Anterior Segment and Corneal OCT Applications
Why the hardware differs
Anterior segment OCT must penetrate the opaque sclera and the tear film/air interface. The usual wavelength is ~1310 nm, which scatters less in sclera and gives better depth penetration than the 840 nm used for retina; the trade-off is slightly lower axial resolution. Some biometry-oriented devices use swept-source at 1050–1310 nm with very fast acquisition. Ultrahigh-resolution corneal OCT reaches roughly 2–5 µm axial resolution but with a smaller field.
Normal corneal appearance
From anterior to posterior: epithelium (thin, with a brighter superficial and basal layer), Bowman layer (thin hyperreflective line), stroma (fairly uniform, mildly reflective with visible keratocyte nuclei), Descemet membrane (thin hyperreflective line) and endothelium (not resolved on most clinical devices). Pachymetry is measured epithelial surface to endothelium. The angle, iris, lens surface, implanted devices (phakic IOLs, phakic implants) and filtering blebs are all assessable.
Clinical uses
Keratoconus shows epithelial thinning over the cone, Bowman layer disruption and stromal thinning; refractive surgery follow-up shows flap or SMILE interfaces as thin hyporeflective lines and enables flap thickness measurement; corneal grafts, scars, infiltrates and oedema are localised by depth; AS-OCT quantifies angle opening for angle-closure assessment and confirms implant position and vault.
Traps: assuming a 840 nm retinal OCT will image the angle well (it will not); interpreting the bright epithelial surface as pathology; calling a LASIK interface a scar; forgetting that a soft contact lens, tear film meniscus or eyelid can create a false anterior boundary; and mixing up anterior chamber depth with lens vault.
Rule of thumb: for cornea and angle, think 1310 nm and a corneal (not retinal) lens adapter; for retina, think 840 nm. If the sclera looks black and featureless, the wavelength is wrong for the task.