Interesting Eye Articles
18 October 2018
The term refraction means to bend light. Your optometrist will use several methods to bend or change the direction of light coming from a distant object, to focus accurately on the retina by introducing different lenses before your eyes. When this is achieved, vision will be clearest. This results in the prescription for your spectacles or contact lenses. Refraction can only be accurate if the letter chart is at least 6 meters or 20 feet away from the patient. Any shorter, and the accuracy drops. To save space, most practitioners will use a mirror to double the distance.
Methods of Refraction
Retinoscopy is used as an objective method to determine the refraction without your interaction, that is without asking you “Which is better, 1 or 2?”
An Autorefractor is a computerised instrument used to determine the refraction of your eyes. Again, this is an objective method that does not require the interaction of the subject. This method may not be sufficiently accurate and should not be used to dispense prescription spectacles unless the subject is unable to respond to other methods.
This is the traditional method used to determine the prescription for spectacle and contact lenses. Lenses are placed before your eyes until the best vision is achieved. Several methods based in science is used to determine when the end point is achieved.
Jackson Cross Cylinder
This method is used to determine the Astigmatism and its Axis.
Red green test
This test is used to determine the short or long sight of the subject. Our eyes optical system splits light slightly into its different colour components. A short sighted eye (Fig A) is too long therefore light is focused before it has a chance to focus on the retina, but because the colours are split slightly, the red light is focused closer to the retina then the green light, therefore the subject will see letters on the red background clearer.
This helps us decide the next lens to place before the eyes. Again the distance of the room is important for accuracy of this test.