Unit cd/Mm² candela per square megametre Luminance Candela per square megametre (cd/Mm²) is a unit of measurement for luminance in the field of optics, specifically photometry. Luminance is the measure of the brightness of a point in a given direction on a surface, which can be a screen, light source, or even an object reflecting light. Basically, it tells us how much light is emitted, transmitted, or reflected from a particular area, and it is perceived by the human eye as brightness. Now, let's break down the unit cd/Mm²: 1. Candela (cd): It is the base unit of measurement for luminous intensity in the International System of Units (SI). It quantifies the intensity of light emitted by a light source in a particular direction. 2. Square megametre (Mm²): It is a unit of area equal to one million meters by one million meters or one trillion square meters (1 Mm² = 1,000,000 m x 1,000,000 m = 1,000,000,000,000 m²). It is a large unit typically used to measure immense areas, like regions on Earth's surface or celestial bodies. So, when we combine these units, cd/Mm² is used to express the luminance of a light source or surface in terms of the emitted or reflected light intensity (in candelas) per unit area (in square megameters). This unit allows us to quantify and compare the brightness of different sources or surfaces on a large scale, and it is particularly useful in applications like astronomy, remote sensing, or large-scale area lighting.