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  • Dr., Marc Lay

What is visual acuity and what is the best way to evaluate it?

Updated: Dec 18, 2019

That is probably a condescending question and title to put forth to a group of optometrists and ophthalmologists! Refining it a little bit more, we know that visual acuity in an eye exam is the ability to resolve the minimum angle of resolution for a given optotype, which is 1/5th of a snellen optotype vertical or horizontal dimension. That is the theory, but we want to evaluate this question and all of its implications from the perspective of a clinician.

Given that we are discussing this in the context of practicing eye care professionals, the more relevant question is: can we do a better job of assessing visual acuity? Surely the projected charts we have used for years are adequate... or are they?

Before we even look at the chart, let's evaluate ambient lighting in the room. Increased ambient lighting will 'wash out' any letters displayed on a silvered screen, thereby effectively resulting in a artificially degraded visual acuity. The optotypes displayed on the chart should have no luminance for a projected slide, but this is simply not true in the real world, as most of the projected charts have imperfect slides or ambient light casting a veil over the chart. A rough analogy is the Ishihara color plates. In a typical office, the lights are turned on and the tech flips through the color plates. However, I personally know that not using illuminant C as indicated makes the test easier for our deuteranomalous patients. Improper illumination results in a false degradation of visual acuity and artificially elevates visual acuity. Our most basic assessment is flawed because of the environment in which visual acuity is evaluated. Furthermore, the need to keep ambient light reduced makes it more difficult to perform some elements of an exam, as with a sensorimotor evaluation. If we are using a digital acuity chart, then the ambient lighting does not affect the desired contrast.

An ideal method for assessing acuity is not only more reproducible, it should also facilitate the eye examination as a whole. Let's look deeper into those examination elements mentioned earlier. When we look at the amount of time spent using the acuity chart during pretesting and refraction, it adds up to a significant portion of the exam. The worst case scenario is using a projector and slide. In this scenario, the examiner must be directly present to manipulate the chart, and while some practitioners have perfected its use, there is no arguing that having to adjust the projector manually is inefficient and cumbersome. Not to mention that a big clunky projector mounted to the wall dates a practice to the mid 20th century! And so enters the automated projection systems. This was a nice boon to efficiency because now you can use the remote while documenting or refracting, and you are released from the chains of manually adjusting the projector. However, as with any slides, the background eventually becomes contaminated with dust, bulb changes are even more of a hassle, and the remotes had a layout that makes a Boeing 747 look like a child's toy.

Thankfully, we now have many options that surpass the dinosaur days of projected acuity charts. Enter the era of digital acuity charts. Sometimes we call them digital acuity systems, digital eye charts, or visual acuity software. Whatever your favorite term is, we can now fully control the optotypes contrast and the background luminance without interference from ambient room lighting. Of course, we can accomplish the same thing with a back-lit physical chart, such as ETDRS, but the digital acuity chart allows us to easily change those optotypes and isolate them as well.

Digital acuity charts, or digital acuity systems, also provide more efficiency during the examination process. There are no more frustrating bulb changes in the middle of a busy examination schedule. Digital acuity charts such as the eSee Acuity System at have an LED back-lit display to ensure long-life, unlike some other digital charts that have a fluorescent back-lit display, which will burn out sooner.

So we finally have arrived at a time where we can have the consistency of a digital acuity chart, but at a price point that is no longer in the stratosphere. We always encourage any doctor to evaluate all of the digital acuity chart options and see if there is really an added value for paying 100% to 200% more for a digital acuity system.

Now we have all of the benefits of accurate, efficient, and reproducible acuity assessment. So, what is next on our list to see if digital acuity is right for your office?

The best way to evaluate an acuity chart is to ... USE IT! This is precisely why you can have a trial unit sent to your office, simply by filling out the form at No gimmicks, shenanigans, or even any payment needed. Simply tell us where to send the chart, and you can evaluate how it works for you. If it is not everything you expect, then we will even pay for return shipping!

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