RGB laser line generator

I have been using a 40mW red laser to take photos here and here and in the Oxford Times here. But the whole redness of it all is just a little too, well, monochrome..

So after considerable deliberation I purchased a 500 mW RGB laser used in video projectors and light shows, with a pair of blue, green and infrared safety goggles.
RGB lasers create a white beam consisting of red, green and blue lasers that are combined using a couple of dichroic mirrors (I did an experiment with a dichroic prism previously here).

500mW RGB laser with cover removed. Notice the glue holding everything in place…

Before firing it up I researched laser safety to calculate the amount of energy that would enter your pupil if the beam entered a night adapted pupil, with the line generator spread at various distances. I also joined the PhotonLexicon forum, which was worthwhile because the first advice was to ensure you don’t mount any flat surfaced optics (eg filters or line generator) perpendicular to the beam because the reflection can fry itself – always mount them at an angle…

So I went about designing an enclosure so that the line generator could be rotated and the beam can be masked off to reduce the possibility of catching your eye in stray beams. Another necessity is an interlock switch so that the laser can’t be switched on without the optics in place.

Design includes rotating line generator attachment, a mask to control the spread of the beam and space to contain an interlock switch (not shown), and an Arduino nano to regulate the beam brightness and hue. On the side is a 1/4” tripod mount. The joys of using components in sketchup are that as you update the model the cut-out plan automatically adjusts. Drill hole alignment is not tedious either…

Cardboard prototype of enclosure showing rotatable line generator holder – stuck on with blu-tack…

Beam test sweep – the horizontal stripes are from the green laser flickering – apparently they need to reach the correct temperature before they stabilise. The vertical stripes are from inconsistencies in the ridged line generator.

Trippy picture from sweeping the beam across my desk while moving the camera. The distortions are intended but the colour variation isn’t.

I needed to see why the beam was so inconsistent and on closer inspection I found the colours weren’t exactly

This is the beam shone at the millimeter graticules on a steel ruler photographed through a 10 stop ND filter. The beams are not identical – the red beam is a 3mm diameter circle, the blue beam is a 1x4mm line and the green laser is well focussed 1mm diameter spot.

Making a Powell Lens line expander:

2015-10-28 14_54_19-FRED 13.90 (powered by Photon Engineering) - [Powell Lens.frd _]

CAD model of Powell line expander

With the help of Graham Stabler and Tim Stephens from Oxford Hackspace looked into making my own Powell lens line expander – Graham cut out some blanks in perspex and using 1500 grit sandpaper and brasso which I polished on a couple of blocks of wood.

First attempt with 1000 grit sandpaper and brasso polish on perspex blank. Shiny but not very smooth…

The line on the first attempt is kind of straight but looks like it’s shone through a cobweb…

That’s as far as I have got.
Next stage – polish a new blank with up to 2000 grit wet sandpaper followed by aluminium oxide polish finish using a polishing jig made on aluminium blocks.
Then hopefully we’ll have a precision polished lens with a more consistent line.

Next step is to integrate a beam controller to reduce the beam intensity to safe levels using pulsed width modulation (PWM) and RGB colour adjusters to make it as close as possible to white.