SparkFun Forums

[metalist]

I have just joined. I spent time following a very impressive topic in this forum, "Trying to understand how Laser Range Finders work" yet I still need your expert advice on selecting a photo detector.

I am prototyping an invention and the task I am solving is this:

Imaging a white screen with a laser dot (from a laser pointer, 650nm wavelength) slowly moving on it in a random pattern. A stationary objective lens is positioned 8 foot from the screen and it has a 12 inch diameter field of view in the center of the screen. A shutter opens for 50 - 150 millisecond to allow light to reach the photo detector.

The task for the photo detector is to reliably detect all instances when the laser dot happens to be within the field of view of the lens. The screen may be also lighted by natural light or any kind of common household electric lighting.

I am concerned that a tiny red dot from the laser will not be resolved against the lighted background in the large field of view.

The speed is irrelevant, the shutter is opening at the rate of 1Hz or less.

While the cost of building the prototype is not important I am looking for a low cost photo detector for a possible commercial viability. Thus Avalanche Photo Diodes ($100) and tight optical band filters ($60, from Andover Inc.) are not very desirable. Will a conventional photo transistor work? Or a photo darlington has a better chance? Or something else?

I am a mechanical engineer and I have no experience with photo detectors. I can just buy the components and try them one at a time but I hope that your advice will guide me in the right direction.

Thank you.

[lyndon]

The problem here isn't sensitivity of the photodetector; it's dynamic range. You're trying to see a small red dot against a possibly bright white background.

My first thought would be a a camera.

[waltr]

The other key is to use an optical filter that only allows the RED LASER light to pass. This really help dynamic range of the sensor. Check Edmund Optical and the Surplus Shed for filters.

For a very sensitive photo diode search for "Ham radio LASER communication receiver" and "Transimpedance amplifier". There is lots of good 'how to do it' info on the web for this.
This one is excellent with several circuit and very good explainations on how they work.
http://modulatedlight.org/optical_comms ... nters.html


A camera is another choice (lyndon's idea). Again, an optical filter will help improve the dynamic range.

[jremington]

A stationary objective lens is positioned 8 foot from the screen and it has a 12 inch diameter field of view in the center of the screen

Have you worked out how big the image of that "12 inch diameter field" will be at the plane of the photodetector?
It certainly won't be a point! Obviously, the image must either match or fit inside the light-sensitive area of the detector.

[metalist]

Thank you all for your helpful comments. I now agree - the filter is a must. The Andover Corp. sells very unique narrow band filters (http://www.andovercorp.com/Web_store/index.php ) but they are pricey.

I will purchase one for 650nm. While I have a ham licence I never heard about HAM laser communications (thanks, Waltr). But so far I see them use conventional photo diodes, but I will go deeper into this. The Surplus Shed is new to me and I plan to explore it soon.

Jremington brings an excellent point: yesterday I experimented with all kind of video camera lenses and I found that the image in the focal plane is quite big. It looks like I will have to deal with a 10mm diameter image. An avalanche photo diode of this size will be $1000+!

I am planning to use the following for my first prototype: UDT-555D (a now obsolete photo diode with a 11mm dia. window and built-in op amp ( http://www.osioptoelectronics.com/Libra ... .sflb.ashx ). What is your opinion regarding this device for my application?

As for the camera, I have quite a few and I may try them but I do not expect them to resolve the dot and I have no idea how to make use of the output of the camera to trigger a relay, for example.

[jremington]

I don't know anything about the big photodiode, but photomultiplier tubes have windows on the order of 1x1 cm, are very fast, can be had for less than US$ 10 (check eBay) and are not hard to use. However they do require a low-current but variable voltage supply of 700-1200 volts for operation. If you decide to experiment with a photomultiplier, never expose one to room light with the power on, or it will be destroyed.

There are some very inexpensive cameras that follow colored objects (called blobs) in the visual field and are popular with robotics hobbyists. See the CMUCam: http://www.cmucam.org/ SFE sells one: https://www.sparkfun.com/products/10032

Thanks for posting the link to the Andover filters. They are quite a bit cheaper than at Edmund Scientific, where I've bought bandpass filters in the past.

[waltr]

Try a silicon photodiode or even a photo-voltaic cell. These are much cheaper and since you really don't need high speed one of the transimpedance amp circuit (from the LASER comm link) should give you sensitivity you need. The link I listed is a really good read as much is explained and discussed about the circuits, and how to modify them for different characteristics.

You might want to try other lenses. The Surplus Shed is a good place to get optics cheap. Edmund Optical is a good place to read about various optics.
Good luck and please keep us posted on your progress as this sounds like an interesting project.

[Renate]

Well, I know that this is your invention and that you don't want to be too explicit, but...

The point is just to detect if the spot is anywhere on the card, not to track where it is?
If that's the case, modulate the laser beam, it will be easy to detect.

[metalist]

Photomultipliers... I am scared of vacuum tubes and high voltages. I need something simple, ideally with no more than 3 leads, I really have little practical experience assembling complex electronics. Same applies to modulated lasers, they are beyond my skill level. Besides I do not quite see how it would help: if a static beam is washed out in the 12 inch dia. field, same will apply to the modulated beam. I may be unclear on the concept.

I am quite skeptical about a PV panel to detect my small laser spot but I will play with this idea. But first I will explore the photo diode option, the one I described above.

And I will check how a camera sees the dot, this will be the easiest test of all but, again, I will not be able to process the video output to trigger a relay.

... Speaking of optical filters, the Schott Inc. is another high quality maker, as you undoubtedly know ( http://www.us.schott.com/advanced_optic ... index.html )

Thank you, my friends, for your advice and for your care. My device may at some point require an Adruino and this will be something new for me to learn. As the most ignorant member of this forum it is unlikely that I will be able to reciprocate and contribute, unless the topic will relate to something mechanical.

I think that within 2 weeks I will have some experimental results and if they are positive I will share them here.

Once again, I appreciate your help a lot.