Indirect ophthalmoscopy employs a head lamp device to shine a very bright light into the eye. This is generally done by an ophthalmologist.
Direct ophthalmoscopy is in more common usage. The examiner uses a device the size of a flashlight to examine the eye. The device consists of a concave mirror and a battery-powered light (contained within the handle). The operator looks through a single monocular eyepiece into the patient's eye. The ophthalmoscope is equipped with a rotating disc of lenses to permit the eye be examined at different depths and magnifications. This may be enhanced by drugs that dilate the pupil and enlarge the opening into the structures within the eye.
The ophthalmoscope is invaluable in many fields of medicine, including cardiology (cardiovascular disease), hematology, medical genetics, neurology, neurosurgery, rheumatology, family medicine, pediatrics, internal medicine, and geriatrics. Visualization of the retina is important in conditions such as the effects of glaucoma, diabetes, and hypertension and in conditions such as brain tumors and meningitis/encephalitis which may cause increased intracranial pressure.
The invention of the ophthalmoscope is often erroneously credited to the great German physician-scientist Hermann von Helmholtz (1821-1894). Helmholtz demonstrated the principle of the ophthalmoscope in 1850 using a crude device (cardboard, glue, and microscope glass plates). By means of his ophthalmoscope, Helmholtz could place the eye of the observer in the path of the rays of light entering and leaving the patient's eye, allowing the patient's retina to be seen.
However, Helmholtz was neither the first person to look into the living retina nor the first to fashion a device for viewing the retina. In 1823 Johannes Purkinje observed the back of the eye and in 1847 Charles Babbage fashioned an ophthalmoscope similar to the one later developed by Helmholtz. The work of Purkinje and Babbage lay largely unknown. Helmholtz rediscovered the ophthalmoscope and immediately realized and communicated its importance.