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The binocular indirect ophthalmoscope, or indirect ophthalmoscope, is an optical instrument worn on the examiner's head, and sometimes attached to spectacles, that is used to inspect the fundus or back of the eye. It produces an stereoscopic image with between 2x and 5x magnification. It is valuable for diagnosis and treatment of retinal tears, holes, and detachments. The pupils must be fully dilated for it to work well.
In a dark room, the examiner orientates his/her head so that light from the internal light source is directed into the patient's eye. A positive-powered condensing lens is held by the examiner at its focal length from the patient's eye, serving two purposes (Figure 1):
The viewing system of the instrument (Figure 2) consists of a pair of low-powered convex lenses. This design affords the examiner a stereoscopic view of the virtual image. The +20D lens is the standard lens for general examination offering 3x magnification and a field of view of approximately 45°. A +30D lens will offer 2x magnification along with a field of approximately 65°. These higher powered lenses are commonly used to examine small children and those with small pupils. They can be thought of as more forgiving than the lower-powered lenses, and as such are often advocated as a good choice of lens for those new to the indirect ophthalmoscope.
Indirect ophthalmoscopes use halogen bulbs as the light source although many newer models use LED light sources which operate much cooler and last much longer. The newer models may incorporate battery packs that can be worn on the examiner's belt or can even be incorporated into the headband itself. These make it possible to use the indirect ophthalmoscope without the movement restrictions caused by power cables.
The indirect ophthalmoscope offers some advantages over the direct ophthalmoscope: