Grow bulb?

[JlaPinska]

so i have a fixture meant to hold a 6500k tube bulb, but its been so long since i used it, i have no idea what size bulb goes inside it. how do i measure that? the fixture itself is 21.5" long. but it looks to hold a much shorter bulb maybe an 18 im just guessing

 

[Syreptyon]

I would also guess 18", but maybe try taking a tape measure and measuring the distance between the part where the light bulb actually fits into

 

[JlaPinska]

I would also guess 18", but maybe try taking a tape measure and measuring the distance between the part where the light bulb actually fits into

i thought to do the same thing and i did. comes out to 17" ...do 17" tube grow bubs even exist any more???