aRTist has a feature to automatically set the correct exposure time, which is usually enabled. This is the reason that all images were acceptably exposed so far. For film radiography, the exposure time is set such that the darkest point in the image has an optical density of 2.5.

1. Move the mouse cursor over the radiograph in the image viewer. The optical density under the mouse is displayed in the top row. The darkest point with a density of 2.5 happens to be in the gas pore, while the density in the base material is around 2.3.
2. Usually, a certain optical density is requested in the base material of the weld. In the exposure frame, click on the mouse cursor icon . The image viewer requests you to pick a reference point, as in the screen shot above.
3. Click on a point in the base material. The image viewer returns back to normal state.
4. Enter 2.5 in the "set to" field and compute a final image
5. Use the mouse to explore the optical density. Now you should find a mean density of 2.5 in the base material, approximately 2.7 in the gas pore and 2.0 in the root. Note that due to the film grain, the optical density is not perfectly constant but displays small variations.

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