The autorefractor is one of the first instruments a patient encounters during a comprehensive eye exam. In most practices, the paraoptometric operates the autorefractor as part of pre-testing -- gathering objective data about the patient's refractive error before the doctor begins the subjective refraction. Getting a clean, reliable autorefractor reading saves chair time and gives the doctor a solid starting point.
For the CPO and CPOA exams, you need to understand how the autorefractor works, how to operate it properly, what the printout means, and -- equally important -- why autorefractor readings are a starting point and not a final prescription. The exam tests not just procedural knowledge but your understanding of the instrument's capabilities and limitations.
This article covers the operating principles, step-by-step procedure, printout interpretation, factors that affect reliability, and the common issues you will encounter. If you run the autorefractor in your office, most of this will be familiar. If you are studying for the exam without much hands-on experience, pay close attention to the limitations section -- that is where exam questions like to live.
How an Autorefractor Works
An autorefractor projects a beam of infrared light into the eye through the pupil. This light travels to the retina and reflects back out of the eye. The instrument's sensors analyze the pattern and wavefront of the returning light to determine how the eye refracts (bends) light. From this analysis, it calculates the sphere, cylinder, and axis needed to correct the eye's refractive error.
The measurement is objective -- it does not depend on the patient telling you what looks clearer. This is one of its major advantages: it works the same way whether the patient is highly communicative or unable to give reliable subjective responses. The infrared light used is invisible to the patient, so it does not trigger accommodation the way visible light might (though accommodation can still occur from the proximity of the instrument).
Modern autorefractors use fogging systems to relax accommodation. The internal fixation target -- often a picture of a house, balloon, or landscape -- appears to move farther away during measurement, encouraging the patient's eye to relax its focus. This helps produce readings closer to the patient's true distance refractive error rather than an artificially myopic result.
Types of Autorefractors
Table-Mounted
The standard in most optometry practices. Patient sits at a chin rest and looks into the instrument. Typically combined with keratometry (kerato-refractometer). Provides the most reliable readings due to stable head position and precise alignment.
- Most accurate and repeatable
- Often includes keratometry
- Built-in printer for results
Handheld
Portable devices that the operator holds and aligns with the patient's eye. Useful for pediatric patients, wheelchair-bound patients, or bedside assessments. Readings may be slightly less precise due to movement.
- Works with uncooperative patients
- Portable for off-site use
- Can measure through small pupils
Operating Procedure
1. Patient Positioning
Seat the patient comfortably at the instrument table. Adjust the table height so the patient can rest their chin in the cup and press their forehead firmly against the headrest bar without straining. Both feet should be flat on the floor. Remove glasses -- the autorefractor measures the uncorrected eye. Contact lenses should generally be removed as well, unless the doctor specifically requests a reading over contacts.
2. Alignment
Using the joystick, align the instrument with the patient's right eye first (standard protocol). Most autorefractors display a live image of the eye on a screen. Center the crosshairs or alignment indicator on the pupil. The instrument will often auto-detect when alignment is correct and either take the measurement automatically or indicate readiness with an audible or visual cue.
3. Instruct the Patient
Tell the patient to look at the target inside the instrument and try to keep it in focus. The target may appear to move -- this is the fogging mechanism working. Ask them to keep both eyes open (even though you are measuring one at a time) to reduce accommodation. Tell them to blink normally between measurements but to hold steady during the brief measurement itself.
4. Take Multiple Readings
Most autorefractors take three readings per eye automatically and average them. If the readings are inconsistent (high standard deviation or low confidence indicator), take additional measurements. Move to the left eye and repeat the process. Some instruments measure both eyes sequentially without repositioning; others require you to move the joystick to the other eye.
5. Print and Review
Print the results and review the printout before the patient leaves the pre-test area. Check that readings were obtained for both eyes, that the confidence indicators are acceptable, and that the values are within a plausible range. Flag any readings that seem unusual -- extremely high cylinder, vastly different readings between eyes, or poor confidence scores -- for the doctor's attention.
Interpreting the Autorefractor Printout
A typical autorefractor printout shows the following for each eye: sphere (S or SPH), cylinder (C or CYL), axis (A or AX), and often pupillary distance (PD). Most instruments also report keratometry readings if they are kerato-refractometers. A confidence or reliability indicator may appear as a number, letter grade, or bar.
SPH (Sphere)
The spherical refractive error. Negative values indicate myopia; positive values indicate hyperopia. Expressed in diopters (D) in 0.25 steps.
CYL (Cylinder)
The amount of astigmatism. Can be reported in minus or plus cylinder form depending on instrument setting. Higher absolute values mean more astigmatism.
AX (Axis)
The orientation of the cylinder correction, from 1 to 180 degrees. Only meaningful when cylinder is present. Determines the direction of astigmatism correction.
PD
Pupillary distance measured by the instrument. May be less accurate than a dedicated pupillometer. Reported as binocular total or monocular values.
K Readings
Keratometry values showing corneal curvature in both meridians. Expressed in diopters and/or millimeters. Used for contact lens fitting and astigmatism evaluation.
Confidence
A reliability indicator showing how consistent the readings were. Low confidence suggests the reading may be unreliable due to poor fixation, small pupil, or media opacities.
Practice instrument and pre-testing questions
Opterio covers autorefraction, keratometry, and all CPO/CPOA pre-testing topics with detailed explanations.
Factors That Affect Reliability
Accommodation (Instrument Myopia)
The biggest source of error, especially in patients under 40. When the patient accommodates while looking into the instrument, the reading shifts toward more minus (myopia). Fogging targets help but do not eliminate this completely. Young patients may show 1-2 D more myopia on autorefraction than on subjective refraction.
Pupil Size
Very small pupils (miosis) limit the amount of reflected light available for analysis, reducing accuracy. Patients on miotic medications or in very bright rooms may have this issue. Very large pupils can also introduce more aberrations into the reading.
Media Opacities
Cataracts, corneal scars, or other opacities in the optical media scatter the infrared light and degrade the return signal. Dense cataracts may prevent any reading at all. Mild cataracts may produce readings with low confidence scores.
Irregular Corneas
Conditions like keratoconus, pellucid marginal degeneration, or corneal scarring create irregular astigmatism that autorefractors cannot accurately characterize. The instrument assumes a regular corneal surface when calculating refraction.
Patient Cooperation
Patients who cannot hold still, have nystagmus, or do not fixate on the target will produce inconsistent readings. Excessive tearing or dry eye can also affect the optical surface and alter results.
Why Autorefraction Is a Starting Point
This is a point the certification exams emphasize repeatedly: the autorefractor provides an objective starting point for the doctor's subjective refraction, not a final prescription. The autorefractor measurement and the final Rx may differ for several reasons.
Accommodation can make the autorefractor reading more myopic than the patient's true refractive error. The autorefractor does not assess binocular balance -- how the two eyes work together. It does not account for the patient's visual comfort, adaptation to their current prescription, or specific visual demands. And it cannot determine the near add for presbyopic patients.
The subjective refraction -- where the doctor places lenses in front of the patient and asks which option provides clearer vision -- refines the autorefractor estimate into a prescription that accounts for all of these factors. The autorefractor gets the doctor in the right neighborhood quickly; the subjective refraction finds the exact address.