L19 Flashcards
(19 cards)
What is an optometer?
An instrument that measures refractive error, typically without using trial lenses.
On what principle is an optometer based?
Assessing the ‘Far Point’ of the eye.
What are subjective optometers?
Instruments requiring the patient’s input, often using image sharpness or doubling.
What are objective optometers?
Also known as autorefractors, these measure refraction without patient input.
What is the purpose of using a high plus lens in a simple optometer?
To locate the far point of the eye and assess refractive error without accommodation.
What are common problems of simple optometers?
Non-linear scale, large depth of focus, target-induced accommodation, no astigmatic value.
What is the main improvement in Badal optometers?
Linear relationship between target position and vergence; constant angular size of the target.
What is the Scheiner principle?
Uses image doubling through two pinholes to detect focus and identify ametropia type.
What happens in the Scheiner principle for myopia?
Occluding one pinhole makes the other image disappear; direction of disappearance indicates refractive state.
What happens in the Scheiner principle for hyperopia?
Opposite image disappearance occurs, indicating hyperopic refractive error.
How does the Young-Porterfield optometer work?
Uses Scheiner principle with a double slit and a movable marker on a central white line to identify the far point.
What are objective optometers based on?
Retinal image analysis, coincidence principle, and automated optics.
What is the principle of infrared optometers?
Use IR light, photodetectors, and separation of accommodation stimuli to measure refraction.
What are some techniques used in autorefractors?
Scheiner disc imaging, retinoscopy simulation, image quality analysis, knife-edge principle, ray deflection, image size analysis.
What are advantages of autorefraction?
Quick, repeatable, useful for screenings and in patients with communication difficulties.
What are limitations of autorefraction?
Over-minus tendency, issues with small pupils, irregular media, and poor fixation.
What is photorefraction?
Capturing images of fundus reflexes with light to assess refractive error, especially useful for infants.
What are photorefraction features?
Uses white/IR light, images analyzed digitally, reflex size and shape correspond to ametropia.
What are the major principles behind modern objective optometers?
Badal principle, Scheiner coincidence, and retinal image analysis.
