The first thermal imaging camera’s required the user to manually adjust the gain and contrast, but users were not always skilled enough to keep it adjusted properly. This sometimes led to users missing information. Automatic gain control (AGC) was developed to remove this burden from the user so he could concentrate on his work. However, this also led to users missing information .
But wait… how can this be? Can we really have the same problem with and without AGC?
First, how does AGC work?
The camera finds the hottest and coldest temperature in the scene. The hottest temperature becomes white (or red) and the coldest temperature becomes black (or blue). All the objects with temperatures in between take on different shades or colors based on their temperature compared to the maximum and minimum temperatures.
AGC is very good when you are interested in a good overall view of the area. You can concentrate on flying, investigating, or hunting rather than how to use the camera.
AGC can be a problem when you are interested in an object which is a small part of the scene and its max/min temperatures are different from those of the overall scene. For example, imagine you are in a helicopter and you look down on a scene looking for a police vehicle. It just so happens that there is a power transfer station in the scene which is very hot, say 200ºF, even though the day is cold and the ground is 20ºF. You look down to find your car which has a thermal marker on it. The marker may appear 0ºF and the car roof 20ºF. Recall that ΔT is the “temperature difference between two objects.” When the AGC uses the 180ºF scene ΔT to set its colors, the marker may be much harder to see because it has a relatively small ΔT. The roof may be 80% gray and the marker 90% grey. To fix this the operator should take the camera out of AGC mode and adjust the gain and contrast manually. Lower the max temperature to 90ºF and the vehicle marker will be much easier to see.
The same thing could happen when hunting. In AGC mode a pig may look all the same color without detail if a hot campfire is also in the scene. Switch out of AGC mode and you can get a clear picture.
Let’s look at an actual picture and see how adjusting gain and contrast affect it.
In this picture, contrast is set correctly because there are colors from white all the way to black in the scene.
In this photo, contrast is set too wide. The camera is expecting to find objects with higher and lower temperatures than it is finding, and has reserved certain colors it is not using. As a result the thermal resolution suffers. The absence of pure white or pure black from the scene indicates this problem.
In this photo, contrast is set too tight. The camera is expecting to find only objects in a narrow temperature span. As a result, the coldest object appears pure black, and the warmest object appear pure white. As a result we lose all temperature information outside a small band. The absence of significant gray from the scene indicates this problem.
In this picture, gain is set correctly because there similar amounts of white and black in the scene.
In this picture, gain is set too high. The camera is expecting to find cooler objects, and almost everything in the scene is hotter than the max temperature. The serious lack of black and gray in the scene indicates this problem.
In this picture, gain is set too low. The camera is expecting to find hotter objects, and almost everything in the scene is colder than the minimum temperature. The serious lack of white and gray in the scene indicates this problem.
In order to make best use of your thermal imager, practice using manual mode and AGC mode and switching from one to the other. You may find you use AGC mode most often, but regularly switch to manual mode to investigate the details hidden by AGC.