noun Optics.
illumination ( def 6 ).

1940–45; illumin(ate) + -ance Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
illuminance (ɪˈluːmɪnəns)
Compare irradiance Ev, Sometimes called: illumination the luminous flux incident on unit area of a surface. It is measured in lux

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
illuminance   (ĭ-l'mə-nəns)  Pronunciation Key 
The luminous flux per unit area at any point on a surface exposed to incident light. It is measured in luxes. Also called illumination.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Illuminance alone does not consider the disabling glare that reduces visibility for the driver.
Different illuminance levels are required for different applications.
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