Unlike conventional film, CCD cameras have a linear response to light, that is, the registered intensity is proportional to the real intensity for every point.

This is an important characteristic for some scientific tasks -i.e. astrometry and magnitude measurement. However, it produces some unwanted effects when your goal is to get a nice, artistic image in which all possible details can be seen.

The root cause is that the necessary bright and contrast adjustment to see dim objects has the secondary effect of saturating the bright ones. Conversely, if bright objects are not saturated, dim objects disappear.

The practical solution is to digitally simulate conventional film behaviour. Thus, the “S” shaped characteristic curve of conventional film is applied to the image in place of the normal “straight line” curve. This way, dim objects may be enhanced without saturating the bright ones.

Additionally, this method allows to enhance the edges of the objects by means of a method similar to the well known unsharp masking.

You can get additional details in several web sites that describe this process in detail, as for instance Dr. Okano’s own web site.