The author synthesises the ways by which light can be measured in forest, and shows that they are not completely satisfactory. In the case of high elevation stands, he shows that the number of hours of direct sunshine during the May-June period is a pertinent parameter to predict the growth of young Norway spruce seedlings. This duration can be accurately assessed with a simple device, the "horizontoscope", which is described. Examples of the use of horizontoscope aimed at designing regeneration fellings in high elevation Norway spruce stands are given.