to concentrate on a very detailed area. (see internet under 'kijkvenster')

The translation of "kijkvenster" is especially hard here because the sentence explicitly says: This is called a 'kijkvenster'.

Based on the text below, it is possible that the word is used wrongly in your text...:

"Deze sleuven en vlakken functioneren als een soort kijkvenster dat archeologen toelaat om een evaluatie van het terrein op te maken."
http://www.kluizendok.ugent.be/vooronderzoek.html

Based on the reply of a friend archeologist who is director of the Center for Archaeological Research at William & Mary, I would choose for "excavation unit." Depending on the judgment of your text you can expand it to either "initial excavation unit" or "expanded excavation unit."

Below is his reply:
"I can tell you how we would typically refer to what I think you are reading about, though I don't believe there is necessarily a universal set of terminology for these things among all professional archaeologists. The terminology we use is generally consistent with what others use in this region and throughout the US, though, as far as I know.

If I understand you accurately, the "existing excavation" may be what we would call a "test unit" or "excavation unit." What you quote as "traces" [sporen] probably corresponds to what we would usually call "intact cultural deposits" or "features." The former is usually used to refer to a layer of undisturbed cultural deposits (e.g., like a buried topsoil deposit that has artifacts in it, or a buried, undisturbed deposit of oyster shells and artifacts from a prehistoric native american campsite). The term "feature" is used to refer to some kind of undisturbed, historic intrusion into the natural subsoil that is filled with artifacts and cultural deposits like a virtual time capsule of the historic occupation of the site (e.g., postholes from a building or fenceline that extend deep into the subsoil, or a historic cellar pit that's filled with the artifacts and rubble from the historic building that once stood over the cellar, or undisturbed subsurface remains of a historic building foundation, or a filled wellshaft, or a human burial, etc etc).

It's common that archaeologists might identify a portion of a subsurface "cultural deposit" or "feature" in an initial "test unit" or "excavation unit," and then expand the "test or excavation unit" into what we'd call a "block excavation" by adding other "excavation units" immediately adjacent to the first one to expand it into a "block excavation" of contiguous "excavation units," which is in less formal terms a kind of "window" into the intact deposits and features lying below the surface of an archaeological site.

And adding:
" I could add that the initial "excavation units" are often placed across a site in some objective, systematic way (e.g., 1-x-2-m units placed at 10-m intervals) or some other systematic shovel testing to identify specific spots that may have higher potential for features.

Expansion into block excavations would then be in those high-potential areas to open a larger "window." Professionals work under an ethic that emphasizes maximum information-gain from a minimum amount of excavation, given that all such excavation ultimately destroys the part of the archaeological record that is excavated...like ripping out and burning the pages of a rare, one-of-a-kind archival book as you read it. So careful documentation of what one is excavating is critical."