Visible beam yet low power diode

[sam1902]

Hello,

I'm working on a project that requires aiming a laser back to near its source where I placed a photoresistor which senses the laser. When the beam is crossed by a falling object, it triggers an action shortly thereafter. I've already built two functionning iterations of the device but they both use a low power ~5 mW cheap 650 nm laser diode. I power it using an Arduino Nano which has a voltage regulator, itself powered by a 9V battery. You can see what power the current laser diode uses in my other post ( search "How to mesure a laser diode's power ?"). Basically the Arduino can only deliver around 500mA @ 5V.

To use the device, I need to hold and let fall objects right above the beam, but since the beam is invisible, it's hard to know if I'm right above it or not. Something I'd like for the next version would be to be able to see the beam.
The whole setup is indoor, in a dark room, and since there might be 3 people moving around without goggles, it'd be best that this laser diode be safe for eyes (at least reflections of it). Also, the objects might be very reflective (glass, water bottles etc), which is another reason why this should be safe for the eyes. The beam only has to travel around 4 to 5 meters (13ft to 16ft) and should end up with a dot radius of around 6 mm (0.23 inches) thereabout, which is more or less the size of the sensor.

I've looked around and apparently blue and green laser are the best for visible beams, but I remember reading a long time ago that "angel blue" (~480nm) lasers were the best for visible beams because they worked best with Rayleigh scattering. Finally, before receiving this recommendation, I must add that using a smoke machine is not an option for various reasons.

From the post entitled "List of Laser Diodes - 21/06/2020", I've concluded that 475 nm to 495 nm diodes are my sweet spot, but I have no clue as to their beam visibility which is my main concern here. Anything under 470nm is great too because I already have eye protection for these (OD4+).

I will add a few pictures of the project setup.

Thanks for taking the time to help me, again..

 

[FireMyLaser]

A green DPSS laser (advertised at 532nm) will give the brightest beam compared to low output power ratio at a fair price. This is because they typically have very narrow beams (if you keep you beam travel distance reasonably short), around 1-2mm in diameter at aperture. However these are less efficient than direct diode green lasers (510-520nm), so they may a get too close to your 500mA current limit depending on what laser you end up getting. Also, keep in mind that a 9V battery will not be able to continuously supply 500mA anyway. You should get a more sustainable power source.

 

[ZRaffleticket]

I don't think you can get away with anything eye safe without using something to make the beam more visible. If you're putting reflective objects in the beam, the best you'll get is a 5mW 532nm.

Why is a fog machine not an option? How contained is this system? You may want to look into something that can spray a water mist into the beam instead. Ultrasonic humidifiers make it foggy. Or light some candles. I think you should be approaching this problem as if you're stuck with the red diodes and as a last ditch effort only then switch to 532nm.

 

[FireMyLaser]

I use a vape pen with non-nicotine fluid to make beams visible. The fog goes away in a couple of minutes.

 

[Encap]

532nm is visually the brightest based upon normal eye retina sensitivity to wavelengths, not Raleigh scattering.
See relative brightness calculator here: https://slickscreen.github.io/laser-tools/brightness/

At same output power in mW 532nm green is visually 8X brighter than 450nm blue and 16X brighter than 650nm red.

PS Laser diodes need drivers that deliver constant current, not constant voltage like for a typical LED.
A 9V battery is not a good choice as mentioned above

 

[Anthony P]

Just a thought... a convex lens on the sensor will allow for greater range and the use of lasers with larger beam diameters.