# How can I create laser detecting flexible surface

What I'm trying do is creating a game like laser tag for larger areas, I want to detect laser beams on a flexible/wearable surface like a vest. That surface will be connected to an Arduino board.

So small photodiodes will won't work for me, because they are too small to hit from distance. I've read these: Using a solar panel for laser light detection, Detecting Multiple Lasers Also creating an array of diodes would not be so flexible.

But what I'm thinking is more like photosensitive mesh, like when hit by a laser creating a small current, so I can tell is it hit by a laser. Next step is detecting laser's color by the current's level. But this is not necessary, rather than that, modulation of the laser can be the better solution.

Simply what I trying to achieve is, a flexible, wearable surface that can tell is it hit by a laser. Source count does not matter.

• It's unclear what you're trying to achieve. Do you want to just know which laser source is hitting the detector or do you want to determine the wavelength(color) of the laser source. If you want to distinguish between a couple of different sources you'd be better off modulating the output of the diodes and using that to determine the source. If you're actually trying to determine what the wavelength of the source is you're basically after a small spectrometer which is no small task. – Mark Dec 18 '13 at 23:59
• @Mark What if you only care about distinguishing a few colors (say two or three), rather than a continuum? – Kaz Dec 19 '13 at 0:52
• @Kaz If you only wanted to distinguish between a few different sources a set of cheap filters might work. You'd need several photodiodes. Would need to make sure the transmission curve of the filters didn't overlap enough to cause problems but even cheap plastic filters like edmundoptics.com/optics/optical-filters/color-dichroic-filters/… might be ok if you carefully picked your sources and filters. That said you're not going to get much spectral information from but should be able to distinguish between a couple different sources. – Mark Dec 19 '13 at 1:01
• @Mark the different color lasers gives different scores, so I would like to determine color, source count does not matter. My main problem is this setup should be wearable. – siniradam Dec 19 '13 at 8:18
• Perhaps you can defocus the laser for a small receptor (i.e. wide beam + small receptor instead of narrow beam + wide receptor). – RedGrittyBrick Dec 19 '13 at 14:38

Assuming that the laser tag variant will be played in relative darkness, not in daylight, a thin film photovoltaic cell (TFPV) or thin film solar cell of requisite thickness and flexibility could serve as a wide-area photodetector.

TFPV cells are available that use very thin, yet tear-resistant polyimide substrate, which can be cut and sewn into, for instance, the top surface of a jacket or cap.

The challenge will be distinguishing a relatively small area exposed to the laser, as opposed to the overall photovoltaic surface being exposed to ambient light. Clearly, if the intensity of the laser is not significantly higher than the combined ambient light that may be incident on the entire surface of the clothing, this won't work. Unfortunately increasing the laser intensity leads to laser eye safety concerns.

Permissible eye-safe laser power is pretty low, typically single-digit milliwatts for visible lasers, and fractional milliwatts for IR or UV lasers - keep in mind that eye safety concerns apply even to brief accidental exposure to the laser from very close, such as if a player accidentally triggers the laser tag gun while looking into it.

The solution is to use a TFPV sheet that consists of a multitude of separate cells, rather than one large photosensing area, for instance:

(source)

The detection mechanism would need to sense individual PV cells within the mesh. The cell which produces an anomalously high voltage would be the one with a laser beam incident upon it - all the other cells would have lower baseline voltages representative of the ambient light upon each.

Color differentiation would not work for source identification, if using the TFPV approach: Consider a tiny red laser spot, a tiny green laser spot, and a larger surface area illuminated by the laser tag arena lighting: The PV cells would not be able to distinguish between these.

Another challenge with using TFPV material is that signal rise times are pretty slow - and slower for larger cell sizes. Using multiple smaller cells mitigates this somewhat.

Therefore, traditionally used signal modulation for distinguishing between different light sources, e.g. 38 KHz pulse modulation as used in infrared remotes, will not work. A much slower modulation frequency, and identification of distinct code sequences from different emitters, would be the way to go.

If you're using a simple photodiode without any filters you won't reliably be able to detect what wavelength (color) the laser is. What governs how much current a photodiode generates is the optical power absorbed by the active area of the photodiode and the responsivity (current/optical power at the appropriate wavelength) of the diode.

Theoretically you could shine a known amount of power at some wavelength, $P(\lambda)$, on a photodiode and based on the current generated you could roughly determine what the wavelength is. However, as you can see from the responsivity curve below, there are two wavelength solutions for a given responsivity. Also beam drift, source fluctuations, misalignment, etc. will all reduce the optical power incident on the photodiode to the point that it's not really usable as a makeshift spectrometer.

If you're attempting to differentiate between several different sources but don't care what the wavelength is you're better off modulating the diodes differently so that you can easily see which source is illuminating the detector.

• Thank you for your answer, I have to walk before run :) So before detecting colour of laser, creating a foldable/wearable surface for detecting laser beams is more important for me. Maybe instead of different color variations I can modulate the laser. I really don't know right now. – siniradam Dec 19 '13 at 15:01
• @siniradam I suggest you edit your question to narrow the scope of it. – Phil Frost Dec 19 '13 at 16:50