The study of the measurement of light in terms of the visual response (either the eye or optical instruments, e.g., light meter) is referred to as photometry. This chapter seeks to highlight the most important units, measurements and numbers in photometry. The content of this chapter is not necessarily the subject of clinical optometry or ophthalmology; however, you might encounter some of the units and measurements in the user manuals of common clinical tests, e.g., color vision tests, or visual acuity charts.