Peephole is also commonly written as peep-hole as in the Leo entry (mis)translated as Guckfenster.
One can also write "peep hole" as two words but it is perhaps not necessary to add this.
Peephole has a much wider range of meanings than Türspion and are well covered by das Guckloch.
"Bitte beachten Sie: Nach DIN 18105 beträgt die Normhöhe für ein Guckloch ("Türspion") in Wohnungstüren 155 cm." http://www.polizei.propk.de/vorbeugung/diebst...
"This would be by way of one way glass, door viewer (peephole) or CCTV coverage."http://www.semweb.com/may04/alarms.htm
The (wide angle) (security) door viewer corresponds to the Türspion.
Compare for example http://www.thehardwarehut.com/door_viewers.php
could be omittted since Guckfenster is a relatively uncommon word whose meaning is obvious from its component parts and is not the same as peephole.
Strangely Leo does not have the entry
to peep -- gucken
which is much more apposite than
to peer -- gucken
but perhaps this is a suggestion for the new entry forum.