I just had a friend bring up a similar idea though (Alec t). If I were to use a broader spectrum light, I could detect the signature of water reflecting. It is another avenue, but worth looking into. The only problem I see is having a broad spectrum light beam that is intense enough to reflect off a water droplet. That was the whole reason I wanted to go with a laser.

 

Can the difference in time be detected from the reflection? That seems almost impossible. Is that what you are saying or do I not understand?

 

Is your question directed to my laser-panning idea? If so, no, timing is not the issue. The panning will cause the laser to hit anything in its plane, possibly repeatedly if the droplet is hanging there long enough to be hit by multiple swings of the panning laser. This would cause a flash or repeating flashes of light.

It's common to additionally modulate the transmitted beam so that all ambient light that is not at the modulation frequency is screened out at the receiver. This hugely increases the signal-to-noise ratio and allows you to crank up the sensitivity of the receiver.

So whether the room is dark or not, if anything that can scatter light enters the beam plane, it'll get detected.

 

I see now. I guess that leaves me with trying to find a very sensitive receiver. Is there such a thing as modulating a single frequency? Maybe that would be the wrong term, but I am thinking about "mapping" the area around the drop so any inherent, background reflection of the same frequency is also cancelled out.

 

Not sure I get your question. The light is in a narrow spectrum of wavelengths, all at very high frequencies. Your detector must be capable of detecting that wavelength and colored filters are often used to reduce or eliminate other colors of light from confusing the sensor.

Modulating the light means varying the intensity of the light at much lower frequency than the carrier, the light. So for instance a TV remote flashes its IR LEDs at 80kHz. The TV filters out visible light with a piece of dark plastic (that passes the IR signal) and the IR detector behind the filter further filters out any signal that is not pulsing at ~80kHz. The digital signal being transmitted is at a still lower frequency, and the receiver reconstructs the digital pulses. These techniques allow the TV to sense a dim LED all the way across a brightly lit room.

 

If you can get light into the drop, I believe that it will act as a retroreflector. My optics knowledge is not good enough to predict what the acceptance angle will be for reflectance.

http://en.wikipedia.org/wiki/Retroreflector

 

OK, Yes, I see what you are saying about modulation (wayneh). But I am trying to figure out how the light would be detected, modulated or not, if it were reflected. The reflection would be immediate. There would be no way to tell that the reflection was not at *khz. I guess I do not understand that if a light was emitted at 80khz, and was detected by a device, it would be anything different than 80khz? Or are you just saying that the detection device would simply be looking for an 80khz signal that would be reflected back? I may have been looking to far into your initial comment since my mind is trying to think of a lot of alternative possibilities right now.

 

I looked up your link RicharO and I think the retroreflector would work if I had a constant, defined place for the water, which is what I am looking for right now, but would not help me in the future since I am trying to make this work in a plane eventually.

 

Have a rotating polygon mirror scan a plane with a laser beam modulated at some high frequency.
90 degrees off-axis, have some high-gain optics (big lens) gather the scattered light, synchronously demodulate the resulting optical signal. (lock-in amp)

This would be my first stab at a viable solution.

 

That sounds like where I need to go. I have a lot of learning to do.

 

Yup, that's what I'm visualizing. I'm not so sure a lens will be needed. I picture myself in a dark room with this. Would I see a flash when the laser hits a droplet? I think so. That means a light sensor will see a flash too.

 

It'd be easier to just set up an experiment. Handheld laser pointer, dripping faucet.

I second that!
Nothing like hands-on to get a reality check.

In a quick test in a moderately lit bathroom, there's ample reflection from a drop with a cheap pointer - the power of the pointer will determine the amount of course, but it should be quite easy to detect.

If modulation is used, go with at least 38kHz, as the time of reflection will be less than 2ms - perhaps try an integrated IR receiver, it might work with a red LASER on such short range (I have used a red LED in a remote to fire my camera on short range).

 

I have thought about the IR receiver, but as you brought up, I think it would be better in a shorter range. I want to be able to detect as far as possible. But I have recently been in the yard at night with a dripping hose and a green laser, and there is no doubt, you can see the water.