The Absorption Medium
Absorption is controlled with the absorption map, which defines how quickly light is absorbed while passing through a medium. A setting of 0.0 means no absorption. The higher the value the faster light is absorbed by the medium.
The absorption texture is multiplied with the Scale parameter. This allows setting a wide range of values more easily (see figure 4). Mediums are affected by the scale of the scene and the objects that have the shader applied. Use the Scale slider in the Medium node to tune the effect. In some cases you may not see much of the effect unless you lower the Scale setting. Fig. 1 shows the absorption node in the Hypershade.
Figure 1: The parameters for the Absorption medium node in the Attribute Editor.
Invert Abs inverts the absorption color so that the absorption channel becomes a transparency channel. This helps visualize the effect of the specified color since a neutral background shining through the medium will appear approximately in that color.