Light ruler

[OS74]

I have had a project to make a "light ruler" on back burner for a while. But I'd like to float it here to see if you have any ideas.

The idea is to have two (or more) absolutely parallel lasers fixed on a bar at a known distance (10 cm). This is then fixed to the camera tray.

When filming or photographing the beams and dots will give an instant scale reference on the subject when documenting artefacts.

I've had a few 5mw red modules laying around, but I don't have equipment to machine housings for them.

So, do you have any suggestions of available dive-proof laser pointers that could be used as is or with modification alternatively, small simple but waterproof dive lights that could donate the shells?

Cheers
/O

 

[hanachan]

hi, OS74
i had made two waterproof cross lasers to measure object's length underwater.

the cheap lasers were got from DX. they were not so good. some good, some bad.
put a laser into polyvinyl chloride pipe and sealed it with clear acrylic plate.

fixed at camera frame(15cm,10cm)

IMO,
a 5mw RED laser has not enough power. it may not be visible in the bright sea.
over 25mW green laser suitable.

my project has stopped because i rarely dive recently.

 

[OS74]

Thanks,

Is there any special reason for one or the other color if you disregard power? In my home waters, there won't be much ambient light anyway...

Your solution to seal the lasers seems good! What did you do with the power though, a separate battery holder? My thoughts were that it would be simpler to seal in the battery too to avoid cable glands.

About the power, if I stick to a dot there should less requirement on power as the output is concentrated in one point, wouldn't you think?

Cheers
/O

 

[hanachan]

absorption rate of the water (and sea water) is low between blue and green.
outside of this range it is high.

only a laser body not include battery is compact. so it is easy to fix the laser to camera tray.
but either (include battery or not) is good.

it is somewhat difficult to recognize the dot's position in bright condition. ->possibly dangerous.

it is difficult to evaluate the real object's length from the distance between 2 point in some occasion.
in my upper figure two horizontal red lines are parallel but 2 vertical lines are not.
this implies upper horizontal line is nearer to the camera than lower line.

 

[jspeybro]

please be aware that a 25mW visible laser is a class 3 laser and has some safety concerns!
You seem to be using line-generating lenses so this reduces the risks but don't assume it will be safe to the eyes.

green will appear brighter to the eye, but camerass are more sensitive to red light so I wouldn't rely only on what your eyes percieve as more intense.

 

[Klem]

Agree with jspeybro...the human visual spectrum is most efficient in the middle; ie, on the visual 'rainbow' ROYGBIV, green sits in the middle as it requires the least amount of light energy for the cone cells (daylight cells) of the eye to fire. Red sits on the edge of what we are capable of seeing and therefore requires more energy to be perceived. For mechanical recording devices this is does not apply.

For example, green firearm aiming lasers can be seen in most light conditions including daylight whereas red is confined to indoor and twilight/night shooting only. As I understand it, red lasers are the cheapest/simplest to manufacture which is why they are so ubiquitous, otherwise I am sure we would all be using green.

5mW is the power limit the US authorities will allow as 'eye safe' without a Class III license. In Australia thanks to a few 'oxygen-thieves' hanging out in the carpark of Macca's shining 100mW lasers in the eyes of pilots the authorities have banned the importation of them all without a firearms license.

 

[Packhorse]

Excellent project. Will be watching!

 

[lucca brassi]

Contour HD cameras have such system http://contour.com/products/compare_cameras

http://contour.com/stories/children-and-laser-pointers how it looks

 

[hanachan]

green will appear brighter to the eye, but camerass are more sensitive to red light

really??
a (digital) camera is to be made to catch the light as human (eyes) perceives it.
if not so, the camera is not good or aimed at special use.
to my knowledge a CCD for digital camera has twice green sensors (R1,G2,B1)
to get closer to human (eyes) sensitivity.

anyway any color may be ok, if it can be visible and recorded by camera.

 

[jspeybro]

really??
a (digital) camera is to be made to catch the light as human (eyes) perceives it.
if not so, the camera is not good or aimed at special use.
to my knowledge a CCD for digital camera has twice green sensors (R1,G2,B1)
to get closer to human (eyes) sensitivity.

anyway any color may be ok, if it can be visible and recorded by camera.

Good point. I wasn't aware of this because I only work with cameras aimed at machine vision. Cameras use silicon sensors which are most efficient at the edge between red and infrared.

 

[OS74]

God discussion!

For safety reasons I'd like to stick to 5mw or below if it is enough. BTW my home waters are green enough :-)

As long as there are only laser dots, I think that much higher power would not be necessary. Anyone with experience of laser pointer underwater that could confirm or reject this assumption?

Cross-type or other patterns will diverge the beam and will probably require more power, I have been thinking of a combination though in order to show difference in distance and angles.

Cheers

 

[hanachan]

hi,
i made a 'quick & dirty' laser ruler and tested it the other day.

two red cross laser (5mw) at corners & one line green laser (30mw) at center

on land

photos taken from distant & near position. paper size is 13x15cm

under water (bathtub)

at this time a green laser might be flooded.:crazy:

under water (sea)

upper: original captured from the video (TZ7)
lower: enhanced red color
can you see the red line on the fish?

visibility was good (15-20m) but (or so) it was very difficult to see the laser line.

my impression:
a 5mW red laser is too weak to apply a moving object at some meters distant.
it may be usable for macro objects near camera within about 50cm distance.

a line laser is better than cross laser because line is brighter than cross
and it is difficult to make both vertical and horizontal lines be equally bright.

next ill make the laser ruler for macro objects.

Anyone with experience of laser pointer underwater that could confirm or reject this assumption?

ten years ago i made a underwater laser pointer.

the articles is here 1, 2, 3. sorry Japanese only.
it was very very difficult to see the red point position under sea. no useful.

some years ago i saw the video taken by the Japanese research group searching the Coelacanth.
they used the two line green lasers (maybe very powerful) to measure its length.

 

[eheupel]

God discussion!

For safety reasons I'd like to stick to 5mw or below if it is enough. BTW my home waters are green enough :-)

As long as there are only laser dots, I think that much higher power would not be necessary. Anyone with experience of laser pointer underwater that could confirm or reject this assumption?

Cross-type or other patterns will diverge the beam and will probably require more power, I have been thinking of a combination though in order to show difference in distance and angles.

Cheers

I use both off the shelf consumer (less than 5mw) underwater lasers (mainly this one: http://www.scuba.com/scuba-gear-31/026283/Trident-Underwater-Laser-Pointer.html ) and serious custom built licensed lasers for my underwater research. The consumer lasers work well enough for night and dark diving especially. Max distance I have used them (and seen them) is ~12' in near pitch black conditions. In the green you dive in (like my diving grounds) I routinely use them in 3 to 8' range. They are sometimes hard to spot beyond about 6' though, they are there but dim and hard to see. Even in clear water I doubt 5mW pointers would be useable more than about 15'. Anything more and you really do need to crank up the power which has significant dangers for dive buddies and fish etc.

The dot's are all we ever use, whether on our 2km capable ROV's with serious Class IIIb, 500mW+ lasers or on the consumer lasers (less than 5mW) I use on video camera rigs. Lines and cross patterns in lasers are decreasing the effective output compared to spot.

I have two of the trident lasers rigged to a PVC base with grooves cut into it to hold them snug & parallel. We adjusted the lasers to ensure they remain parallel (at least to 30') and we measured the beam center to beam center distance (5cm) so now I can use them to measure anything perpendicular (or nearly so) to the lasers. The base can be bolted to the bottom of a video camera plate or with bungee cords & velcro worn on my wrist like a goodman light. I've gotten pretty good at eyeball estimating in the water using the lasers (+/- 5cm on a 50cm fish) , but using the video after the dive we can get more exact measurements. I have repeatedly taken these down to 100' with no issues. They aren't perfect but they are cheap and work ok for what i do.

Only real problem is that fish radically change their behavior around the dots, chasing them and following them. Fun for first 2 minutes, but after that...not so much.

 

[OS74]

Ah, thanks.

I have a few small laser <5mW modules lying around that I was planning to house but maybe I should shop around for ready made ones instead like your trident. I haven't seen waterprof/dive proof variants from the usual dealers in HK though. So the Trident may be the best option then.

Cheers
/O

 

[hanachan]

new version

5mW line red lasers from DX. ill use this for small objects near video.

it is not easy to fix the laser lines (really plane) parallel.
because the frontal lens is movable.

first rotate a frontal lens focused,
second fix the line lens to the body with adhesive,
third fix the bodies to make their lines parallel.

photo while adjusting

the interval between lines is 15mm.

i noticed the line laser is really a 'plane' laser.