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# HP 42S/DM42/Free42: Room Cavity Ratio, Luminous Intensity and Illuminance

HP 42S/DM42/Free42:  Room Cavity Ratio, Luminous Intensity and Illuminance

Room Cavity Ratio

The room cavity ratio is used to assist in calculating lighting efficiency in determining where to place ceiling lights in an office room.  The formula is:

RCR = INT( ( 5 * H * ( L + W ) ) / ( L * W ) )

where:
L = length of the room
H = height difference between floor cavity and ceiling cavity
W = width of the room

HP 42S/DM42/Free42:  Solver RCR

LBL “RCR”
MVAR “H”
MVAR “L”
MVAR “W”
MVAR “RCR”
5
RCL * “H”
RCL “L”
RCL + “W”
*
RCL “L”
RCL * “W”
÷
IP
RCL – “RCR”
END

Examples

Example 1:
Input:  H = 5.5, L = 16.8, W = 13.7.   Result:  RCR = 3

Example 2:
Input: H = 5.5, L = 16.8, RCR = 4.  Result:  H = 7.1650

Sources:

Dilouie, Craig.  “Lighting Design:  Example of Role Surfaces Play in Lighting Efficiency”  LightNOW  https://www.lightnowblog.com/2010/06/example-of-role-surfaces-play-in-lighting-efficiency/  June 16, 2010.  Retrieved March 31, 2019

“Room Cavity Ratio, RCR” Illuminating Engineering Society.  https://www.ies.org/definitions/room-cavity-ratio-rcr/  July 5, 2018.  Retrieved April 28, 2019

Luminous Intensity and Illuminance

The follow equation relates the luminous intensity (measured in candelas, cd) and illuminance (measured in lux) of a light source.  The equation assumes the light source radiates a spherical matter.

E = I / R^2

E = illuminance
I = luminous intensity
R = radius of the sphere’s light (meters)

HP 42S/DM42/Free42:  Solver ILSPH

LBL “ILSPH”
MVAR “E”
MVAR “I”
MVAR “R”
RCL “E”
RCL “I”
RCL “R”
x ↑ 2
÷

END

Examples

Example 1:
Input:  I = 10 cd, R = 2 m.  Result:  2.5 lux

Example 2:
Input:  R = 3.65 m, E = 30 lux.  Result:  I = 399.6750 cd

Sources:

Daryanani, Sital  Building Systems Design With Programmable Calculators Architectural Record Books.  McGraw-Hill Book Company: New York.  1980.  ISBN 0-07-015415-5

Zumtobel “The Lighting Handbook” Zumtobel Lighting GmbH.  Dornbirn, Austria.  6th Edition: 2018  https://www.zumtobel.com/PDB/Ressource/teaser/en/Lichthandbuch.pdf

Eddie

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