Re: Quite believeable
Well yes, and no.
The dew sensor was a flat resistive element that did change it's value with moisture present. The lamp to which you refer was used to detect start and end of the tape. Subsequent end of tape detection was done with infra red LEDs - which were much more reliable.
A nice little earner was to be made in the 80's replacing those filament lamps though. I've still got some in storage ! :-)
In a former life, I was a production engineer for a major Japanese VCR manufacturer, and as part of finished goods QA, we used to remove the top cover and blast in some moisture from a modified humidifier, just to check that the dew detection circuitry was functioning correctly.