Daylight Optical Testing
That is, optical communications in the middle of the day...


Normally, one would do optical communications testing at night - and for some pretty obvious (and not-as-obvious) reasons:
Figure 1:
  A zoomed-in view of the distant transmitter located at the QTH of N0KGM at a distance of about 21.3km (13.25 miles.)
Bottom:  A wide-angle view of the above picture.  Both images have been contrast-enhanced to overcome some of the effects of haze.
Click on either picture for a larger version.
                    optical transmitter as viewed from a distance of
                    23km, across the Salt Lake valley.
Wide-angle view showing the distant optical
                    transmitter across the Salt Lake valley

With all of these in mind, it is not surprising that daytime optical communications is a bit "trickier" than that done at night.  It is also to be expected that the ultimate range would be reduced compared to that obtainable at night.

A few techniques:

In addition to the techniques applied for "normal" nighttime optical communications, there are a few other things that can help when daylight optical communications are attempted:
June 12, 2011 daytime optical experimentation:

It was during the "June VHF Contest" (50 MHz and "up") that we decided to try our hand at daytime optical communications.  At that time, the mountains near Salt Lake City were still full of snow and many of the higher-altitudes roads were still closed, so we settled for a fairly short "across the valley" shot of about 21.3km (13.25 miles) between the QTH of Robb, N0KGM and a location west of my house in West Jordan, Utah.  Because the geography is that of a "bowl" and since each of us were partway up our respective sides, we had a clear, line-of-sight shot, traversing across some heavily-populated portions of the Salt Lake valley.

For this contact, Ron went over to Robb's house with the optical gear and set it up on a west-facing deck and after a bit of fussing around trying to figure out where each other was located (I'd forgotten to bring a mirror!) we finally spotted each others' lights and completed the contact as can be heard in the audio clip below:
Figure 2:
A brief YouTube video clip showing what the distant light looked like from about 21.3 km (13.25 miles) distant.  Sorry about the rather poor video quality!

As is typical across the Salt Lake Valley, there was a bit of a thermal layer which caused a bit of scintillation (flickering) of the distant light and this can be heard in the above audio clip as slight, rapid fading.  Perhaps most dominant in the audio clip is the "hiss" - which is what the sun sounds like at optical frequencies!  It is this "hiss", caused by the extraneous reflected sunlight falling on everything within the field-of-view of the receiver and from the haze in the air that is the limiting factor for daylight operation.

The receivers used at each end were unmodified "Version 3" optical detectors, in front of each was placed a piece of red gel of the sort used in theatrical lighting.  In other testing it was determined that the use of this red gel improved the signal-noise ratio by about 6dB but it is expected that significant benefit would be obtained from more-selective filtering!  Testing was also done with my APD-based optical receivers and despite a bit of distortion caused by the amount of light driving the receiver's electronics into a nonlinear range, good results were obtained.

How did it sound?

As can be heard from the clips, signals are reasonably good!  A "current extinction" test was also carried out in which random words were said as the LED current was gradually reduced and it was noted that audio was still fairly easily copyable even when the LED current was reduced to less than 10% of the original value - a reduction of 20dB - indicating that we could more than double our distance with the current configuration (e.g. no receiver improvements):  If Morse were used, "naked ear" copy down to just 2-3% of the original LED current would have been possible and a "sound card" mode such as QRSS or one of the modes in the WSJT suite would have permitted, perhaps, another 10-15dB in reduction.  Interestingly, it was also at around the 10% level that the distant light became difficult to spot with the naked eye.

Although it is difficult to tell from the pictures, the light from the distant end when operating a maximum current was very obvious to the casual observer and of a striking, brilliant red color reminiscent of a strong specular reflection from, say, a mirror - except, of course, that it was red!

What's next?

Having gotten our feet wet with daytime optical communications, we went away having learned a number of things - namely those mentioned above - and we hope to try for even longer distances in the near future.

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If you have questions or comments concerning the contents of this page, or are interested in this circuit, feel free to contact me using the information at this URL.
Keywords:  Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, daylight, daylight optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector
This page and contents copyright 2011-2013 by Clint, KA7OEI.  Last update:  20130724
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