Principles of Brightness, Contrast, Gamma

Brightness

Contrast

Gamma adjustment in CCD imaging

What is Gamma ? Think of a CCD sensor capturing an image. If there's a linear relationship between light intensity and CCD output signal we define this state as Gamma 1. We can draw this as a graph with the light intensity on the x-axis and CCD output on the y-axis:

Now we modify the graph so that it has a steeper slope >1 for the low light areas and a flat slope <1 for the highlight areas. This is called a higher Gamma (than 1):

This will enhance details in the shadows of the original image and compress the highlights, thus this setting is usefull for objects with much details in the darker areas.

A lower Gamma is the opposite: The slope of the graph is <1 for the shadows and steeper for the highlights:

This will enhance details in the highlight areas and compress the shadows.

Instead toa light-intensity/CCD-signal graph, gamma can also aplied to an existing image. Input/x-axis are the existing pixel values and output y-axis are the gamma adjusted pixel values.

Conclusions for astro imaging

Gain

Chapter 3