Sigma

The Greek letter Sigma. It should be a capital Sigma, and stand out in bold against a plain white background. The sigma is black, of a font that is simple and easy to read such as Arial or Times New Roman, in a size that fills the majority of the image. The edges of the Sigma are sharp and well-defined. The background is uniform, without any shadows, smudges, or marks. There is plenty of space around the Sigma, so it isn't touching the edges of the image.