Functional visual acuity (FVA) has been suggested to reflect an individual's performance in relation to certain daily activities involving visual tasks. The concept of FVA has been thought applicable to the detection of masked impairment of visual function in patients with dry eye who complain of decreased visual acuity despite normal conventional visual acuity. A stable tear-film layer over the surface of the cornea is essential for a smooth ocular surface and the formation of clear visual images. In dry eye, the ocular surface tends to dry out when normal blinking is suppressed during gazing, and patients with dry eye may have problems maintaining clear vision while gazing. The FVA measurement system is a device to measure changes in continuous visual acuity over time. Visual maintenance ratio (VMR) is the ratio between FVA and baseline visual acuity. FVA and VMR are indices for the assessment of functional vision. FVA has been thought applicable to the detection of changes in functional vision in relation to the tear film over the ocular surface. It may also be useful for the assessment of subjective visual complaints in diseases that do not involve dry eye.