Mislim da se moze reci prozirni prozor ili prozorcic bez obzira na njegov oblik (kvadratic ili traka, itd.), a evo nekih primjera...

The cover also includes a transparent sight window located adjacent its lower end for observing the level of the contents of the bottle, a light window adjacent its upper end for admitting ambient light into the closed dispenser and a push bar for activating the pump to dispense material from the bottle.

www.freepatentsonline.com/7086567.html

A sight window 36 is provided on the lower front fascia 27 of cover 25 to allow the user to gauge the liquid level inside the collapsible bag or bottle. Another sight window 37 is provided in the top surface 30 of the cover 25 to allow the user to further gauge the level of the liquid in the collapsible bag or bottle. Furthermore, the back surface 23 of the base mounting

[BH417] Bottle Holder with Drink Level Window
http://www.sunluxhk.com/bottle_holder.htm


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Note added at 1 day1 hr (2009-09-03 22:52:32 GMT)
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Ja sam imala jednu ustvari prskalicu od 0.5 l koja je dosla sa sredstvom za sobne biljke, ali je imala prozirni prozor od vrha do dna sa crtama (dozer) pa se u tome razrijedjivalo sredstvo tipa 2 crte ovoga 1 crta vode. Kako god, meni prozor za provjeru nivoa najbolje zvuci...