Realistic image synthesis is the process of computing photorealistic images which are perceptually and measurably indistinguishable from real-world images. In order to obtain high fidelity rendered images it is required that the physical processes of materials and the behavior of light are accurately modelled and simulated. Most computer graphics algorithms assume that light passes freely between surfaces within an environment. However, in many applications, ranging from evaluation of exit signs in smoke filled rooms to design of efficient headlamps for foggy driving, realistic modelling of light propagation and scattering is required. The computational requirements for calculating the interaction of light with such participating media are substantial. This process can take many minutes or even hours. Many times rendering efforts are spent on computing parts of the scene that will not be perceived by the viewer. In this paper we present a novel perceptual strategy for physicallybased rendering of participating media. By using a combination of a saliency map with our new extinction map (X-map) we can significantly reduce rendering times for inhomogenous media. We also validate the visual quality of the resulting images using two objective difference metrics and a subjective psychophysical experiment. Although the average pixel errors of these metric are all less than 1%, the experiment using human observers indicate that these degradation in quality is still noticeable in certain scenes, unlike previous work has suggested.