The poor man’s red/green test

One morning, as a few faculty members were squeezing in a workout before work, the topic arose of how to use the red-green (Duochrome) test most efficiently in clinic. As we talked further, I related an experience and posed a question to Dr. Rick Savoy, who happens to teach a portion of the optics curriculum at SCO. This conversation led us to cowrite this article.

Gasoline signage

My experience unfolded with a trip that my wife and I routinely make back to our hometowns several times per year. During these long treks, my mind occasionally takes me back to Dr. Susan Kelly’s first-year “Sensory Aspects of Vision” course at the Illinois College of Optometry. Call me a glutton for punishment, but I enjoy relearning old material in a novel manner as a way to help my patients and to better explain concepts to my students in clinic and the classroom.

On one of these long trips, my vision was great as we started with some daylight left. However, as the hours passed and the light waned, I began to notice interstate highway signs becoming more blurry and that I had a harder time reading them than I usually did in the regular daylight hours.

As I passed one of many gas stations I drive by on every trip home, I noticed a discrepancy in the clarity of the signage of that particular gas station’s fuel prices (Figure 1).

My eyes were drawn to the green-colored numbers, but I could barely make them out without squinting. However, when I looked at the red-colored numbers, they were much clearer to me.

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In my head, I immediately was taken back to my first year in optometry school with Dr. Kelly, and I diagnosed myself with night myopia. How is this possible? After my discussions with my workout partners, it dawned on us that we might have just stumbled upon a “poor man’s” duochrome test.

Chromatic dispersion and aberration

Optometry’s standard red/green test (duochrome or bichrome) that most of us routinely forget to employ in clinic is based on the refractive nature of light and on one peculiar property of refracted light known as longitudinal (or axial) chromatic aberration.

Chromatic dispersion is a variation in a material’s index of refraction depending on the specific wavelength of incident light entering that material. Different wavelengths are focused at different axial and transverse planes, leading to longitudinal and transverse chromatic aberration, respectively.^1,2

I’m sure that we’ve all been taught this, but how often do we go back and review how and why this aberration occurs? Probably not too often.

Explained a little differently, the underlying principle with chromatic aberration is when white light passes through a refracting surface or refracting system (such as a lens or the human eye) and gets refracted, shorter wavelength colors (such as blue) travel more slowly and are bent more dramatically than longer wavelengths colors (such as red).³

The duochrome test takes advantage of this phenomenon to help us “balance” our prescriptions.

Blue and red wavelengths are the furthest apart dioptrically with longitudinal chromatic aberration. Why do we use red and green for the duochrome test instead of logically using red and blue?

First, blue light is inherently darker in comparison to green light; therefore, the color green is interpreted as “brighter” than blue by most patients.² If blue were used for the duochrome test, interpretation of the test may be difficult because some patients might inadvertently choose which side is brighter instead of which side is clearer.

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Patients might incorrectly assume the darker blue image would be the most blurry when compared to the brighter red side, potentially compromising the reliability and usefulness of the test. The duochrome test needs colors that are approximately the same brightness levels when subjectively being compared by patients; hence red and green are better employed than red and blue.

Secondly, the “dioptric” center of the visible spectrum of white light (approximately yellow) is closer to the midway point between red and green than it is between red and blue.² When patients report that both sides of the duochrome test are “equally clear,” we interpret the test as balanced, meaning the focal plane of the yellow portion of the visible spectrum is focused on the retina (Figures 2-4).

This is fortuitous because humans perceive yellow wavelengths as being the brightest, most sensitive to our eyes and therefore the “easiest” color to see. Arguably, our eyes are finely tuned in to yellow colors, making the duochrome test a good way to maximize vision in many cases.

For a lens system, the shorter wavelength colors focus before the longer wavelength colors resulting in a dioptric “spread” between the two points of focus. The magnitude of the dioptric range produced by longitudinal chromatic aberration can be quite significant, particularly when dealing with a high-powered refracting surface/system.

Remember that an eye is a great example of a high-powered converging (“plus”) system. The power of the human eye is usually estimated to be approximately +60.00 D. Because of this high power and the phenomenon of longitudinal chromatic aberration, blue and red wavelengths of white light entering an average eye typically manifest an approximately 1.25 D difference between the focal points of blue and red wavelengths of light.²

Estimates vary, and of course, actual values can vary dramatically for different eyes; nevertheless, this discrepancy explains why the duochrome test works. An eye will often be unable to simultaneously focus both pure red targets and pure green targets regardless of the refractive error or accommodative status of the patient, especially when in a dark field or when viewing a dark background (like night driving).

This phenomenon partially explains my inability to see both red and green gas prices clearly and is depicted in Figures 2-4.  

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Night myopia

Why was the red side of the fuel prices always clearer than the green side at night but not necessarily during the day? That’s due to another intriguing optical phenomenon is known as night myopia.

Night myopia is an increase in the refractive power of the eye in many patients under conditions of reduced illumination, as compared with their vision in bright light.⁴ As a result, many patients become relatively more myopic in dim light than they normally are in bright light.⁴

The magnitude of night myopia appears to vary widely among individuals and across different studies.⁴ Values ranging from negligible to as much as −4.00 D of myopic shift have been reported.⁴ Average values in most studies are usually approximately −1.50 D, a significant figure that would severely degrade the quality of the retinal image.⁴

However, a study published in 2012 found an average increase of myopia of only -0.32 D to -0.81 D in low illuminance levels.⁴ This led these researchers to suggest that perhaps the practical importance of this night myopia finding is more limited than was commonly believed.⁴

Nonetheless, a myopic shift of approximately -0.25 D to -0.75 D could definitely impact some patients’ sight at night, so it is worth considering in patients complaining of reduced night vision.

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Of note, spherical aberration has traditionally been implicated as the major factor in night myopia. At low levels of illumination, the eye’s pupil dilates, allowing for a greater contribution from peripheral rays that focus more anteriorly than axial rays when generating the retinal image.

More recent research suggests that spherical aberration likely does not play as significant a role in night myopia as classically taught.⁴ For instance, one study concluded that the average relative defocus at low luminance conditions (night myopia) was completely accounted for by the errors in accommodation. The study supports the postulate that the main factor for the defocus shift occurring in dim light (night myopia) is due errors in accommodation instead of spherical aberration, although the etiology is still likely multifactorial.

Regardless of its cause, night myopia is an accepted refractive anomaly. Traditionally, eyecare practitioners routinely try relatively few things to compensate for night myopia when induced by low-light conditions.

One suggestion is adjusting the dioptric power of the spectacle prescription to balance the duochrome test under low light conditions by adding minus powers. For some patients suffering from night myopia, slightly increasing their minus power so the green portion of the Snellen chart is in slightly better focus than the red under traditional refractive conditions can help. Also, you could simply prescribe one pair of well-balanced glasses during the day and prescribe a slightly more myopic prescription based on the results of your in-office duochrome test for the patient to leave in her vehicle for night driving use.

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Moving forward

After realizing my need for an updated glasses prescription, I scheduled an appointment at our clinic. The student clinician did a great job and nailed my refraction that revealed more minus than my previous prescription. This validated my own poor man’s duochrome test on that long road trip.  

Perhaps now we have a new question to ask our patients who complain of trouble seeing at night: Which gas price is clearest for you at night, the green one or the red one?

During the day, the patient will likely be unable to clearly focus both the green and the red letters at the same time even though they may be equally blurred. But at night, the combination of night myopia confirmed by chromatic aberration may suggest the need for a stronger prescription. Depending on the patient’s answer, we now have a good direction of where to go refractively, and we can confirm his answer in clinic with our standard duochrome test.

We realize this trick is not awe-inspiring by any means and should be performed monocularly for best effect. Nevertheless, finding a new use for an old concept excites us and helps keeps things fresh. We hope this poor man’s red-green test helps clinicians better care for their patients’ vision needs.

References

1. Zhao H, Mainster MA. The effect of chromatic dispersion on pseudophakic optical performance. Br J Ophthalmol. 2007 Sep;91(9):1225-9.

2. Rubin ML. Chromatic aberration, spherical aberration. In: Optics for Clinicians: 25^th ed. Gainesville, FL: Triad Publishing; 1993;279-84.

3. Vinas M, Dorronsoro C, Cortes D, Pascual D, Marcos S. Longitudinal chromatic aberration of the human eye in the visible and near infrared wavefront sensing, double-pass and psychophysics. Biomed Opt Express. 2015 Mar 1;6(3):948-62.

4. Artal P, Schwarz C, Canovas C, Mira-Agudelo A. Night myopia studied with adaptive optics visual analyzer. PLoS One. 2012; 7(7): e40239.