http://downloads.bbc.co.uk/rd/pubs/reports/1990-15.pdf
One popular style is the 2D Quadratic Residue diffusor (often called the BBC diffusor, after a paper published by the British Broadcasting Corporation). This design is based upon a complex set of calculations involving prime numbers whose results are then quantized into four "steps" for ease of manufacture. Mathematical proofs and theoretical explanations can be found in the BBC paper, but understanding them is not necessary to build the panels. The practical results are outstanding. Unlike many diffusors, the BBC design works equally well in both the lateral and vertical dimensions and boasts effectiveness even with modest 20% coverage. In field tests, the design demonstrated effectiveness in near field as well as far-field applications and, to a large extent, even at oblique angles.
