This paper will go back 30+ years to look at the work that led toward the lithographic models we use today. Although lithography was a very old art, going back to the 1840's, it was relatively new to what we now know as microelectronics. While circuit designers were already beginning to use sophisticated design and modeling techniques, most process models were extremely simple and often not very accurate. For those working in photolithography, there was almost nothing except for optical image models which, while helpful, were not a model of the lithography process. The original publication of this work was in a series of articles in the IEEE Transactions on Electron Devices, Vol ED-22, July 1975.