optical infinity for all position of the target
Laser speckle optometer
Laser light reflected from a matte surface or transmitted through a diffuser causes an interference pattern at the retina that was perceived by an observer as a speckle pattern
Laser speckle optometer for myopic eye
The speckle pattern appears to move in the same direction as the moving screen
Laser speckle optometer for a hyperopic eye
The speckle pattern appears to move in the opposite direction of motion is observed
How does the emmetropic eye see the laser speckle optometer
As a “boiling” pattern
Newtons relationship for the speckle laser optometer
xx’=ff’
What is the vergence for the speckle laser optometer
V=1/x’=x/ff’
Application of the speckle laser optometer
- measurement of subjective refractive error in dim conditions
- investigation of aspects of accommodation, like determining the resting place of accommodation
Eye focuses when there is nothing to focus on
Resting place of accommodation
