Hello everyone, today I will be discussing some of the common clinical applications of optical principles. Optics, the study of light, is the central physical science of our profession. It also happens to be one of my favorite subjects so far in optometry school. Unlike my OAT articles, I’m going to have to limit the depth of these explanations, but of course if you have any questions, feel free to leave a comment and the end of the page!
1. Why do eye charts really need to be placed 20 ft. away from the patient?
The answer: Vergence! While it is true that the size of letters on a Snellen eye chart can easily be calculated (a letter on the 20/20 line will subtend five minutes of arc with the details subtending one minute of arc, and a letter on the 40/40 will subtend ten minutes of arc with the details subtending two minutes), one might think that we could place the eye chart closer, and compensate by making the letters smaller (i.e. that 20/20 letter would be fine if we placed it at 10 ft. but made it half the height). This just doesn’t work, because the vergence of light originating from a distance of 10 ft. is clinically significant (it is negative 0.33 diopters). At 20 ft. (or roughly 6 meters), however, the vergence is small enough (negative 0.16 diopters) that we don’t have to worry about it.
Economically, of course, we don’t often want to have 20 ft. long exam rooms, so how do we deal with this? Well, the obvious answer is that most optometrists use a mirror system to make sure that the distance the light actually travels between the projected eye chart and the patient is 20 ft.
The answer: Vergence! I know, I know, it’s the same as last time, but hey it’s a critically important concept. The standard vertex distance for glasses (the distance between the lenses and the corneal plane) is 12 mm, and the vergence of light does actually change as it travels that small distance to reach the person’s eye. For low prescriptions, this doesn’t have much of an effect, but for higher prescriptions it does and it must be taken into account. Converging light (light with a positive dioptric value) will continue to converge as it travels over that distance, and the radius of curvature will decrease, meaning that the power will increase. Similarly, diverging light will continue to diverge as it travels that distance, and the radius of curvature will increase, meaning that the power will decrease. So, for those of you who wear plus lenses in your spectacles, you would actually need contact lenses with a higher power than your glasses in order to compensate for this. For those of you with minus lenses, you will need contacts with less power than your glasses to compensate. And if we are dealing with sphero-cylindrical lenses, each meridian needs to be dealt with separately, the result being that the cylinder power may need to change as well. So, at what point is a person’s prescription strong enough that we have actually have to worry about making these vertex adjustments? It happens once the power reaches about 4.00 diopters.
The Answer: Well, a number of things could be happening. One reasonable possibility is that induced prism, caused by improperly centered lenses, is causing the diplopia. For plus lenses, if the optical center of the lens is decentered out, then base out prism will be induced, and if it is decentered in, base in prism is induced. If it is decentered up, then base up prism is induced, and finally if it is decentered down, base down prism is induced. For minus lenses, induced prism is always in the opposite direction of what it would be for a plus lens. The induced prism is directly dependent upon both the amount of decentration and the power of the lens (Prentice’s rule), so this effect would most likely be noticed in patients with a high prescription (plus or minus). In reality, there are many possible explanations for this patient’s diplopia, but this is a good example of how an optical formula like Prentice’s rule can have a direct clinical effect.
I hope you enjoyed this, and comments/questions are welcome!
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