The book has a high graphics content, usually with a text on one page and explanatory diagrams on the opposite page. Below are two samples from Chapter 1.

In order to understand the effect of the angle of the surface on the illuminance consider the diagram on the opposite page. Both quadrilaterals receive the same flux, but one has the flux distributed over a larger area. The angle of inclination of a surface is called the “normal” and you can imagine it as a line perpendicular to the surface.

When the line of the normal and the direction of the light coincide then there is of course no angle (0°) between the two lines. As the surface is tilted away from the light then the angle increases. So the bigger the angle the larger the area which has to be illuminated. You have therefore less flux (lumens) per square meter, and hence less illuminance (lux).