Dynamic Range of Light in Scenes
Figure 2.1 shows paint samples covering the range of reflectances.
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Figure 2.2 shows a montage of different size and exposure photographs to illustrate two test targets in bright sun and deep shade.
From the white in the sun to the black in the garage the dynamic range is 4.3 log units.
We put the painted board in the middle of a field, in front of a garage in partial shade, with a second reflectance target inside the garage in deep shadow. We measured the light from white and black paints in sun and in deep shadow. The range of this scene is 21,900:1 or 4.3 log units.
This is an unusual scene, but it illustrates well the fact that high dynamic range images are the result of variable illumination falling on different surfaces. The range of reflectances is only 30:1 (1.5 log units) because it is limited by front-surface reflections. Regardless of the amount of light absorbers in the paint, a few percent of the light is reflected from the surface depending on the refractive indicies of air and the media.
By comparison the range of illumination is very large. If you aim a spot meter at a cloud in front of the sun it reads 179,000 cd/m2. Turning around, the white paint in the sunlight is 21,900 cd/m2, and the white in the garage is 1,369 times darker (2.8 log units) than the white on the block.
The photographs in Figure 2.2 were made using variable exposure and camera position and fixed sunlight white balance. In other words, these are conventional photographs, with exposure as the variable. The upper-left photograph is the best exposure for the painted block in the meadow. The garage in partial shade is dark. More exposure for the entire scene renders the sky and trees properly, but overexposes the painted block in the field. Still more exposure lightens the sky, trees and garage, but does not improve the information about things in the garage.
In the second row of photos taken in front of the garage, we can just barely see that there are things inside the garage in the image that overexposes its exterior. The bottom row shows the gray scale and toy vehicle not visible in the other pictures. The light entering the garage is reflected from the meadow, hence the yellow-green cast of the gray paints. We will not attempt to synthesize the best HDR image of this scene. We do not know enough yet. We use these pictures to illustrate the range of light in this very-high dynamic range scene.