Sunlight is Only Part of the Story:

The Ring Heliodon simulates sun and shadow patterns in a manner that is very similar to our real life experience here on earth. An artificial light source sweeps and arcs above a model in a very similar manner to the way that the actual sun sweeps across the sky above a given site through the day and year.

Whichever heliodon is used, however, it is important to understand two sets of ideas: 1) The geometric relationships of sun and Earth that occur during the different seasons of the year, and 2) The important distinction between sun, sky and ground light. The heliodons available in the CERES Heliodon Gallery only provide imaging of shadow-casting beam light from the sun. Skyvault illumination is not accommodated in this instrument setup, and moreover, the significance of ground reflective light is only partially addressed. That is to say, the beam light will bounce off the ground plane or off horizontal elements in the building design (e.g., light shelves, roof scapes or patios), but the reflected light coming from these surfaces is only that which is reflected from the beam light and does not include the integrated component of interreflected indirect light off the skyvault itself. Thus, the heliodons are best for studying sun and shadow patterns but not daylighting effects.

The Overcast Component:

In order to address the issue of the overcast sky or the interreflected light component of a clear day blue sky, it is necessary to do one of two things. Either uses a specialized artificial sky technique to model the individualized or combined influences of beam, vault, and ground illumination, or take a model outside and use nature's own completely integrated, totally accurate light source(s)-sun, skyvault, and ground.