Retina: The light-sensitive layer of tissue that is located like a screen at the back of the eyeball. The images that enter the eye are focused on the retina.
Astigmatism: A defect in the eye that results in distorted images when rays of light do not meet at a common focus on the retina. The spherical shape of the lens is disturbed.
Screening test: A laboratory test that helps in identification of a particular disease, disorder or condition.
Quantitative testing: A laboratory test that helps in detecting a disease, disorder or abnormal condition and classifies the condition according to the type and severity.