Friedrich Tonne established the Institut für Tageslichttechnik Stuttgart in 1952. Tonne’s basic idea at the time was to develop a graphical procedure (based on work carried out by the Swedish architect Gunnar Pleijel et al.) with which complex spatial geometry calculations (astronomical and light technology calculations in this case) could be carried out without using complex formulae. The procedure is based on the stereographic projection of the half-space above a plane onto the plane itself. For so-called sun charts this plane is the horizon, the half-space the hemisphere (see below).

- Sunlight penetration studies
By projecting building openings, adjacent buildings etc. into the correctly oriented sun chart, it is possible to obtain an immediate read-off of the changing sun penetration times during the course of the year at the location of interest. This takes place by means of spatial geometry construction using planning documents or pictures that have been taken on-site (see Horizontoscope).

Stereographic projection

Sun charts

Stereographic records
and projections

Balcony as a rigid panel