Abstract: A vision collimator for self-monitoring peripheral vision loss or central visual field limitation. The collimator includes a hand-held collimator housing having a viewing aperture for placement adjacent to the subject's eye. A lens projects an image to the eye, which image is formed by the terminal ends of the blades of an iris diaphragm. The angular size of the image is adjustable by opening or closing the iris diaphragm to obtain a measurement indicative of the peripheral vision loss or the central visual field limitation. A diffuser is positioned behind the iris diaphragm to provide an even back-illumination, and indicia markings indicate the measurement. The device is particularly suitable for home use by persons with glaucoma or macular degeneration.