A security device for verifying an authenticity of a security document comprises an at least partially transparent substrate with a first surface and a second surface. A first pattern is arranged on the first surface. A second pattern is arranged on said second surface. The first and the second pattern each comprise a plurality of pixels with at least three different gray levels visible from a macroscopic perspective. The first and second pattern cover only gray levels in a range between 20% black and 80% black, in particular between 35% black and 65% black. The first pattern is inverted with respect to the second pattern.