How does an autorefractor work? In a few seconds per eye it shines invisible near-infrared light into the eye, captures what comes back from the retina, and turns that returning light into sphere, cylinder and axis. While it measures, a fixation target relaxes the patient's focusing. This guide is written for the technicians who run the instrument and the practice owners who buy it. It covers the optical principles behind the numbers, the operator steps, and how to read every line of the printout. It also covers what the printout cannot tell you.
This is not a buying guide. For choosing a unit, see our autorefractor buying guide. This post explains what happens inside the instrument, so the readings it hands your doctors are easier to judge.
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The short version: what happens during one reading
A typical table-top autorefractor-keratometer (AR/K) runs the same sequence on every eye:
- Fixation. The patient looks into the instrument at an internal target that is optically placed far away.
- Fogging. The instrument blurs the target slightly past the patient's far point, which encourages the eye to stop focusing (accommodating).
- Alignment. The operator, or an auto-tracking system, centers the measuring head on the pupil and sets the working distance.
- Measurement. A near-infrared beam enters through the pupil, reflects off the retina, and the returning light is analyzed by a sensor. Several readings are usually taken in quick succession.
- Calculation and output. Software converts the measurement into sphere, cylinder and axis at the vertex distance you selected. It then prints or transmits the individual readings, a representative value and a reliability indicator.
On an AR/K, a separate ring projected onto the cornea gives the keratometry readings in the same sitting.
Why autorefractors measure with infrared light
An early Scheiner-principle autorefractor patent explains the choice. The light source is placed in the infrared region "so as to be invisible and hence not as a stimulus to accommodation" (US Patent 4,162,828). A bright visible beam would make the patient react and focus. A near-infrared beam does not.
Published wavelengths show how this works in practice. Topcon's KR-800A user manual lists its measurement LED at 870 nm. It lists separate LEDs at 950 nm for alignment and observation and 940 nm for the keratometry ring. The fixation target LED, which the patient is meant to see, is in the visible range at 530 nm. A 2025 case report in Cureus describes autorefractors as typically using infrared light of 800 to 900 nm.
Infrared light has a cost. The eye focuses near-infrared light differently from visible light. The same case report notes that infrared light focuses behind the retina. A 2023 optics paper calls this a "chromatic focal shift" that would cause erroneous refraction "if left uncorrected." The instrument's software has to account for that difference.
The measurement principles behind the spec sheet
"Autorefractor" covers several different optical designs. A 2022 review of six instruments lists the main principles as "wavefront analysis, Scheiner disc principle, best focus principle, ray-deflection, and photo refraction." The ones you will meet on spec sheets are these:
| Principle | How it works, in brief | Where you see it |
|---|---|---|
| Scheiner (double-pinhole or double-slit) | Light passes through two apertures. On an eye that is out of focus, the two images on the retina do not line up. The instrument adjusts until they do, and the size of that adjustment gives the refractive error. | Automated optometer designs, such as the double-slit Scheiner projection system in US Patent 4,162,828 |
| Retinal image size / ring image analysis | A ring or pattern is projected onto the retina. Its returning size and shape are analyzed across meridians to calculate sphere, cylinder and axis. | Many table-top AR/Ks. One study's methods table lists the Topcon RM-8900 as "Image size assessment using rotary prisms." Another lists Nidek's TONOREF III and HandyRef-K as "Retinal image size." Our Ezer ERK-770 uses motorized rotary prism technology. |
| Shack-Hartmann wavefront | An array of small lenses splits the returning wavefront into many spots. How far each spot is displaced shows the shape of the wavefront, and refraction is calculated from that shape. | Wavefront refractors and some handhelds. Our Ezer ERH-770 lists "Shack-Hartmann aberrometer" as its principle. |
| Eccentric infrared photorefraction | A camera reads the pattern of light in the pupil from an off-axis infrared source. | Photoscreeners, which are screening devices rather than lane autorefractors |
Two further design details explain other spec-sheet lines:
- Minimum pupil diameter. Every design needs a large enough pupil to send light in and get it back. Topcon's brochure says its rotary prism design measures through pupils "as small as 2 mm in diameter." Our Ezer ERK-BH and ERK-770 both publish a 2.0 mm minimum pupil.
- Measurement zone. Some makers measure a wider area of the pupil. Nidek describes a "large pupil zone imaging method" on its ARK-1 series that measures refraction "up to 6 mm diameter."
Fogging and the fixation target: controlling accommodation
An autorefractor measures the eye's refractive state at the moment of capture. If the patient focuses on the target, the reading shifts toward minus. Fogging is the instrument's answer. One autorefractor patent describes the purpose of the fixation target this way: to draw the eye's focus "to its most distant refractive state, and then beyond," which is what fogging the eye means (US Patent 11,877,797). In a Badal optical arrangement, the target can be moved toward optical infinity while appearing to stay the same size.
Manufacturers present this as a feature:
- Topcon's KR-800A brochure notes that a feature which will "fog" the fixation target can be adjusted through the touchscreen, especially for young children.
- Nidek says fogging lets the patient view the target clearly and limits interference with accommodation "even in high astigmatism."
- Our Ezer ERK-770 lists a "follow-up colorful fogging target system."
Fogging reduces the problem but does not remove it. The 2022 review reports that autorefractors with built-in fogging give "less myopic or more hyperopic values" than designs without it. It also notes that closed-field autorefractors "can induce instrument myopia." A separate study describes the table-top closed-view design as presenting a target "at physical proximity" while simulating distance viewing. It calls variable accommodation "a known challenge" with that design. In practice, the reading from a young, strongly accommodating patient deserves the most skepticism.
Keratometry on an AR/K: the second measurement
On a combined autorefractor-keratometer, a ring of light (Topcon lists a 940 nm kerato ring) is reflected off the front of the cornea. The instrument measures the size and shape of the reflected ring and reports:
- R1 and R2: the radius of curvature of the flat and steep meridians, in millimeters
- Corneal power in diopters for each meridian, with the axis
- Corneal astigmatism (CYL) and usually an average
Converting millimeters to diopters needs a refractive index. Keratometry conventionally assumes 1.3375. This is a fictional value that, as the Wikipedia article on keratometers explains, "includes an allowance for the small, yet significant, negative power of the posterior corneal surface." Our ERK-BH spec table states its corneal power range at 1.3375. Instruments set to a different index give different diopter values from the same millimeter reading. For the details, read what a keratometer is and how it works.
How to use an autorefractor: the operator steps
Controls vary by model, so always follow your own operator manual. The sequence below follows Topcon's published KR-800A user manual and applies to most table-top AR/Ks:
- Select the mode and settings first. Choose REF, KER or combined K/R. Confirm the vertex distance and cylinder sign your practice uses (see the printout section below), because these change what is printed.
- Seat the patient. Pull the measuring head back toward you. Have the patient place their chin on the chin rest and their forehead against the forehead rest.
- Set the eye height. Raise or lower the chin rest until the eye-height mark on the chin rest support is level with the patient's eye.
- Ask the patient to look at the target. Ask them to blink before the capture rather than during it.
- Align and focus. Move the head toward the eye being measured until the pupil is centered and a reflected alignment dot appears on the cornea. Topcon describes it as a "vague, reflected alignment dot" that sharpens as you focus.
- Let the instrument fire, or press to measure. In semi-auto mode the KR-800A completes fine alignment and measures automatically once you are close. In auto-shoot mode it starts as soon as the eye is in range. Topcon's settings allow 1 to 10 continuous measurements per eye, with 3 as the initial value. The 2022 review describes Nidek's TONOREF III the same way: three consecutive measurements, averaged.
- Switch eyes and repeat. Slide the head across to the other eye and repeat the alignment.
- If the instrument will not take a reading, check for frequent blinking, a drooping lid or a small pupil. Topcon notes that semi-auto measurement may not be possible with frequent blinks or irregularities of the corneal surface. Its manual also includes a cataract mode for eyes where normal measurement fails because of a cataract.
- Print or send the result. Print it, or transmit it to the phoropter or EMR, before seating the next patient.
Auto-tracking models reduce the joystick work. Our ERK-770 uses automatic binocular positioning with an integrated camera to locate both eyes. The steps above still describe what the instrument is doing for you.
A handheld autorefractor works differently in the hand but follows the same logic: align on the pupil at the correct working distance and hold steady while it captures. Our ERH-770 sends results to a Bluetooth printer. For the full field workflow, see our handheld autorefractor guide.
How to read an autorefractor printout, line by line
Printout layouts differ by brand, but the fields are largely the same. Topcon's printout legend lists vertex distance, cylinder sign, individual right-eye readings ("recordable up to 10 measurements"), a typical value for each eye, pupillary distance and a reliability factor. Here is what each field means and what to check:
| Field | What it shows | What to check |
|---|---|---|
| VD (vertex distance) | The plane the refraction is calculated for. Topcon offers 0.00 mm (contact lens), 12.00 mm and 13.75 mm (eyeglass lens). Our ERK-BH offers 0.0, 12.0, 13.5 and 15.0 mm. | Make sure it matches how the number will be used. A 0 mm reading and a 12 mm reading of the same eye are not interchangeable at higher powers. |
| Cylinder sign | Whether cylinder is printed in minus form, plus form or mixed. Topcon and our ERK-BH both offer -, + and MIX. | Use the same convention as your phoropter and your doctors. A plus-cylinder printout read as minus-cylinder is a transcription error waiting to happen. |
| R / L rows | The individual readings for each eye: S (sphere), C (cylinder) and A (axis, 1 to 180 degrees). | Look at how closely the individual readings agree. If they vary a lot, the averaged value is hiding that spread, so re-measure. |
| Representative value | The single value the instrument reports for each eye from its set of readings. Topcon calls it the "typical value." | This is the figure that is usually carried into the subjective refraction as the starting point. |
| Reliability indicator | Topcon prints a reliability factor from 1 to 9, "in increasing order of reliability," and omits it when reliability is high enough. Keratometry values get a * mark when reliability is too low. | A low number or a * means re-take the reading. Other manufacturers use their own scales or error codes, so learn the one in your manual. |
| PD | Pupillary distance in millimeters, measured between the two pupil positions. Our ERK-BH lists a 10 to 88 mm range. | Useful as a cross-check. It does not replace a dispensing PD measurement. |
| K readings (R1, R2, AVE, CYL) | Corneal radius in mm and power in diopters for the flat and steep meridians, with axis, average and corneal astigmatism. | Check the refractive index setting before comparing with another instrument's K values. |
On screen, Topcon's convention is simple: figures alone mean the measurement worked, and "ERROR" means it did not. Do not transcribe a reading that came with an error flag or a * mark.
What the printout cannot tell you
An autorefraction is a strong starting point, not a final prescription. Authoritative sources agree on this:
- EyeWiki's article on the cross-cylinder technique says a "rough estimate" can be obtained by retinoscopy or automated refractometry. It says cross-cylinder examination "remains invaluable in fine-tuning the refraction for best vision."
- The 2022 review states that autorefraction "has not been able to completely replace subjective refraction," citing accommodation and blur sensitivity among other factors.
- One comparison study reported limits of agreement that "translate into starting points of subjective refraction that may be as far as ±2 D away" from the actual refractive error in clinical settings.
Readings are least dependable when the pupil is near the instrument's minimum diameter, when the patient cannot hold fixation, when they blink through the capture, or when the ocular media scatter the beam. In those cases, re-take the reading, use the instrument's special modes, and give the subjective refraction more weight. The doctor's clinical judgment about the eye itself is outside the scope of this article.
Table-top vs handheld: same job, different optics
The two form factors often use different principles. Table-top AR/Ks such as the Ezer ERK-BH and ERK-770 sit on a chin rest and forehead rest, use a closed-view internal target with fogging, and usually add keratometry. The handheld ERH-770 uses a Shack-Hartmann wavefront sensor, publishes a sphere range of -20 to +20 D and a cylinder range of -12 to +12 D, and is built for screening and work outside the lane. For a side-by-side spec and practice-fit comparison, see our best autorefractor keratometers comparison and the Ezer ERK-BH autorefractor keratometer guide.
Ezer autorefractors from US Ophthalmic
Ezer is US Ophthalmic's own house brand. It is backed by our in-house technical and spare-parts department and by US warranty and service. Three models cover most practices:
- Ezer ERK-BH: a table-top AR/K with five modes (K/R, REF, KER, contact lens base curve and peripheral keratometry), a sphere range of -30.00 to +25.00 D, selectable VD and cylinder form, a 2.0 mm minimum pupil and a built-in thermal printer with auto-cutter.
- Ezer ERK-770: a fully automatic table-top AR/K with camera-based binocular positioning, motorized rotary prism measurement, a fogging target system, a 10.4-inch touchscreen and USB, RS-232C, Bluetooth and Wi-Fi output.
- Ezer ERH-770: a handheld Shack-Hartmann autorefractor with Bluetooth printing for screenings and satellite sites.
Want help matching a model to your lane, your staffing or your EMR? Request a quote and our team will send the full spec sheet, warranty terms and financing options.
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Frequently asked questions
How does an autorefractor work, in one sentence?
It sends near-infrared light into the eye while the patient views a fogged target, analyzes the light reflected from the retina, and converts that into sphere, cylinder and axis at the vertex distance you selected.
Why does an autorefractor use infrared light?
Infrared light is invisible, so it does not trigger the patient to focus during the measurement. The software then accounts for the difference between how the eye focuses infrared and visible light.
What does fogging do on an autorefractor?
It blurs the internal target slightly past the patient's far point, which encourages the eye to relax its focusing. Built-in fogging gives less myopic readings than designs without it, but closed-field instruments can still induce some instrument myopia.
What do SPH, CYL and AX mean on an autorefractor printout?
SPH is the spherical power in diopters, CYL is the cylindrical (astigmatic) power, and AX is the cylinder axis from 1 to 180 degrees. Check the printout's cylinder sign (minus, plus or mixed) before transcribing.
What is the reliability number on an autorefractor printout?
It is the instrument's own confidence in the reading. On Topcon's KR-800A it is an integer from 1 to 9, where higher means more reliable, and it is not printed at all when reliability is high. Other brands use different scales or error codes, so check your operator manual.
Can an autorefractor replace a subjective refraction?
No. Published reviews describe autorefraction as a starting point that has not replaced subjective refraction, because of accommodation, blur sensitivity and measurement variability. The final prescription is still refined subjectively.
What vertex distance should the autorefractor be set to?
Set it to how the result will be used: 0 mm for contact lens power, or the spectacle vertex distance your practice standardizes on, such as the 12 mm or 13.75 mm options Topcon offers. Whatever you choose, keep it consistent across instruments and staff.
Sources
- Topcon KR-800A User Manual and KR-800A brochure (Topcon Corporation)
- Nidek ARK-1 series product page (nidek-intl.com)
- Comparison of six autorefractor designs, 2022, PMC9704684; Padhy et al., Translational Vision Science & Technology, PMC7794271
- Near-infrared vs visible refraction, PMC10093131; case report, Cureus 2025, PMC12326287
- US Patents 4,162,828 and 11,877,797
- Wikipedia: Autorefractor; Keratometer. EyeWiki (AAO): Cross-Cylinder Technique for Subjective Refraction
- US Ophthalmic product specification tables: Ezer ERK-BH, ERK-770, ERH-770
