Use a Consistent Light Source (HI Guide)

Inside Macintosh: Macintosh Human Interface Guidelines / Part 2 - The Interface Elements: Chapter 8 - Icons / Designing Effective Icons

Use a Consistent Light Source

On the Macintosh screen the light source always comes from the upper-left corner of the screen. Therefore icons and other elements have drop shadows on the lower-right side. Use the light source consistently, so that shading is consistent throughout the interface. Figure 8-13 shows some desktop entities that have drop shadows consistent with a light source at the upper-left corner of the screen.

Figure 8-13 A consistent light source

Figure 8-14 shows some desktop images that have different light sources and inconsistent drop shadows.

Figure 8-14 Inconsistent light sources