contrast

How do we quantify contrast?

Sunligh

t differs from minute to minute, hour to hour, day to day and from season to season. This means that there can be visible differences between the inspection of different turbines. Even within one blade inspection as clouds can move in and out. The purpose of an inspection is not to make nice pictures, but to reveal anomalies.

Contrast is defined by the difference in luminance between the object (blade section) and the luminance of the background. Blades are painted in RAL9010 or RAL7035, which is white / grayish type of color. Blades can blend in with the background, depending on the amount of clouds, which impacts the contrast. The amount of contrast is determined by the position of the sun as well. Visually it is usually very apparent if contrast is poor. Our tracking software provides an indication of the Weber contrast. This is expressed as the ratio of the luminance of the object and the luminance of the background. If it equals 0, there is no contrast. It should be noted that this is a dynamic number, it may change during the course of an inspection.

However in case of doubt, it can easily be assessed from the image histogram. Usually there are two distinct peaks in the histogram: one for the blade and one for the background. The distance between the two peaks is a measure of contrast. The larger this distance, the greater contrast. When the two peaks tend to merge, it's an indication of a very little contrast. With sufficient contrast, post processing will reveal sufficient detail even in poor light conditions.

contrast combined

Interested in how we control the quality of our images? Feel free to contact us!