Windows are an essential element in the architecture, as they provide daylight in the inner spaces. However, little is known about the design and proportion of openings to maximize natural light.
In this first chapter on windows I will introduce the basic principles for design. I will set rules very simple and basic, that anyone can follow. In later chapters, we will study the quantification of light by a window.
1. - The daylight coming through a window is very unevenly distributed. The more illuminated surface is observed in the area close to the opening and it extends to a distance equivalent to the height of the lintel. Bellow, I show the lighting study (specifically the daylight factors, a concept to be discussed later) seen in a room with a window that represents 20% of the front surface . As can be seen, the highest enlightenment is next to the window and decreases with increasing distance from it.
2. - Accordingly, it can be concluded that the optimum illumination which is provided by a window is equal to twice the height of the lintel. This is important to remember: the depth of light depends exclusively on the height of the window lintel .
3. - Other studies  conclude that the lighting in a room can be divided into five sections, depending on the height of the window, as shown in the following figure. Thus, for a centered window, the optimum illumination is observed at twice the height window or once the height of the lintel.
4. - Finally, it should be noted  that the bottom of a room may seem dark when the ratio of the illumination of the front half of the room between the illumination of the back half is greater than 3.
And that's all for now. We have much to talk about lighting through windows, I will gradually write more about them.
 Acosta, I., Campano, M., et ál.: Daylight in window design: optimization of proportion and position under overcast sky conditions. Portugal, Lisbon: 5th International Congress on Energy and Environment Engineering and Management, 2013.
 Baker, N., Steemers, K.: Daylight: Design of Buildings. James & James, 2002.
 Martín Monroy, M.: Manual de la Iluminación. Ay. Las Palmas de Gran Canaria, 2006. p. 42.
 Lynes J. A.: Principles of natural lighting. Elsevier, 1968.