Unlike adults, who can easily sit behind a device called a phoropter and reliably tell you whether "Number 1 is clearer" or "Number 2 is clearer," children have their glasses prescription measured in a different way.
By using a handheld device called a retinoscope, the eye doctor can accurately measure not only whether a child needs glasses, but also what the prescription should be.
The physician turns on the retinoscope, generating a bar of light, and then looks at each eye, moving the bar of light up and down and side to side. This creates a reflex of light within the patient's eye. Handheld lenses -- just like the ones in the dials of the phoropter -- are held up in front of the patient's eye, and the physician looks for the light reflex to stop moving as he or she moves the bar of light. The lenses that allow this light reflex to stop moving are used to determine the patient's prescription. In the hands of an experienced examiner, this method of prescribing glasses is very accurate.