Medical interfaces and other medical devices, systems, and methods for performing eye exams

An ophthalmic instrument for performing an eye exam on a subject's eyes, the ophthalmic instrument comprising:

• at least one light source and optics configured to direct a pair of incident light beams onto the subject's eyes and receive reflected or scattered light beams from the subject's eyes;
• a docking portion comprising a contoured receptacle having a pair of apertures through which a pair of respective optical paths from the at least one light source through at least some of said optics of the ophthalmic instrument pass, said contoured receptacle contoured so as to receive a barrier;
• and
• an interlock switch configured to indicate engagement of the barrier with the docking portion and to cause movement of internal components to stop and/or cause optical measurement of the eyes with the ophthalmic instrument not to be performed when the barrier is not inserted or when the barrier is removed during operation of the ophthalmic instrument.