Ophthalmic Imaging

At the assistant level OCT is a machine you position a patient in front of. At the technician level it is an instrument whose physics you can explain, whose settings you choose deliberately, and whose output you are expected to judge before anyone else sees it.

The measurement. OCT is low-coherence interferometry. Light from a broadband source is split: one arm goes to the eye, the other to a reference. Only light that returns from the eye having traveled a path length matching the reference arm interferes constructively, so depth is encoded by path length. A single depth profile is an A-scan; A-scans lined up side by side form a B-scan (the familiar cross-section); a raster of B-scans forms a cube, from which the thickness maps and en face images are derived.

Two resolutions, two different limits. This is the single most-tested piece of OCT physics. Axial (depth) resolution — about 5 μm in a commercial spectral-domain device — depends on the bandwidth of the light source: broader spectrum, shorter coherence length, finer depth discrimination. Nothing about the patient's eye changes it. Transverse (lateral) resolution — roughly 15 to 20 μm — is limited by the pupil aperture and the eye's aberrations. That is why a small pupil or a cataract smears detail sideways while the layer separation stays crisp, and why more frame averaging cleans up speckle noise without ever improving true resolution.