From: Keith Bishop <k.bishop@heptasystems.com>
Sent: Wednesday, October 31, 2018 5:08 PM
To: Considine, Toby <Toby.Considine@unc.edu>
Subject: oBIX unit database recommendation
Toby,
I’m not sure if you are still involved with oBIX (or who is), but I have made a recommendation to change the Project Haystack unit database and since that file is based on the oBIX units, I thought it might be a good idea to pass it along
to you (if this needs to go to someone else, please let me know).
Discussion
The base SI unit for the addition of lighting is the Candela (cd). The cd is the magnitude of luminous intensity (we are calling it luminescence which is contrary to the
International Bureau of weights and measures but I don't think the exact name is important). The cd is roughly the amount of "light" traveling in a certain direction.
The next concept is luminous flux which is the amount of "light" emitted through the available direction. (This available direction is measured in steradians (sr) where a sphere would have a steradian value of 4π sr or roughly 12.57 sr). Luminous flux is
measured by the lumen (lm) which resolves to base units of cd·sr. (The steradian is not technically a base unit but can not be reduced to a base unit because it's math resolves to m²/m² which removes all units)
Now that we have luminous flux defined, we can move on to illuminance (or luminous emittance). To build on the above concepts, this is the amount of lumens passing through and area. The SI unit for luminous flux is the lux (lx) with base units of cd·sr/m².
The SAE unit for luminous flux is the footcandle (which we have listed as a measure of luminescence). The footcandle (fc or ftcd as we have listed) is equal to 10.76 lx
The measure of luminous intensity passing through an area is called luminance. The SI unit for luminance is the candela per square meter (cd/m²) which we have listed in our unit database under illumanence.
Proposal
Based on the above discussion, it is recommended that we change the cd based units in the database to:
-- illuminance (m-2*cd1*sr)
lux, lx; m-2*cd1*sr
footcandle, ftcd; m-2*cd1*sr; 0.092937
-- luminance (m-2*cd1)
candelas_per_square_meter, cd/m²; m-2*cd1
-- luminescence (cd1)
candela, cd; cd1
-- luminous flux (cd1*sr)
lumen, lm; cd1*sr
Please let me know what you think.
Keith Bishop
Director of Analytics
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