Broadband behavior of an optical-digital focus-invariant system

We present experimental evidence that demonstrates a greatly increased depth of field in a broadband-illuminated imaging system through the use of a special-purpose phase mask and digital filtering. The phase mask is of a simple rectangularly separable cubic-phase design, whereas the digital filtering used was a simple object-independent least-squares inverse filter. We demonstrate successful operation of the system over a range corresponding to 30 times the Hopkins criterion for allowable misfocus. (C) 1996 Optical Society of America.