Optical window

For the optical element, see Window (optics).

The optical window is the portion of the optical spectrum that is not blocked by the Earth's atmosphere. The window runs from around 300 nanometers (ultraviolet-B) up into the range the human eye can detect, roughly 400–700 nm and continues up to approximately 2 μm.^[1][2] Sunlight mostly reaches the ground through the optical atmospheric window;^[3][4] the Sun is particularly active in most of this range (44% of the radiation emitted by the Sun falls within the visible spectrum and 49% falls within the infrared spectrum).^[5]

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

Definition

The Earth's atmosphere is not totally transparent and is in fact 100% opaque to many wavelengths (see plot of Earth's opacity); the wavelength ranges to which it is transparent are called atmospheric windows.^[6]

Disambiguation of the term 'optical spectrum'

Although the word optical, deriving from Ancient Greek ὀπτῐκός (optikós, "of or for sight"), generally refers to something visible or visual,^[7] the term optical spectrum is used to describe the sum of the visible, the ultraviolet and the infrared spectra (at least in this context).^[8][9]

Optical atmospheric window

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

The optical atmospheric window is the optical portion of the electromagnetic spectrum that passes through the Earth's atmosphere, excluding its infrared part;^[10] although, as mentioned before, the optical spectrum also includes the IR spectrum and thus the optical window could include the infrared window (8 – 14 μm), the latter is considered separate by convention, since the visible spectrum is not contained in it.^[11]

Historical importance for observational astronomy

Up until the 1940s, astronomers could only use the visible and near infrared portions of the optical spectrum for their observations. The first great astronomical discoveries such as the ones made by the famous Italian polymath Galileo Galilei were made using optical telescopes that received light reaching the ground through the optical window.^[12] After the 1940s, the development of radio telescopes gave rise to the even more successful field of radio astronomy that utilized the radio window.^[13]

See also

• Infrared (atmospheric) window
• Optical window in biological tissue
• Radio window

References

1. Dwivedi, Ravi Shankar (2017). Remote Sensing of Soils. Springer. p. 13. ISBN 978-3-662-53740-4. OCLC 959595730.
2. Thorne, Anne P. (2012). Spectrophysics. Springer Science & Business Media. p. 3. ISBN 978-94-009-1193-2. OCLC 906664124.
3. Fluorocarbons, Environmental and Health Implications: Environmental Impact Statement. Food and Drug Administration. 1978. p. 79. OCLC 4611045.
4. Stergis, Christos G. (1966). Rayleigh Scattering in the Upper Atmosphere. Air Force Cambridge Research Laboratories, Office of Aerospace Research, United States Air Force. p. 273. OCLC 1037802615.
5. "Climate Science Investigations South Florida - Energy: The Driver of Climate". www.ces.fau.edu. Retrieved 2021-12-26.
6. "The Atmospheric Window | US Department of Commerce, NOAA". www.weather.gov. Retrieved 2021-12-26.
7. "Definition of OPTICAL". www.merriam-webster.com. Retrieved 2021-12-27.
8. Pedrotti, Frank L.; Pedrotti, Leno Matthew; Pedrotti, Leno S. (2018). Introduction to optics. Cambridge University Press. pp. 7–8. ISBN 978-1-108-42826-2. OCLC 1029320342.
9. Mazda, F. F. (2013). Electronics Engineer's Reference Book. Butterworth-Heinemann. pp. 9/3. ISBN 978-1-4831-6106-8. OCLC 1100870227.
10. Kartalopoulos, Stamatios V. (2011). Free space optical networks for ultra-broad band services. Oxford: Wiley-Blackwell/IEEE. p. 33. ISBN 978-0-470-64775-2. OCLC 773844977.
11. United States Congress | Committee on Science and Astronautics (1973). 1970 NASA Authorization. hearings before the United States House Committee on Science and Astronautics, Ninety-First Congress, first session, on Mar. 4, 5, 1969. Part 1. Washington: U.S. G.P.O. p. 981. OCLC 968587432.
12. Drake, Stillman (1978). Galileo at work : his scientific biography. Internet Archive. Chicago : University of Chicago Press. p. 146. ISBN 978-0-226-16226-3.
13. Wilson, Thomas (2016). Tools of Radio Astronomy. Springer-Verlag GmbH. pp. 1–2. ISBN 978-3-662-51732-1. OCLC 954868912.