Snellen eye chart - to test a person's visual acuity Snellen chart is familiar to everyone since everyone who goes to doctor to get their eye checked reads the chart to find out the corrective numbers.

Snellen chart is named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862. The chart shows letters in a decreasing size. If you are able to read the last line of the chart, your vision is considered 20/20 meaning that you can read from 20 ft what a normal person can read from 20 ft. The further down you can read the chart, better your visual strength is. If the lowest line that you can read is the fourth from the bottom than your vision is considered 20/40, meaning what you can read from 20 ft, a normal person can read from 40 ft. Thus, by changing strength of lenses, ophthalmologist determines what lens power you need to make your vision closest to 20/20 if not 20/20.

Glossary of the terms:

Aberrations: deviation of rays from what is ideally expected

Accommodation: ability of change focal lengths to focus objects at different distances

Astigmatism: difference in focal length for rays coming in different planes from an off-axis object

Cornea: focusing element of the eye, responsible for two - thirds of the focusing

Diopter: measure of curvature

Direct Ophtalmoscope:

Emmetropic: when the rays from a distant object and a near object can come to a focus on the retina

Focal length: distance beyond the parallel light rays converge

Hyperopia: unable to clearly see a nearby object yet is able to focus objects that are further away

Keratometry:

Myopia: unable to clearly see a distant object yet is able to focus near objects

Presbyopia: loss of eye's ability to accommodate with age

Refraction: the bending of the rays when light travels from one medium to the other

Retinoscope: Visual Acuity: ability to see clearly or the resolution of the eye

Different models of the eyes made through BEAM2.

- Simple model of the eye with cornea bulged out and crystalline lens

- Objects from further away focusing onto the retina (normal, myopic and hyperopic eye)

- Objects from nearby focusing onto the retina (normal, myopic and hyperopic eye)

Information about the machine:

Keratometry: Keratometry finds out the curvature of the cornea and corneal astigmatism by finding out the maginification of the bright object.

Direct Ophtalmoscope: Direct opthalmoscope also uses light that is sent to the pupil of the eye which allows the internal of the eye to be observed. Since this method has to add up corrective error from the examiner to the retina to find the correct error, this is the least used method in today's world.

Retinoscope: measures the red reflex while the retinoscope beam is moving inside the eye. The goal is find when the reflex moves infinitely fast or when the motion reverses and the patient's refractive error is the value of the interposed lens that gave the reflex motion plus some working distance correction.