The measurement of intraocular pressure (IOP) is an important vital sign for the eye, particularly for the diagnosis of glaucoma. Procedures for measuring IOP have been used by eye care professionals for over 100 years, but those without access to such professionals often go undiagnosed. We present a smartphone-based system that can be operated by minimally trained users to measure IOP. The system emulates fixed-force tonometry using a low-cost mechanical attachment to the smartphone. Video is captured through the attachment and then processed in real-time to provide an absolute estimate of the patient's intraocular pressure. Our preliminary assessment with two ex vivo porcine eyes demonstrates that the system follows a baseline physical model with correlations of 0.89 and 0.88.