The actual housing itself. If there is nothing there than no surface to mark and the laser would have nothing to project to. However once I get up there with the housing and put it in place, then I can use it to position the housing while the housing is occupying that space, and mount it right then right there.
It's not that I need the precision, although I did say that I want "precision" what I was looking for was convenience without having to measure and mark onto a non-surface since I already did all the markings on the floor already. Another poster mentioned those old fashion projector laser pointer than we all used to use in the old days and I think I have one of those if I can find them I will just use that. Put it on the floor mark facing up and I should have a good reference on the ceiling to mount. If it's off a bit no big deal.
"I have a laser level that does a line on the wall or floor, but no way to project a precise point from the floor < to the ceiling.>" so please excuse us if we don't picture your project perfectly. Yes, you said the drywall was off, but nothing about a hollowed sloped attic until now.
re: There is nothing to mark onto unless I start doing masking tapes across studs for the housings I need to put up which then I have to tear off.
I'm sure you know they're not "studs"...
Anyway, in the grander scheme of everything else you are doing, is taking off a few pieces of masking tape really that much of a big deal? It's not like it's gonna stick to bottom of the joists very well anyway. Get a tall stick that you can rest on the marked floor and make sure it's plumb. Tape from joist to joist right above the stick and put an X on the tape.
Another option is a piece of 1 x 4 long enough to span several joists. If the light pattern is an evenly spaced grid (or even if it's not) you could use one board to span several joists and mark several locations on the board using either a laser pointer or the long vertical stick mentioned in the preceding paragraph. The board would even serve as a place to rest the housing and keep them flush with the bottoms of the joists, or extended below by using some spacers.
What's it going to take - a couple or three drywall screws to hold the board onto the studs while you mount the lights?
BTW, the PLS-5 laser I mentioned above is self leveling, a real time saver. It would spot the location perfectly for every one of your 42 lights in 5 seconds or less. I used mine on a recent project to get the roof location for a furnace vent. The pipe went up perfectly plumb. Next up, same thing for a plumbing vent stack. Great for checking joist level before drywall, too.
If it is in the budget, buy one that will do the most. This time you need it for this, next time, who knows what you'll need it for. They are handy handy tools, and you will find its value when it saves you hours of work, or some relocation of misplaced widgets. Good ones can be bought used or at pawn shops for very reasonable prices. I have a POS Ryobi that so far will only throw one plane of light vertically. I have e mailed them, but so far they give me indecipherable answers. It levels with bubble levels, so is not very accurate. It is useful for a few limited things, but next time, it will be a self leveling with multiple features. It's the kind of tool you use once a year, but well worth it even at that rate. Maybe you'll use it more than that, and it sure saves time and work.
It is real. Just like the sun. If you stare into it, it will damage your eyes. But there are so few people who actually stare at the sun until blindness, it rarely ever happens. Just like lasers. The smaller lasers WILL harm your eyes. The big ones just burn anything they touch. The smaller ones will only harm you with excessive exposure.
So, unless you're going to stare into it for long periods of time, there is no danger.
If you're dumb enough to stare into it for some minutes, injury is possible, even with the low outputs of the devices. So is staring into the sun at high noon. And arc welding without a helmet. AFAIK, shining lasers at airliners can get you in trouble, but the incidents of office workers being blinded by laser pointers is close to zero. All the instruments have printed warnings. People who lack the common sense to heed the warnings will always suffer the consequences. Like in politics.
Well I have not started the project yet. I always do a two to three week look-ahead, so I am not hanging lights, stop, come to my computer, sit idle and ask a few questions. I try to plan ahead to make sure I have all the information and pieces I need, then when I execute I have minimum disruptions. Does not always work out that way. So while I am discussing lasers, my current project is actually wiring my internet cable and doing formwork for my outside planters. The lights will come in a few weeks and I think a laser will help a great deal.
I have already done the tape measuring, snapped chalk lines etc... on the floor. I just want an easy way to hang the lights into the hollow space between the joists by transferring the location from the floor to the ceiling without having to put stuff there to mark that will become an obstacle when I do the hanging.
The laser will do the trick, I found one for about $50 will do it just fine and will be a great help on other projects down the road.
Laser levels, laser tools in general and laser pointers are usually either Class II or Class IIIa. (There are a few higher power laser pointer-like devices of Class IIIb, which are not supposed to be retail-available.)
Class II and variations of Class II are quite safe, especially if red in color. At worst, it is "supposedly safe" (though *not recommended*) to stare into any Class II laser of any wavelength/color for up to 1 second.
The "caution label" for Class II lasers is "CAUTION - Do Not Stare Into Beam".
Class IIIa is "supposedly safe" to stare into for at least .2 second or "blink/aversion reflex time". Red lasers in this class are safer than that, since different wavelengths are not equal in ability to damage the retina and red is the least damaging range of visible wavelengths.
If the laser pattern is a line rather than a dot, if you are far away enough from the laser for the line to be at least a couple inches long, and if the laser is Class IIIa, then at such distances it is effectively down to Class II.
I would not worry if your eye takes a brief hit and you have an afterimage lasting even several minutes or occaisionally more. Such effects are normal with exposure only a couple to a few percent of the amount that would cause actual damage.
There is such a thing as a Class I laser, though that is not common. Class I means safe to stare into continuously, according to 21 CFR
1040.10 and 1040.11. At upper limit of Class I, looking into a laser beam still to some extent puts a spot in your vision. A laser product that is properly certified as Class I and having proper label requirements is otherwise not subject to regulation.
Keep in mind that for visible wavelengths, Class II has a range of 2500 to 1 ratio of power or exposure intensity. Since looking into a laser beam at upper limit of Class I can put a spot in your vision and 2500 times as much laser power as that is safe or "at least very likely safe" to stare into for 1 second (more for red wavelengths), I would not worry about brief eye strikes of laser beams up to and including Class IIIa.
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