Preliminary experiments are done in the daylight chamber of the university of Delft in order to make an estimation of the size of the illuminance in underground spaces. The artificial sky simulates a CIE-overcast sky. The illuminance Ehorf in the daylight chamber is 6800 lux. Experiments are done with scale models of scale 1:20 because the height below ground level in the daylight chamber is only 0,8 m in reality (fig. 1). The dimensions of the standard underground space was taken to be 4 x 4 x 4 m3 with a tube of 1 x 1 x 3 m3 on scale.

Figure 2 shows this standard space and the different variants of which the level of illuminance has been tested. The dimensions of the cylindric model (H) and the half sphere

(I) are chosen in a way that the surface of the walls is the same as in the standard model.

All scale models of the underground space are made of white foam, so that internal reflections are taken into account. The inside material of the tube was a material of high specularity in order to send as much daylight as possible to the underground space.

The effects on the illuminance in relation to the dimensions and the form of the space and the tube are investigated. The illuminance is measured at different points on the floor and the wall. On the floor the distance between the grid points was 0,5 m on scale (fig.3). On the wall there were two rows of measurement points: one row on 1,5 m and one on 3 m height, with distance of 0,6 m between the points (fig. 4).

Table 1 and 2 show the results of the measurements. On the floor almost all the models show illuminances between 150 lux and 450 lux and on the wall illuminances between 80 lux and 180 lux. Raising the height of the space or the tube reduced the illuminance in the space, but raising the height of the tube has the most impact. From the symmetries of the spaces it can be concluded that the measurement uncertainty is below 10 lux.

The measured illuminances are too low for offices and livings, but these buildings are yet forbidden below ground level by the building norms in the Netherlands. But exposition spaces can be built underground. Illuminances of 150 lux on walls are very well suited to expose paintings, and illuminances of 300 lux — 450 lux on the floor are suited to show 3D art objects like statues [2,3].

fig. 1. Measurements below ground level in the daylight chamber.

A. Standard

Ci^£aceJieic]hU5m

E. Space length 6m

F. Space length 8m (1 tube)

D. Space height 6m

Bi^£acejieic]ht_3m

SHAPE * MERGEFORMAT

fig.2. Different shapes of the scale models used in the daylight chamber.

0,5 m

fig.3. The gridpoints on the floor of the underground space, distance 0,5 m (scale 1:20).

3 m

1,5 m

0,6 m

fig.4. The gridpoints on the wall of the underground space, distance 0,6 m (scale 1:20).

table 1. Illuminances on the floor of the underground space for the different variants.

lux	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
A	260	250	225	190	252	243	219	189	229	223	200	174	197	190	172	152
B	370	358	305	235	352	337	295	228	300	291	255	204	243	242	215	182
C	200	195	183	162	197	193	181	162	190	184	173	155	179	174	161	144
D	161	157	150	141	160	156	150	141	154	152	146	139	148	144	140	128
E	222	217	195	158	219	213	193	157	203	198	178	149	180	177	159	135
F	204	198	178	141	201	196	176	140	188	183	166	133	169	166	149	123
G	221	219	243	265	219	218	240	261	210	208	232	248	193	192	205	220
H	249	241	218	188	242	233	214	178	227	219	197	169	201	197	179	162
I	436	396	290	226	426	404	309	245	350	328	260	218	279	267	230	192
J	381	370	342	293	383	369	342	296	366	357	326	293	343	335	306	279
K	196	183	155	130	181	170	149	123	151	146	127	108	134	131	117	95
L	251	245	221	189	249	240	219	186	229	211	223	176	202	198	180	157
M	259	250	226	189	255	246	223	189	234	227	208	178	212	207	187	159
N	260	251	230	195	256	247	225	194	235	228	208	183	210	206	196	168
O	265	258	229	203	259	253	229	199	239	228	212	189	213	205	197	165

table 2. Illuminances on the wall of the underground space for the different variants.

lux	1	2	3	4	5	6
A	169	167	154	132	128	116
B	185	173	151
C	150	139	123	130	129	122
D	123	123	116	107	108	104
E	137	131	116	98	97	85
F	118	112	95	86	84	75
G	160	160	169	129	132	141
H	161			123
I	163
J	196	181	162	141	135	128
K	147	141	132	132	130	121
L	165	160	144	128	125	110
M	167	165	155	130	126	116
N	168	167	154	132	125	114
O	172	170	151	143	136	128