The SI unit of luminous intensity is the candela (abbreviated cd), which is a primary unit. The derived SI unit of luminous flux is the lumen (abbreviated lm). These are based on the sensitivity of the human eye to light. Light sources are often evaluated based on their luminous efficacy, which is defined as the luminous flux divided by the power consumed and is measured in lm⋅W⁻¹. In a physical or online store, find manufacturer’s specifications for representative incandescent, halogen, high temperature discharge, LED, and fluorescent lamps of similar luminous flux and compare their luminous efficacy.