OK, last night I took the DVD laser outside for a test. At about 9:30 pm I could see the beam, though it wasn't completely dark outside. Later at 11:00 pm, I could see the beam clearly. It wasn't anywhere near as bright as the 20 mW green, but it was OK.
For comparison, I also tried out my 2 mW red pointer (690 nm) and my 10 mW HeNe. surprisingly, the beam from the red pointer was actually visible, although to see it I needed to keep it still, and stare at it for a few seconds. If I moved my eyes, it would disappear and I would have to search for it again.
The 10 mW HeNe was much brighter, and vsible without any effort. The 100mW DVD laser was considerably brighter than the HeNe. None of these reds is visible at all indoors.
It definitely seems that red is way more sensitive to the presence of moisture or particulates in the air than green.
For this test, I used the Meredith Instruments brass collimator with the glass lens.
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