Louis I. Kahn's philosophy of what a room must be and do is expressed by light. With Kahn there is exalted light, functional light, glaring light, indirect light, soft light, cold light - each of which functions differently in his architecture. In order to make the effect of the "Kahn interior" palpable, the author provides exact analyses of the entire spectrum of light modulation possibilities which Kahn attempted, sophisticated, and nothing short of dramatized: the ceiling-framed window, the lookout slot, the keyhole window, the many variations of brisesoleils, sliding wooden panels which provide day windows and night windows, the double space envelope, the light cylinder, and an unlimited variety of skylights. The text represents an introduction to Kahn's architectural language.