The 25 cm square calibrated diagnostic panel configuration

It All Started Over a Game of Blind Cricket

The greatest development in the field of low vision assessment in decades was a direct result of observing an international competition for blind cricketers. In October 2007, I was invited to Brighton, England, to join an Advisory Committee on Vision Standards for the World Blind Cricket Council (WBCC). Visual acuity served as the primary classification criteria to balance team compositions across three distinct impairment tiers. However, one of the rules relied on the highly unscientific metric of being “unable to recognize a human hand at any distance.” It was immediately obvious that a profound change was required.

I began my career as a low-vision clinician in 1972, rejecting the primitive clinical practice of counting fingers (CF) and using hand motion (HM) when vision is too poor for standard charts. These traditional methods are neither well-defined nor well-controlled. My early preference was to bring a Bailey-Lovie or Feinbloom chart very close to the patient, calculating a score from the viewing distance and letter size. It wasn't until late 2007 that I directed serious attention toward engineering an alternative for the extreme poor end of the visual resolution scale.

The structural catalyst occurred two weeks after the WBCC meeting, during an invitational seminar organized by Art Jampolsky at the Smith-Kettlewell Eye Research Institute (SKERI) in San Francisco, titled “Visual Prostheses: Toward Standards for Outcomes and Their Evaluation.” Researchers evaluating early visual electronic retinal implants were still defaulting to primitive CF and HM scripts. Michael Bach had developed the computer-based Freiburg Acuity and Contrast Test (FrACT), which provided excellent psychophysical thresholding algorithms.

However, drawing from my experiences testing children at Schools for the Blind in Malawi, I recognized a critical need for an acuity tool that was simple, entirely portable, free of electrical power or batteries, and operable in any environmental condition. I initially built a PowerPoint script using Single Tumbling E’s (STEs), square wave gratings, and quadrant localization targets. While functional in the UC Berkeley clinic, it was slow and clumsy. This led to a year-long collaboration on sabbatical with Jonathan Jackson from Belfast—cutting, pasting, and testing cardboard prototypes alongside Robert Greer and Marlena Chu in Berkeley, while consulting regularly with Hasan Minto in Pakistan.

Limitations of the Acuity Range with Letter Charts

The logMAR chart design by Bailey and Lovie (1976) standardized visual acuity testing using 5 letters per row, a logarithmic size progression, and proportional spacing. The universal gold-standard ETDRS chart (1982) adapted these principles to a 4-meter testing distance with Sloan optotypes. At 4 meters, the top row represents a logMAR value of 1.0 (20/200), subtending a visual angle of about 7.5°.

When a patient cannot read this row, the common practice is to move the chart closer. However, 1 meter is the absolute closest distance an ETDRS chart should be presented. At 1 meter, the top row subtends an angle of approximately 30° (±15°). Any wider, and the patient must employ compensatory head turns or body twists to read from edge to edge. Furthermore, at 1 meter, there is a 3.5% distance variation between the eye to the center versus the outer margin of the chart, introducing edge distortion.

At 1 meter, the top 40M row represents a logMAR value of 1.60 (20/800). Because the top row lacks an upper bounding row, its letters are less embedded or cluttered, presenting a slightly altered visual task. Therefore, the absolute physical limit of chart-based testing is reached when a patient fails to read the second row from the top at 1 meter—meaning their acuity is poorer than logMAR 1.60 (20/800). When a patient falls below this threshold, the task structure must be simplified.

Practical Dimensional Considerations

To manage a balance between broad range coverage, practical card volume, and manageable sizing steps, we standardized all cards to **25 cm (10 inches) square**. This uniform dimension makes the kit highly portable and allows the physical card edges to serve as a precise measuring tool to verify the 25 cm testing distance.

Recommended Standard Protocol Execution

The standard testing sequence for the BRVT is highly intuitive if the clinician remembers that, for patients with ultra-low vision, tasks must be simplified progressively and viewing distances must be systematically shortened:

Determining the Visual Acuity Score

Each Single Tumbling E and Grating card features calibration metrics printed in fine text along its margin, expressing values in logMAR, Visual Acuity Rating (VAR), decimal, and Snellen equivalents for both 20 feet and 6 meters.

Because each Single Tumbling E presents 4 alternative choices, a success rate of **4 of 4** demonstrates definitive resolution at or above that tier. For extended testing over 6 or more variations, a success rate **greater than 50%** (such as 4 of 6 or 5 of 8) is required to score. For the two-choice Grating Acuity test, the forced-choice threshold requires a success rate of **6 of 6**, or at least **80%** over extended presentations (e.g., 7 of 8 or 8 of 10).

The Clinician's Professional Responsibility

A primary obligation of all ophthalmic clinicians is to preserve and monitor the vision of their patients. When sight is too poor to be measured with standard wall charts, counting fingers and hand motion are simply not serious or sensitive diagnostic practices.

Quantifying residual function is critical to track underlying disease progression, modify medical or surgical interventions, and provide realistic advice to guide rehabilitation. As emerging electronic micro-arrays and biological gene therapies expand to restore sight to the profoundly blind, sensitive measurements with fine increments are mandatory to capture clinical success.

This rigorous metric definition is why international bodies—including the International Paralympic Committee (IPC) and the Blind Sports Association—mandate the BRVT for classification decisions to maintain competitive sports equity. In my opinion, it is professionally negligent to rely on finger counting or hand waving when accurate, inexpensive, and efficient tests like the BRVT are readily available to serve the visually impaired population with integrity.