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Functions Overview

The toolbar of our NEOD Acuity App provides access to all important functions:

  • Settings   to customize the settings of the acuity testing (acuity units, optotype appearance, chart type, acuity scoring, termination criterion and distance units),
  • Units Conversion   to convert between various acuity units,
  • Mail   to email/export the acuity data.
How to start a measurement session?

Follow these steps to start a session:

  • Create or select a patient entry,
  • Specify the current session (tested eye, correction, notes),
  • Specify the units to be used to report visual acuity and define viewing distance,
  • Specify the test to carry out in terms of protocol and procedures,
  • Select the optotypes to be used to assess visual acuity,
  • Insert the iPad mini inside our NEOD Phoropter Mount.


Here are a few important recommendations to follow before carrying out a test using the iPad mini with Retina display:

  • Set the testing luminance (in "Settings" section Settings) to an appropriate level (the International Council of Ophthalmology and the American National Standards Institute for ETDRS recommend a minimum photopic test level of 85 cd/m^2) and make sure the "Auto-Brightness" option is turned off in the iPad "Brightness & Wallpaper" settings,

  • Always verify that the viewing distance to the iPad is correctly specified because the precision of the acuity measurement critically depends on it,

Standard Optotypes

NEOD Acuity uses standard optotypes to test visual acuity in literate & illiterate people as well as preschool children, including the Landolt C and Tumbling E symbols considered as the reference optotypes by the International Council of Ophthalmology in its Visual Acuity Measurement Standard. Standard optotypes are specifically designed to appear equally recognizable contrarily to those used in the Snellen chart. The available optotypes are the following:


TIP: Touch the icon corresponding to the desired optotypes
to switch to the chart panel and start the testing
  • The Landolt C is a symbol that has only one element of detail and varies only in its orientation. The broken ring symbol is made with a "C" like figure, which subtends 5 minutes of arc in the 20/20 optotype and has an opening (oriented in the top, bottom, right or left) measuring 1 minute of arc. Edmund Landolt proposed the Landolt C in 1888 based on the fact that not all of Snellen's optotypes were equally recognizable. In its Visual Acuity Measurement Standard, the International Council of Ophthalmology considers the Landolt C the purest research standard and requires all other research approaches to be calibrated against the Landolt C. It is the recommended reference optotype for testing visual acuity, and the preferred visual acuity measurement symbol for laboratory experiments but gained only limited acceptance in clinical use.

  • The Tumbling E follows the same design principles as the Landolt C, but uses a stylized letter E instead. Charts based on this single optotype in various orientations were created by Hugh Taylor in 1976 to test visual acuity of Australian Aborigines. They have become standard for testing of illiterates and populations not familiar with the Roman alphabet or too young to read letters since they simply need to indicate the orientation of the symbols.

  • The Sloan set consists in 10 letters (C, D, H, K, N, O, R, S, V and Z) specifically designed by Louise Sloan in 1959 in order to avoid the problem that not all letters are equally recognizable in the Snellen chart. The Sloan chart was also specifically designed to follow a geometric progression of letter sizes.

  • The HOVT set consists in only 4 letters (H, O, T and V) which are more appropriate for testing visual acuity in preschool children under 5-6 years old. These optotypes are thought to be equally recognizable and maximally distinguishable. A child should be first taught the four symbols and then tested against this eye chart.

  • The Kid set consists in outlines of 6 figures (apple, house, circle, square, heart and star). It is adapted from the LEA™ Symbol and Patti Pics™ charts used to measure visual acuity in preschool children too young to perform the HOTV test. This chart follows the same logMAR design as the other charts.

  • The Numbers set consists in 8 digits (2, 3, 4, 5, 6, 7, 8, 9) found in the "Feinbloom Number Chart" designed by Dr. William Feinbloom in 1935 to test people with low vision. This chart follows the same logMAR design as the other charts.

  • Optotype Settings

    NEOD Acuity provides options in the "Settings" section Settings to customize the optotypes presentation:

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    • Appearance specifies whether the optotypes appear as black letters on white background or white letters on black background. If there is no counter-indication, acuity testing should be performed preferentially with black letters on white background.

    • Orientations of the Landolt C and Tumbling E optotypes can be randomized among 4 or 8 directions. The use of 8 orientations for the Landolt C can improve the reliability of the acuity measurement.

    • Confusion Bars can be added around the optotypes to investigate the crowding effect which is particularly important in Amblyopia.

    Acuity Units

    NEOD Acuity provides measurements in various standard units which are available in the "Settings" section Settings:

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  • Foot units are used in the US: visual acuity is expressed as Snellen fractions in foot relative to 20/20, the standard definition of "normal" visual acuity, that is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. If you have a visual acuity of 20/x, then if you stood 20 feet away from an object and the "normal" person stood x feet away, you would both see the same thing. If x is more than 20 feet, you have worse eyesight than normal, and if it is less than 20 feet, you have better than "normal" vision. Note, however, that despite being referred as the normal or standard visual acuity, 20/20 does not indicate a perfect vision but should be thought of as the lower limit of the normal visual acuity, the maximum acuity of a healthy human eye being approximately 20/16 to 20/12.

  • Meter units are used in UK: visual acuity is expressed relative to 6/6 (meters), equivalent to 20/20 (feet).

  • Decimal units are used in France: acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A value of 1.0 is equal to 20/20. Values lower than 1.0 mean worst acuity, and values higher than 1.0 mean better acuity than "normal".

  • logMAR units are used in clinical research where acuity is expressed as the logarithm of the minimum angle of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures visual acuity loss: positive values indicate vision loss, while negative values denote normal or better visual acuity. A value of 0.0 is equivalent to 20/20. Though this scale is rarely used clinically, it is more frequently used in statistical calculations because it provides a more scientific equivalent for the traditional clinical statement of "lines lost" or "lines gained", which is valid only when all steps between lines are equal, which is not usually the case.

  • VAR (Visual Acuity Rating) units provide a more intuitive scoring: the VAR scale is similar to the logMAR scale, but represented as a percentage. A value of 100 is equivalent to 20/20. Values lower than 100 mean worst acuity, and values higher than 100 mean better acuity than "normal".

  • CPD (cycles per degree) units express visual acuity in term of angular resolution. A value of 30 cpd corresponds to a resolution of 2 arc minutes per line pair (ie a 1 arc minute gap in an optotype) and is equivalent to 20/20. Values lower than 30 cpd mean worst acuity, and values higher than 30 cpd mean better acuity than "normal". The upper limit is directly related to the resolving power of the cone photoreceptors in the retina center (fovea) and by the imperfect optics of the eye: for a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD (equivalent to 20/12).

  • M-unit system (introduced by Louise Sloan) is generally used to measure near visual acuity and with low-vision patients, where 1 M-unit subtends 5 minutes of arc at 1 meter; it follows that a letter of n M-units will subtend 5 minutes of arc at n meters. M-units are commonly used on metric letter charts and in reading cards.

  • The selected acuity unit in Settings is the default unit used in all acuity tests. Note that the Tools section provides a tool for easily converting between the different visual acuity units.

    Viewing Distance

    The units for the viewing distance can be selected in the "Settings" section Settings and can be either in meters or feet:

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    The viewing distance (from the patient eyes to the device display) can be provided either in meter or foot units, and can be set to some preset values (25 cm, 33 cm, 40 cm, 63 cm, 1 m, 1.25 m or 10 in, 13 in, 16in , 25 in, 40 in, 4 ft). The viewing distance can be also set to any other value using the bottom slider or by taping on the distance label and entering a new value under the chart panel. This last set value is used every time the application is launched.

    Testing with NEOD Acuity

    Visual acuity is the most important test used to evaluate eyesight. It measures the eye's ability to resolve details at near and far distance. It usually involves reading letters or looking at symbols of different sizes (optotypes) on a wall chart, like the famous Snellen chart found in most physician offices and developed by Dutch ophthalmologist Herman Snellen in 1862.

    The most reliable acuity tests used today follow the logMAR design recommended by the National Eye Institute (NEI) and the International Council of Ophthalmology (ICO) to address design flaws in the 150 year-old Snellen chart: the charts provided by NEOD Acuity follow this logMAR design by implementing a geometric progression of letter sizes and proportional spacing between letters, with standard optotypes specifically designed to appear equally recognizable and appropriate for testing literate & illiterate people as well as preschool children.

    Moreover, to measure visual acuity in a precise and efficient way NEOD Acuity provides unique interactive logMAR charts that are randomly generated to prevent the subjects to memorize the test, and takes advantage of the enhanced display resolution of the iPad mini with Retina display to access near visual acuity in a precise and accurate way.


    With NEOD Acuity several protocols are available based on a combination of chart types, acuity scoring, and termination criterion. The current protocol is selected by customizing these options in the "Settings" section Settings:

    • 3 types of charts:
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      • Multiple lines
      • Single line
      • Single letters

    • 2 acuity scorings:
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      • Simple
      • ETDRS

    • 3 termination criteria:
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      • 1 incorrect letter
      • 3 incorrect letters per line
      • 1 incorrect line


    The general procedures for assessing visual acuity follow these basic steps (though methods may vary from practice to practice):

    • Position the patient at the appropriate distance,
    • Cover the eye not being evaluated (starting with the worst of the two eyes if known),
    • Ask the patient to read the optotypes from right to left,
    • Decrease the size of the optotype until the patient is unable to read or misses some of the characters on any one line,
    • Note the last line successfully read and record the incorrect result,
    • Repeat the above procedure for the fellow eye, then for both eyes together with and without corrective lenses.

    With NEOD Acuity, the experimenter interacts directly with the iPad through touch interactions with its display, and should follow these steps to measure near visual acuity:

    1. customize your protocol using the options in the "Settings" section Settings,
    2. customize the session options (tested eye, correction, notes) in the "Current Session" section of the subject panel,
    3. press one of the optotype icons to switch to the chart panel,
    4. set precisely the viewing distance using the bottom slider or using one of the preset values,
    5. adjust the vertical slider on the right to select a range of letter sizes that can be easily seen by the patient,
    6. press the + and - buttons to fine tune the letters size by small steps,
    7. touch and release the smallest line the patient can fully or partially read to save the corresponding acuity score,
    8. optionally indicate the number of errors to improve the accuracy of the acuity score,
    9. A test can be aborted at any time by pressing the Info Button button in the top left corner of the Chart panel.

    TIP: maintain and drag the touch along the multiple lines of a chart to shift the indication bar, release outside the chart area to have the indication bar go away

    Using our NEOD Phoropter Mount

    When the practitioner is setting the desired test distance they must align the inside marking (closest to the patient) on the NEOD mount window to the desired test distance on the rod if using the iPad mini or if using a near card must align the central marking on the mount window with the desired test distance on the rod. The reason for this is that the plain of the LED aligns with the inside marking on the mount window and the plain of the near card aligns with the central marking on the mount window. Once desired test distance is obtained the practitioner can fasten the mount.