#14. Exposure Meters Measure Illuminance
True or False?
All exposure meters measure illuminance.
Illuminance is the only quantity of light that can be measured. Luminance, which is needed in the Exposure Equation for exposure calculations, is an intensity that is calculated from measured illuminances just as other physical intensities, such as pressure and temperature, are calculated from other measured quantities, such as a change in volume.
Incident-light exposure meters measure the ambient illuminance and calculate a representative luminance for the scene using the Reflection Equation.
Ls = ( 1/π ) Rs Ea
Reflected-light exposure meters measure illuminances from the scene and calculate a representative luminance for the scene using the Illumination Equation,
Es = ( π/4 ) T qs Ls / A2
which can be rearranged for the calculation of luminance.
Ls = ( 4/π ) A2 Es / T qs
These representative luminances are then used in the Exposure Equation to calculate the camera settings.
The derivation of the Illumination Equation is discussed in the book Photographic Exposure Calculations and Camera Operation.
Copyright 2008 Michael G. Prais, Ph.D.
For a readable but in-depth analysis of this concept along with many other concepts associated with photographic exposure, take a look at the book Photographic Exposure Calculations and Camera Operation. This book provides insight into the equations that govern exposure, exposure meters, photosensitive arrays (both solid-state and emulsion) and the Zone System as well as concepts associated with resolution, dynamic range, and depth of field.
The book is available through Amazon.com (ISBN 978-1-4392-0641-6) where you can Search Inside!™.
Check http://michaelprais.info under Photography for the table of contents, an extensive list of the topics and subtopics covered, the preface describing the purpose of the book, and a diagram central to the concepts in the book.
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