Autorefractors & Phoropters Flashcards
(31 cards)
What is the definition of an autorefractor?
An instrument designed to automatically measure refractive error
What is an autorefractor also known as?
An automated refractor or an objective optometer.
What is an optometer?
A device to measure refractive error that obviates or restricts the use of trial lenses
What are the uses of an autorefractor and why?
Screening - Especially pre-screening in optometric practice Where speed and ease of use are required
Widely used in the developing world - Vision Aid Overseas
Measuring accommodation in research studies
Does the job of a retinoscope
Why does a px look at a distant target in an autorefractor?
To relax their accommodation as much as possible.
What is a simple optometer?
What are the disadvantages of a simple optometer?
- Large depth of focus
- The target can stimulate accommodation ( making the Rx wrong)
- Scale is non-linear ( in terms of progression)
- The apparent size of the target varies ( as you bring it closer)
- Proximal accommodation
- Astigmatism ( Simple optometers cannot measure astigmatism)
- Relies on subjective response
[All of these problems apart from Proximal accommodation, are largely solved in modern objective instruments]
What is proximal accommodation?
The awareness that a object is close to us thus we accommodate
What is the Scheiner principle and how is it used to measure vision?
It involves placing a Scheiner disk in front of the eye.
For an emmetrope when an object is placed at the far point, they will only see one line. A coincident image is placed on the retina.
For a myope they will see 2 crossed lines as image forms in front of the retina.
For a hypermetrope, they will see 2 uncrossed lines as the image forms behind the retina.
What is the advantage of the Scheiner disk over the simple optometer?
It allows for a better assessment of focus.
What are the disadvantages of the Scheiner principle?
- Target can stimulate accommodation
- Scale is non-linear
- When are images coincident- i.e. the images touching doesn’t necessarily indicate the end point
- Proximal accommodation
True or false - Modern autorefractors still use the Scheiner principle.
True
True or False - Both the simple optometer and modern autorefractors use visible light to measure refraction.
False - Modern autorefractors use infrared radiation
Why do modern autorefractors use Infrared radiation?
The retina reflects infrared radiation better than visible light.
Infrared light is invisible to the human eye - so no effect on px comfort or pupil diameter.
True or false - light of different wavelengths will focus at different lengths in the eye
True - some colours such as blue focus in front of the retina whereas other colours such as green focus behind the retina.
True or false - green light wavelength focuses on the retina.
True - that’s why we ask our px to look at the green of the duochrome when we refract them.
(It relaxes accommodation).
What is transverse chromatic aberration and what does it cause?
Transverse chromatic aberration is an aberration caused by different wavelengths focusing at different lengths in the eye. As a result colour fringes may be seen.
What calibration is needed in a modern Autorefractor as a result of using Infrared light as a pose to visible light?
An adjustment of around -0.50D to -1.00 D needs to be made to the ocular refraction due to using a different wavelength.
True or false - IR light is reflected from deeper in the fundus than visible light.
True - Visible light is reflected fro the ILM whereas IR is reflected from the sclera.
This requires a correction of around -0.50D. ( The machine applies this correction factor).
True or false- Autorefractors measure astigmatism
True
It measures power in six different positions ( whereas realistically it would only need to measure in three) in order to increase accuracy.
The modern autorefractor provides us with the sph/cyl and axis
What are typical specifications of autorefractors?
- Can measure Up to ± 25D of sphere
- Can measure Up to ± 10D of cyl
- Minimal Pupil Diameter needed is at least 2.0mm
- They use an Autofogging technique to relax accommodation
- Some have subjective refraction facility –Internal targets for duochrome, x cyl, fan and block, subjective refraction
- Some may also carry out keratometry
How can we attempt to control proximal accommodation?
By giving them a target that appears in the distance ( e.g. a hot air balloon in the distance of a picture).
Modern autorefractors use automatic progressive fogging in order to try to control this.
What is the benefit of the handheld autorefractor?
Can be used for Pxs who are disabled and cannot move to the machine.
What are the drawbacks of IR autorefractors/optometers?
▪ Pupil diameters of 3mm or less produce may not give a reading ▪ Cost - they are expensive
▪ Requires good fixation - children may get bored
▪ Retinoscopy provides information on cataract assessment – retro-illumination - this is not provided by autorefractors
▪ Proximal accommodation remains an issue
▪ Tends to overminus - particularly with children
