AAPM TG18-QC Pattern
This image shows QUALITY CONTROL three times. Once very close to black, close to white, and in the middle.
Each letter is slightly darker than the previous letter.
There are 18 squares each testing a different level. The first and last square has a small square inside.
The remaining squares have small squares in the corners that are + or – one level.
You should see black smoothly shifting to white with no steps. There should not be lines in the image.