As more cities in America legalise the drug, attention has switched to the pungent smell that wafts from the joint itself.
Denver has passed a new “odour ordinance” with a potential $2,000 (£1,247) fine for anyone found guilty of polluting the atmosphere.
The need to draw up standards emerged because of the confusion over the legal position of whether somebody smoking marijuana in their own home could be committing an environmental offence when the smell seeps into the street.
Under the new law an offence is committed if the odour is detectable when the smoke is mixed with seven times the volume of clean air.
The nose telescope – also known as an olfactometer – is the device used to measure the concentration of cannabis in the air.
It has something of a Heath-Robinson appearance with a telescope type device attached to a nosepiece.
Denver Police have opted for something called the “Nasal Ranger”, costing $1.500 (£935).
However the more sophisticated version of the technology, which also records both the concentration and GPS coordinates of where the offending whiff was recorded costs $3,500 (£2,183).
This does, however, include a subscription to a patented odour tracker programme.
Denver is one of a number of cities across the USA to have legalised pot.
Last week recreational use of the drug was legalised for adults over 21 in Portland, Maine at the other end of the country.