In part 1 and part 2 of this series I described how differences in the location of the built-in camera flash can provide useful information in the forensic analysis of an image. Specifically, I described how we can determine if cast shadows are consistent with the location of the camera flash. Here I will describe another aspect of flashes that can be informative.
Shown below in the top panel is a computer generated image in which the scene is illuminated with a flash. Shown in the bottom panel is the same scene illuminated with sunlight (and a little bit of ambient light). You will immediately notice that the pattern of shading on the walls and ground are quite different. In the scene illuminated by the flash, the side walls and ground get darker as they recede away from the camera. In addition the back wall is brighter and more uniformly lit as compared to the side walls. In the scene illuminated by sunlight, each surface is uniformly lit, and the ground is brighter than the back wall which is brighter than the side wall (the wall on the right is in shadow because the sun is positioned above and to the right).
The primary reason for these differences is that the amount of light striking a surface decreases as the angle between the surface and light increases. (More precisely, the amount of light that strikes a surface is proportional to the cosine of the angle between the surface and the light, which decreases as the angle gets larger).
Shown to the right is an illustration of this effect. The vertical surface on the left is illuminated by a nearby light source (yellow circle). As you can see, the angle at which a light ray reaches the surface increases as the surface recedes away, and so the illumination on the surface decreases accordingly.
In contrast, the surface on the right is illuminated by a distant (infinite) light source. As a result, the light rays reach the surface at the same angle, and the surface is equally illuminated.
There is a secondary effect at play here as well. The amount of light that reaches a surface decreases as the distance to the surface increases. (More precisely, the amount of light reaching a surface is inversely proportional to the square of the distance between the light and the surface.) For a nearby light such as a flash, this exaggerates the difference in illumination between nearby and distant parts of the surface. For a distant light such as the sun, this effect is minuscule.
In a forensic setting, these effects can be quantified and used to verify that the pattern of illumination on a surface, object, or person is consistent with the illuminating light source. See, for example, Exposing Digital Forgeries by Detecting Inconsistencies in Lighting.
By the way, this fact about how light interacts with surfaces is the reason why it is colder in the winter than in the summer.