beergod
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perceived brightness of split lasers
Hi, I am trying to wrap my head around something
So you have a standard green consumer laser(532nm, 5mW) shining through a set of 10 90 degree beam splitters (microscope slides). The first beam
should be 50% of the power of the original beam, the second should be 25%, the third 12.5%, etc.
As far as the eye is concerned would the last reflected beam still be visible do to the high power of the laser? Or is the perception of light
logarithmic and the last will appear very dull?
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peach
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I know that hearing works close to logarithmically, in that you can hear very faint sounds easily, but can't really tell the difference between very
loud sounds. To make an amplifier sound twice as loud, you will need to be putting ten times more power through it. Guitar amplifiers, jet engines and
explosions all sound similarly ear piercing, yet involve quite different amounts of power.
Light intensity, I suspect, works in the same manner. If you let your eyes settle at night, you can see very faint stars. But trying to tell the
difference between two arcs from a welder by simply staring at the arc is next to impossible.
A bigger problem for your laser is the contrast if you're viewing all of those together. The beam it's self is going to be around 500 times brighter
than the last, and it's initial intensity may be enough to compress the level of contrast your eyes can discern to. If you're viewing the last beam on
it's own, I would expect you'd be able to see that in a dark room. You can buy visible LEDs and lasers with just half a mW of output or less.
Also, because your intensity is halving with each microscope slide, that would be an exponential decline, as opposed to logarithmic.
What is it you're thinking of doing out of interest?
Tiiiiiiiiny LEDs from the 'X-box ring of LEDs' instructable
This 0402 from LED1.de is just 0.04mW
[Edited on 21-2-2011 by peach]
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unionised
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The 10th reflection would be about 0.1% of the original beam, but I'm certain it would still be visible.
A 1 mW laser will just about light a whole room (very dimly). That's covering several square metres of walls with light. The original beam only covers
a few mm squared so the room is about a million times dimmer than the bright spot.
I suggest you could use a stack of 20 reflections and still see the beam. The eye is very sensitive. That's the problem with it when you start playing
with lasers- it's sensitive, but it's easy to damage.
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bquirky
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A peice of unsilvered glass like a microscope slide will typically reflect about 4% and transmit aproximatly 96% from each air-glass interface
as has been previously mentioned eyes are logorithmic in sensitivity in order to percive a change in brightnesss whould require a loss of about %50
of the available light.
Give that and a quick tickle of Excel
starting with 5mw and loosing %4 at each interface you can see that it takes about 17 splits before the power level has been reduced by half (or 3db)
Given that each glass slide is actuly 2 air glass interfaces you should be able to split your beam with about 8 slides before noticing a difference
in brightness.
or go through 16 glass slides before your 5mw laser looks like a 1mw laser
as 1 micro watt is quite visible you could got well beyond 100 slides before you can no longer see the beam with your eyes.
interface # Microwatts
0 5000
1 4800
2 4608
3 4424
4 4247
5 4077
6 3914
7 3757
8 3607
9 3463
10 3324
11 3191
12 3064
13 2941
14 2823
15 2710
16 2602
17 2498
Regards.
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beergod
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Thanks so much for the responses guys, really cleared up my questions!
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IrC
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How sensitive is also a function of frequency, and whether rods or cones are doing the seeing. The eyes are most sensitive to green which is why my
blue fire starter is barely visible at night pointing it up in the sky, but with my 200 mW green I can see patches of glow in the clouds on a low
overcast night and of course the beam itself even in daylight. I can see the red glow in the low clouds as well but nowhere nearly as bright as the
green. This sensitivity VS color difference makes it a real pain in the ass trying to scan all three producing nearly white lines since it is
difficult to control the intensity of three diode lasers to get a white balance on the wall by mixing the beam out of all 3 Galvos. However at .2
watt it barely lights a match even though the green is much brighter to the eyes. Of course this is partly due to using adjustable glass lenses for
the red and blue, so far I only have a plastic adjustable lens for the green. If any of you are into fire starters believe me pay the extra for the
glass lens, and non adjustable laser diodes are relatively useless other than for pointers. My 1/4 W red starts fires, is very visible at night
although the rods are coming into play in the dark with the red one. Very hard to find adjustable focus green diodes and so far I have not found one
with an adjustable glass lens in the low cost units. Off topic a little but I thought as long as I was bringing up the lasers I would add a few
observations I have had. I should add if any of you are into high power Yag rods do so in a room without curtains, you cannot always see where the
beam hit. I had curtains smoldering one night in a place I was unaware of from a reflection of the invisible beam going where I though it was not. Now
I use night vision goggles to play with the Yag rods so I can see it in real time.
"Science is the belief in the ignorance of the experts" Richard Feynman
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