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Design Standard for Street
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Zonal Cavity Method for Indoor Calculations Methods of calculating illuminance
In order to design a luminaire layout that best meets the illumina requirementss of the job, two types of information are generall requirement illuminance level and illuminance level at a given point. Calculatio specific points is often done to help the designer evaluate the ligh especially when using luminaires where maximum spacing recom supplied, or where task lighting levels must be checked against a
If average levels are to be calculated, two methods can be
1. For indoor lighting situations, the Zonal Cavity Method is us coefficient of utilization table.
2. For outdoor lighting applications, a coefficient of utilization c CU is read directly from the curve and the standard lumen fo
The following two methods can be used if calculations are determine illuminance at one point.
1. If an isofootcandle chart is provided, illuminance levels may this curve.
2. If sufficient candlepower data are available, illuminance leve from these data using the point-to-point method.
The following sections describe these methods of calcu Zonal Cavity Method
The Zonal Cavity Method (sometimes called the Lumen Method) is the method for calculating average illuminance levels for indoor areas, un tion is radically asymmetric. It is an accurate hand method for indoor a takes into consideration the effect that inter-reflectance has on the lev Although it takes into account several variables, the basic premise tha to luminous flux over an area is not violated.
The basis of the Zonal Cavity Method is that a room is made up o cavities. The space between the ceiling and the fixtures, if they defined as the “ceiling cavity”; the space between the work “floor cavity”; and the space between the fixtures and the work cavity.”
Once the concept of these cavities is understood, it is possible to c relationships called “cavity ratios,” which can be used to determi reflectance of the ceiling and floor cavities and then to find the coe
There are four basic steps in any calculation of illuminance 1. Determine cavity ratios Sign effective up to vote this title cavityon reflectances reflectance s 2. Determine 3. Select coefficient Useful of utilization Not useful 4. Compute average illuminance level
Step 1: Cavity ratios for a rectangular space may be calcula following formulas:
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ing cavity reflectance) and
ρfc
ρcc
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Step 2: Effective cavity reflectances must be determined for the ceiling cavity and for the floor cavity. These are located in Table A (see below) under the applicable combination of cavity ratio and actual reflectance of ceiling, walls and floor. The effective reflectance values found will then be
Design Standard for Street
Step 4: Computation of the illuminance level is perf standard Lumen Method formula.
(effective ceil-
Footcandles = (maintained)
(effective floor cavity reflectance) . Note that if
the luminaire is recessed or surface mounted, or if the floor is the work plane,
# of fixtures x lamps per fixt x lumens per lamp x CU x area in square feet
When the initial illuminance level required is known and the nu ed to obtain that level is desired, a variation of the standard lum
the CCR or FCR will be 0 and then the actual reflectance of the ceiling or floor will also be the effective reflectance.
maintained footcandles desired x area in sq. ft. lamp/fixture x lumen/lamp x CU x LLF
# of luminaires =
Step 3: With these values of ρcc, ρfc, and ρw (wall reflectance), and knowing the room cavity ratio (RCR) previously calculated, find the coefficient of utilization in the luminaire coefficient of utilization (CU) table. Note that since the table is linear, linear interpolations can be made for exact cavity ratios and
The total light loss factor (LLF) consists of three basic facto depreciation (LLD), luminaire dirt depreciation (LDD) and b initial levels are to be found, a multiplier of 1 is used. Ligh with the total lamp lumen output, vary with manufacture luminaire and are determined by consulting the manufactu
reflectance combinations. The coefficient of utilization found will be for a 20% effective floor cavity reflectance. Thus, it will be necessary to correct for the previously determined ρfc. This is done by multiplying the previously determined CU by the factor from Table B (pg.12).
Ballast factor (BF) is defined as the ratio between the publis the lumens delivered by the lamp on the ballast used. Typic vary between .9 and .95. Holophane ballasts are designed t
CU final = CU (20% floor) x Multiplier for actual ρfc. If it is other than 10% or 30%, interpolate or extrapolate and multiply by this factor.
Occasionally, other light loss factors may need to be appli applicable. Some of these are luminaire ambient temperat and room surface dirt depreciation.
You're Reading a Preview Table A Unlockreflectance full access with a free trial. Percent effective ceiling or floor cavity reflectance for various combinations. % Ceiling or floor reflectance % Wall reflectance
90
80
Download With70Free Trial
90
70
50
30
80
70
50
30
70
50
30
Cavity ratio 0.2 0.4 0.6 0.8 1.0
89 88 87 87 86
88 86 84 82 80
86 84 80 77 75
85 81 77 73 69
78 77 76 75 74
78 76 75 73 72
77 74 71 69 67
76 72 68 65 62
68 67 65 64 62
67 65 63 60 58
66 63 59 56 53
1.2 1.4
85 85
78 77
72 69
66 62
73 72
70 68
64 62
58 55
61 60
57 55
50 47
70
50
50
50
30
10
49 48 47 30 29 48 47 45 30 29 Sign47up to 45 vote 43on this 30 title 28 47 44 40 30 28 Useful useful27 46 43 38 Not30
28
29 26 25 24
28 26 25 23 22
23 22
21 19
30
30
70
45 45
41 40
36 35
30 30
27 26
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Zonal Cavity Method of Calculating Illuminance Levels Table B Multiplying factors for other than 20 percent effective floor cavity reflectance % Effective ceiling cavity reflectance, ρcc
80
% Wall reflectance, ρw
70
50
70
30
10
70
50
30
50
30
1
50
30
10
50
30
10
10
50
1.070 1.057 1.048 1.040 1.034 1.030 1.026 1.024 1.022 1.020
1.064 1.048 1.037 1.029 1.024 1.020 1.017 1.015 1.014 1.012
1.059 1.039 1.028 1.021 1.015 1.012 l.009 1.007 1.006 1.005
1.049 1.041 1.034 1.030 1.027 1.024 1.022 1.020 1.019 1.017
1.044 1.033 1.027 1.022 1.018 1.015 1.013 1.012 1.011 1.010
1.040 1.027 1.020 1.015 1.012 l.009 1.007 1.006 1.005 1.004
1.028 1.026 1.024 1.022 1.020 1.019 1.018 1.017 1.016 1.015
1.026 1.021 1.017 1.015 1.013 1.012 1.010 1.009 1.009 1.009
1.023 1.017 1.012 1.010 1.008 1.006 1.005 1.004 1.004 1.003
1.012 1.013 1.014 1.014 1.014 1.014 1.014 1.013 1.013 1.013
1 1 1 1 1 1 1 1 1 1
.939 .949 .957 .963 .968 .972 .975 .977 .979 .981
.943 .957 .966 .973 .978 .982 .985 .987 .989 .990
.948 .963 .973 .980 .985 .989 .991 .993 .994 .995
.956 .962 .967 .972 .975 .977 .979 .981 .983 .984
.960 .968 .975 .980 .983 .985 .987 .988 .990 .991
.963 .974 .981 .986 .989 .992 .994 .995 .996 .997
.973 .976 .978 .980 .981 .982 .983 .984 .985 .986
.976 .980 .983 .986 .988 .989 .990 .991 .992 .993
.979 .985 .988 .991 .993 .995 .996 .997 .998 .998
.989 .988 .988 .987 .987 .987 .987 .987 .988 .988
. . . . . . . . . .
For 30 per cent effective floor cavity reflectance (20 per cent = 1.00) Room cavity ratio 1 2 3 4 5 6 7 8 9 10
1.092 1.079 1.070 1.062 1.056 1.052 1.047 1.044 1.040 1.037
1.082 1.066 1.054 1.045 1 038 1.033 1.029 1.026 1.024 1.022
1.075 1.055 1 042 1.033 1.026 1.021 1.018 1.015 1.014 1.012
1.068 1.047 1.033 1.024 1.018 1.014 1.011 1.009 1.007 1.006
1.077 1.068 1.061 1.055 1.050 1.047 1.043 1.040 1.037 1.034
For 10 per cent effective floor cavity reflectance (20 per cent=1.00) Room cavity ratio 1 2 3 4 5 6 7 8 9 10
.923 .931 .939 .944 .949 .953 .957 .960 .963 .965
.929 .942 .951 .958 .964 .969 .973 .976 .978 .980
.935 .950 .961 .969 .976 .980 .983 .986 .987 .965
.940 .958 .969 .978 .983 .986 .991 .993 .994 .980
.933 .940 .945 .950 .954 .958 .961 .963 .965 .967
Example: A typical lecture hall is 60' long and 30' wide with a 14’ ceiling height. Reflectances are: ceiling 80%, walls 30%, floor 10%. Four-lamp Prismawrap (coefficients of utilization shown below) is to be used on 4' stems, and the work plane is 2' above the floor. Find the illuminance level if there are 18 luminaires in the room.
Solutions: (1) Calculate cavity ratios as follows: CCR = 5(4)(30+60) = 1.0 30 x 60 RCR =
5(8)(30+60) = 2.0 30 x 60
FCR = 5(2) (30+60) = 0.5 30 x 60
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(3) Knowing the room cavity ratio (RCR), it is now possible to find the coefficient of utilization for the Unlock full access with a free Prismawrap luminaire in a room having an trial. RCR of 2.0 and effective reflectances as follows:
(4) Illuminance level can now be ca the number of units to be used and rating.
# of fixtures x = 62%; ρw = 30%; ρfc = 20%. By interpolation x lumens/lam Download With Free Trial FC initial = between boxed numbers in the table this CU is .55. Note that this CU is for an effective reflectance of 20% 18 x 4 x 315 while the actual effective reflectance of the floor ρfc is FC initial = 60 x 3 10%. To correct for this, locate the appropriate multiplier in Table B for the RCR already calculated (2.0). It is FC initial = 67 .962 and is found by interpolating between the boxed Sign up to vote on this title number in Table B for 70% ρcc, 30% ρw, and 50% ρcc, Notspacing usefulof luminaires. Useful Check 30% ρw at an RCR of 2.0. A possible arrangement for these fi Then: columns of six fixtures spaced ten f CU final = .55 x .962 = .53 each direction. The Spacing Criterio ρcc
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