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.
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.
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