For ocular double vision, in addition to the usual ophthalmic tests such as visual acuity, optometry and eye movements, a red glass test is required to distinguish between monocular diplopia and binocular diplopia by looking at a lighted area with one or both eyes in a relatively dark room. If the patient sees two separate lights, red and white, after covering one eye with red glass glasses, this indicates binocular diplopia. If the lights are seen with only one eye, overlapping lights can be seen, usually indicating monocular diplopia. Depending on the patient’s other symptoms, further tests such as radiographs and blood tests will then be carried out to clarify the exact cause.