Optical window

• In astronomy, the optical window is the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground. Most EM wavelengths are blocked by the atmosphere, so this is like a window that lets through only a narrow selection of what is out there, though the sun is particularly active in the passed wavelengths. It is called "optical" because the wavelengths we can see are all in this range. The window runs from around 300 nanometers at the short end up into the range the eye can use, roughly 400–700 nm and continues up through the visual infrared to around 1100 nm, which is in the near-infrared range. There are also infrared and "radio windows" that transmit some infrared and radio waves. The radio window runs from about one centimeter to about eleven-meter waves.
• In medical physics, the optical window is the portion of the visible and infrared spectrum where living tissue absorbs relatively little light. This window runs approximately from 650 to 1200 nm. At shorter wavelengths, light is strongly absorbed by hemoglobin in blood, while at longer wavelengths water strongly absorbs infrared light.
• In optics, it means a piece of transparent optical material that allows light into an optical instrument. A window is usually parallel and is likely to be anti-reflection coated, at least if it is designed for visible light. An optical window may be built into a piece of equipment to allow optical instruments to view inside that equipment.