The Ocular Barraquer Applanation Tonometer is a handheld surgical instrument used to measure a patient's intraocular pressure (IOP) directly inside the operating room. It operates on Maklakov's applanation principle, utilizing direct corneal contact where an engraved ring reticle or meniscus ring is evaluated to estimate the target pressure. It is lightweight, reusable, and designed to be autoclaved or sterilized between uses.