Selecting Acuity Systems for your practice.
Simple? Acuity systems have evolved since Herman Snellen developed his chart in 1862. However, one of Dr. Snellen’s conclusions has persisted… the average human retina can resolve a small line that subtends 1’ of arc.
Since then, research has resulted in dramatic improvements in all aspects of vision testing. Optotype selection, construction, spacing, contrast, luminance of charts, equivalence have all been refined and perfected (?)
Over the years, the “state-of-the-art” gets to ANSI (American National Standards Institute) and, ISO (International Standards). Before a standard is adopted or changed, it goes through extended, exhaustive, mind-numbing, analysis and debate on the merits and impact of the proposed standard or change. Then, they argue some more… (NCI was there).
For “Visual Acuity” charts we have the ANSI Z.80. This applies to printed charts, trans-illuminated cabinets, chart projectors and, yes, computerized vision testing systems.
Optotype Construction and Equivalence: The standard requires that the optotype performs the same as the “reference optotype”, the Landolt Ring. We currently construct the optotype in a 5x5 grid with each segment (lines or spaces) subtending 1’ of arc.
Spacing: ANSI Z.80 requires optotypes that are spaced no less than 1 optotype width apart, lines spaced no less than the height of the line above and, THAT THE SPACING IS CONSISTENT THROUGHOUT THE CHART. There’s a reason for this. It’s called “crowding”. The “gold standard” for acuity, the ETDRS charts, are spaced at exactly one optotype width … At NCI, we use this for all charts.
Contrast: The ANSI Z.80 requires no portion of the stroke to exceed 15% of the background luminance. The majority of acuity systems currently sold use “grey-scaled” fonts to produce their optotypes. A skilled programmer can put something that looks like an acuity chart on the screen in 15 minutes. The luminance across the width of the optotype stroke can be as much as 30% of the background luminance. It’s much, much more difficult to program optotypes in pure black on a pure white background but, all NCI systems, since 2001, have used this technology.
Luminance: Low luminance is LOW CONTRAST! If the contrast is reduced, the acuity of the optotype is reduced. A customer reported that few of his customers could read the 20/20 line at “best corrected” with his old chart projectors but, nearly all of them could with his new systems. Obviously, there was something wrong with the new systems. NCI has measured luminance with a calibrated photometer at a rather large clinic and found projectors that ranged from 9 cd/m2 (at worst) to 50 cd/m2(at best). They recorded the room where a patient’s acuity was taken …because they couldn’t get the same result in a different room. NCI systems are calibrated… you’ll get the same results in every room.
A note on ETDRS charts: The Early Treatment Diabetic Retinopathy Study (ETDRS), began in the 1960’s, immediately recognized that no two study sites could produce the same acuity scores. They developed special charts, using only the “Sloan” letters (C,D H,K,N,O,R,S, V), which were all within 3% of the difficulty of each other. They also discovered that each character was more or less difficult depending on its placement in the line. With extensive testing, they attempted to equalize the difficulty of each optotype and make each line within 1% of the difficulty of every other line. REAL ETDRS charts come, either from a test site or the NIH-NEI sponsors of the study. A LogMAR chart is NOT an ETDRS chart, even if it uses only the Sloan letters! Any chart that uses the letters “A,B,E,F,G,J,L,P,Q,U,T,X,Y,Z” is NOT an ETDRS chart! Real ETDRS charts required extensive validation. NCI’s ETDRS charts come from the NIH-NEI ETDRS chart set. There are several other validated versions… otherwise, IT”S NOT ETDRS!
Does this matter? Maybe not. You can refract a patient with a large Campbell’s soup can. If you want “state-of-the-art” in your exam room, a soup can probably won’t look good. An MD, Phd military surgeon did her doctoral thesis using a competitor’s system. When the review board asked if she had validated the acuity system, she found that the optotypes were grey-scaled and could not comply with ANSI standards. The military is picky. She had some… military words for the competitor. Depends on what you expect your vision testing system to do
My recommendations to the Z80 ANSI committee (Yes, NCI was there) was, that if we could not meet or exceed the precision of a properly illuminated paper chart, we had no business selling acuity systems. The recently adopted standards reflect this.
If anybody tells you “WE COMPLY WITH ALL THE ANSI STANDARDS” , ask them if they comply with ANSI Z4.3 …that’s the standard for portable toilets.
Thank you for reading!
Steven B. Nordstrom, President
NCI Vision Systems, Nordstrom Consulting, Inc.