mamma mia, un pezzo di modernariato!
era una tecnica per fare le fotocopie prima di quella che usiamo oggi. ne ho visto alcuni fogli quando ero piccola. impressionanti.
ecco qui la descrizione
http://en.wikipedia.org/wiki/Ozalid_process

The ozalid process is a process of printing positive images on paper from patterns on film or other translucent media. It is akin to creating a photogram, using chemically treated paper.

A transparent film with the pattern to be printed is placed on a diazo compound coated paper. This sandwich is exposed to ultraviolet light. After a 15 sec to 5 min exposure, ammonia vapour is used to develop the film image on to the paper.

Its main use was for making copies of electrical, mechanical and civil engineering drawings.




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Note added at 11 hrs (2008-07-29 21:23:11 GMT)
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aggiungo qualche dettaglio per spiegare un pochino meglio, ecco qui una foto dell'eliocopiatrice "incriminata":
http://usato-computer.vivastreet.alice.it/usato-computer-inf...
si tratta di una eliocopiatrice NEOLT SUPERDIAZO
infatti le Ozalid usano il sistema "diazo" per copiare/calcare

attenzione - da non confondere con i blueprints, in quanto:
Traditional blueprints have largely been replaced by more modern, less expensive printing methods and digital displays. In the early 1940s, cyanotype blueprint began to be supplanted by diazo prints or whiteprints, which have blue lines on a white background; thus these drawings are also called blue-lines or bluelines. Other comparable dye-based prints are known as blacklines.
http://en.wikipedia.org/wiki/Blueprint
infatti nella pagina sul Ozalid process parla proprio del WHITEPRINT - http://en.wikipedia.org/wiki/Whiteprint
..."commercial terminology to describe document reproduction using the diazo chemical process. It is also known as the blueline or blue-line process...."



here they call them heliographic machines
http://www.neolt.it/pdf/Superdiazo_dry_en.pdf

some call them simply ammonia printers http://www.largedocuments.com/copiers/index.shtml , whereas the neolt website (still produces them apparently!) calls them WHITEPRINT MACHINES
http://www.draftingsupplies.com/neolt/neoltrepromachines.htm

credo che si possa usare uno qualsiasi di questi termini, forse il più adeguato sarebbe "whiteprint machine" oppure copier, in quanto Ozalid era (o è ancora) la ditta che aveva inventato il procedimento (un po' come Tippex o post-it).
HTH