The IES method for daylight calculation is consistent with the Lumen method for uniform electric lighting calculation.  It treats the window as if it is a side luminaire, and uses “light loss factor” and coefficients of utilization” to arrive at the required illuminance value.  Required daylight information includes exterior horizontal and vertical illuminance.

The method has been developed from the same methodology described earlier, which uses numerical integration of equations (3.4.5), (3.4.11), (3.4.12) and (3.4.13).

Consistent with tabulation based methods, there are assumptions on room configuration, the work plane, and the sky luminance distribution.

The Lumen method adopts 5 sky luminance distribution models. 

3.5.1 Assumptions and Characteristics of the Method
The sky distribution models are given in Table 3.3.6

a)   The method applies to rectangular-shaped room with uniformly distributed windows.  If the room has irregular shape, prudent judgment must be used.
b)   The method does not accommodate direct sunlight.  The effects of shading devices can be accommodated through accounting for view factor of the window to the sky.
c)    Provisions are allowed for different glazing type and vertical and horizontal blinds.
d)   The following figure illustrates the configuration for the application of the method.

Figure3.5.1 Configuration of the room in the Lumen method.

• The ceiling reflectance is assumed 70%.  The ceiling surface extends from the part of the wall above the window.
• The wall reflectance is assumed 50%.  The wall surface extends from the window sill to the top of the window.
• The floor reflectance is assumed 30%.  The floor surface extends from the floor to the window sill.
• The workplane is 0.75 m. from the floor.  The window sill is at the workplane.

If the window sill height is within 0.30 m. (1 foot) from the workplane, no adjustment is necessary, if the window sill is below the workplane, say, extends down to the floor, then the light contribution from the part of the window below the sill is discounted.  But if it is required superposition method can be used to assess the contribution of the additional light contribution.  If the sill is too far above the workplane, then there is an unavoidable error in using the method.

The method gives illumination values for 5 points on the workplane, at 10, 30, 50, 70 and 90% of room depth, measured from the window.

3.5.2 Calculation Procedure
Let E_kv   and E_kh   be the illuminance values on an exterior vertical plane, and an exterior horizontal plane respectively.

Step 1  Determine the effective light transmittance through the window, T_e

Let              T_e    =   (T_w  )(V_f  )(LLF   )

where:  T_w   = transmittivity of the window glazing, see sample values in table.

             V_f  = effective view factor of interior of window frame to the sky, accounting for shading devices, and blinds

            LLF  = light loss factor accounting mainly for loss in visibility and light to surrounding, see recommended values in table.

Step 2  Determine the exterior horizontal illuminance E_kh   and vertical illuminance E_kv   to be used for the calculation.

Step 3  Determine the coefficient of utilization: sky component CU_k  , and the coefficient of utilization: ground component CU_g   from tables.

For CU_k  , the values are tabulated with the following parameters;

• the ratio of E_kv  /(E_kh  /2)
• W_w  (window width)/H_w  (window height)
• D  (room depth)/H_w  (window height)

For CU_g , the values are given for (D /H_w  ) and (W_w  /H_w  ) ratios.

Step 4  Calculate the interior illuminance at the workplane, at each of the 5 points, by the relationship

The ground reflected illuminance E_gv  is obtained from E_gv = rE_xH /2, where r = ground reflectivity, and E_kH  =  total sky and sun horizontal illuminance.

Example 3.5.1  Given a room as shown

Figure 3.5.2 The room configuration of example 3.5.1.

For 12:00 hour June, and 8:00 hour December, the exterior sky and sun illuminance (lux) for Bangkok are given as follows:

Table 3.5.1 the exterior sky and sun illuminance (lux) for Bangkok

	Sun horizontal	Sky horizontal,Ekh	Sky vertical,Ekv
12:00 hr. June	20,000	46630	23315
08:00 hr. December	10,000	9392	4696

The window is unshaded.  The glass transmittance is 0.8, and the ground reflectivity around the exterior is 0.2.  Assume the surrounding area is clean.

Given H_w   = 1.0 m. and W_w   = 4 m., the daylight illuminance at 10%, 30%, etc. of the room depth from the window can be calculated as in the followings.

Step 1
                   T_e    =   (0.8)(1.0)(0.9)
                           =   0.72

Step 2  The exterior illuminance are already given,

	Ekv  /(Ekh /2)
12:00 hr. June	1.0
08:00 hr. December	1.0

W_w  /H_w   =   4/1  =   4
D  /H_w     =   4/1  =   4

Step 3  From the table, the following are obtained.

Table 3.5.2

P  (%)	CUk	CUg
10	0.563	0.183
30	0.247	0.159
50	0.126	0.103
70	0.083	0.071
90	0.070	0.060

Step 4 
For              E_vg  =   (r)E_kH  /2

                           =   (0.2)(2E_kh  +E_s  )/2, where

                   E_s    =   horizontal illuminance from sun.

The following table and graph show the results from the calculation.

% from window	E kh (CUk )		E kv (CUg )		E DL
	12:00 June	08:00 December	12:00 June	08:00 December	12:00 June	08:00 December
10	13126	2644	1219	355	10328	2159
30	5159	1160	1059	308	4477	1057
50	2938	592	686	200	2616	570
70	1935	389	473	138	1734	379
90	1632	329	400	116	1463	320

Table 3.5.6 (Continued)

	Depth( %)	Window width/Window height
		5	1	2	3	4	6	8	Infinite
8	10	.135	.238	.326	.353	.362	.366	.367	.452
	30	.016	.034	.058	.072	.080	.090	.094	.116
	50	.008	.017	.027	.034	.039	.045	.048	.059
	70	.006	.013	.021	.026	.028	.032	.035	.043
	90	.005	.012	.019	.023	.025	.029	.031	.038
10	10	.090	.165	.244	.272	.283	.290	.291	.395
	30	.009	.020	.036	.045	.052	.060	.064	.087
	50	.005	.010	.019	.023	.026	.030	.033	.044
	70	004	.009	.015	.018	.020	.023	.025	.033
	90	.003	.008	.014	.016	.018	.020	.022	.030

Source: IES RP-23-1989' reprinted with permission.

Table 3.5.7    Coefficient of utilization from window without blinds; Ground component.

	Depth (%)	Window width/Window height
		5	1	2	3	4	6	8	Infinite
1	10	.105	.137	.177	.197	.207	.208	.210	.211
	30	.116	.157	.203	.225	.235	.241	.243	.244
	50	.110	.165	.217	.241	.252	.267	.269	.270
	70	.101	.162	.217	.243	.253	.283	.285	.286
	90	.091	.146	.199	.230	.239	.290	.292	.293
2	10	.095	.124	.160	.178	.186	.186	.189	.191
	30	.082	.132	.179	.201	.212	.219	.222	.225
	50	.062	.113	.165	.189	.202	.214	.218	.220
	70	.051	.093	.141	.165	.179	.194	.198	.200
	90	.045	.079	.118	.140	.153	.179	.183	.185
3	10	.088	.120	.157	.175	.183	.185	.163	.167
	30	.059	.107	.154	.176	.187	.198	.193	.198
	50	.039	.074	.114	.134	.146	.157	.163	.170
	70	.031	.055	.085	.101	.111	.122	.127	.130
	90	.028	.047	.070	.083	.092	.107	.113	.115
4	10	.073	.113	.154	.174	.183	.187	.176	.184
	30	.040	.082	.127	.148	.159	.170	.177	.185
	50	.025	.049	.078	.094	.103	.113	.117	.123
	70	.020	.036	.054	.065	.071	.079	.083	.087
	90	.019	.032	.046	.054	.060	.069	.073	.076
6	10	.056	.106	.143	.164	.175	.184	.173	.194
	30	.021	.050	.081	.098	.107	.117	.123	.138
	50	.013	.027	.041	.049	.054	.060	.064	.072
	70	.011	.021	.029	.033	.035	.039	.041	.046
	90	.011	.020	.026	.030	.032	.035	.037	.042
8	10	.036	.082	.122	.143	.156	.166	.170	.208
	30	.011	.029	.050	.062	.070	.078	.082	.101
	50	.007	.016	.024	.028	.031	.035	.038	.046
	70	.006	.013	.018	.020	.021	.023	.025	.030
	90	.006	.013	.017	.019	.020	.022	.023	.028
10	10	.024	.061	.109	.120	.131	.144	.147	.200
	30	.006	.017	.034	.040	.046	.053	.056	.076
	50	.004	.010	.016	.018	.020	.023	.024	.033
	70	.004	.009	.013	.014	.015	.016	.016	.022
	90	.004	.009	.013	.013	.014	.015	.016	.021

Source: IES RP-23-1989' reprinted with permission.