Computerized Eye Testing
The computerized eye- checking machine is called the Auto-refractometer. It looks something like this:
The actual technology in each machine varies widely but basically all Auto-refractometers have the following three things:
- An Infrared light
- A fixation target
- A Badal lens
The schematic diagram demonstrating the basic principle of the auto-refractometer is in the figure below:
Infrared light is used in these machines because it has the least scatter potential so it can travel long distances without scattering. This is ideal in a machine like this (which measures difference in polarization).
A fixation target is used to relax the accommodation of the eye (the focusing power of the eye). It uses the fogging method to accomplish this action. It blurs the image of the target before it takes objective measurements.
The Badal lens is one which does the measurements. The relative distance between the Badal lens and the eye has a linear relationship with the refractive power of the eye. (See the scale below the lens in the diagram)
The “very” simplified version of the working
The actual process of measurement involves sending a fine ray of infrared light into the eye to be reflected back from the retina. The light sensor in the machine detects the polarisation of the reflected light and compares it against the incident light. The badal lens now moves to equalize the polarization. The analog movement of the badal lens is captured by the computer and converted to digital reading.
Also one needs to understand that the Auto-refractometer gives an objective measurement of refractive error but spectacles and contact lenses are given after subjective refraction- that is the power that the patient is comfortable checked by the Doctor at Supriya Eye Hospital, with lenses.