retroillumination

Retroillumination is an illumination technique used in microscopy and ophthalmology in which light is directed from behind the subject so that it passes through the specimen and returns toward the observer. This back-illumination enhances contrast in translucent or semi-transparent structures, making features that are difficult to see with direct illumination more conspicuous. In microscopy, retroillumination is commonly used to study transparent specimens such as pollen grains, crystals, fibers, and thin tissue sections. It emphasizes edges and subtle internal details and is often employed as a simple, effective contrast method.

In ophthalmology, retroillumination refers to a slit-lamp or ophthalmoscopic technique in which light is aimed toward the posterior part of the eye so that illumination travels through the pupil and back out. The phenomenon highlights irregularities or opacities by altering the back-reflected light against the red reflex. Clinicians use it to detect corneal scars, lens opacities such as posterior subcapsular or cortical cataracts, and iris irregularities. It is particularly useful for assessing posterior capsule opacification after cataract surgery and for identifying subtle anterior segment pathology that may be less apparent under direct illumination.

Limitations include dependence on clear ocular media and proper instrument alignment; media opacities or edema can obscure the retroillumination image. The term derives from the retrograde direction of illumination, and the technique is related to other forms of oblique or transmitted illumination used to enhance contrast.