III.5.6 Brief History
The first paper to present ray-tracing for computation of energy-time responses of real room was presented by Krokstad et al. in 1968 [1]. While the original work dealt with only specular reflections, Kuttruff presented how to implement diffuse reflections in ray-tracing [2]. Heinz was the first to use shadow rays to enhance computation of the diffuse part by shadow rays, the technique also called as 'diffuse rain' [3].

The main principles of cone-tracing were presented by Haviland and Thanedar in 1973 [4]. It was later further developed by van Maercke [5], and also by Vorländer [6], whose technique applied receivers with increasing radius. The pyramidal beams were introduced originally by Walsh et al in 1980 [7], and used also by Stephenson in 1996 [8]. The concept of frustum tracing was presented later in 2007 by Lauterbach et al [9].