This is one of the many security features used to identify counterfeit bills. It is almost impossible for counterfeiters to obtain the same grade of paper with its unique blue and red fibers. These features have been supplemented by security threads made of a special polymer, which glows yellow under ultraviolet light. Furthermore, a watermark of the portrait is visible from both sides when held up to any light. Several features have been added to help people with visual impairments, which also have the capacity to thwart counterfeiters.
Larger portraits are easier to identify and harder to duplicate.
Larger numbers on a white background are also planned for the future as a low-vision aid. At the same time the Treasury department is adding micro-printing which is difficult for anyone to see. For instance the Great Seal which appears on the back of a dollar bill has a storied and somewhat secret legacy. While the bald eagle on the right side of the bill is perhaps familiar to everyone, the picture on the left is more mysterious.