Thanks for the complements, Steve.
The answer is that the
RED (AlInGaP) 'truncated inverted pyramid' chip Luxeon III is a very different device to the
WHITE (InGaNP) Luxeon III. The RED device is more linear, much faster in rise time, and it can be driven to
much higher current densities than the WHITE Luxeon. Every doubling of current through the WHITE Luxeon, by contrast, produces much less than double the light output. It will therefore reproduce a picture with very low gamma, unless the WHITE Luxeon is placed in a non-linear driving circuit such as the one shown by Klaas Robers here:
http://www.taswegian.com/NBTV/forum/viewtopic.php?t=571
All WHITE Luxeons can be dimmed by varying their series current, but the dimming
will not be linear (this is probably the reason that Philips recommends the pulse-width dimming approach) and that probably applies to high-output standard WHITE LEDs more than it does to Luxeon sources.
For
very long distance (100km+) atmospheric communication, simple amplitude modulation of the Luxeon is, in general, to be preferred to PWM, FM or any other system of modulation requiring broad receiver bandwidths and/or receivers employing Schmitt trigger systems and a subcarrier demodulator. Refer the section of this page of Clint's for an explanation (fairly well down the page) under the heading "Why not FM subcarriers?":
http://www.modulatedlight.org/optical_c ... index.html
The elimination of atmospheric scintillation by optical, rather than electronic means is probably to be preferred. This can be done by increasing the comms beam diameter to straddle several atmospheric turbulence cells and 'aperture average'. It can also be done by using multiple beams and spatial diversity - easily expedited by wiring several transmitter Luxeons in series, and collimating them on the receiver with separate, preferably spaced Fresnel lens collimators. If, however, you're interested in pursuing the PWM approach - although we hardly found it worth the extra complexity - you can refer to Clint Turner's PWM design web page here:
http://www.modulatedlight.org/optical_c ... lator.html
The K2 Luxeon is
not quite capable of the same luminous output as the Luxeon Star or Luxeon Emitter. The Luxeon "emitter", lacking the metal-backed board of the "Star" can be crowded more closely onto a heatsink to provide a Luxeon bank for a bright scanning disc receiver source, but a large and efficient heatsink will be needed. These "emitters" should be glued to their backing heatsink with
heat-conductive epoxy of the automotive type - I think this is retailed in America under the trade name of "JB Weld" but there are probably similar proprietary heat-conductive epoxy adhesive products available in other countries.
Incidentally, an illustration of the improved performance of a large-area Luxeon beam over that of a laser may be obtained from this 3.7 MByte downloadable .mpeg clip from our San Jose presentation, which shows the instantaneous intensity of the beam-cross section after passing through a 15 km atmospheric path, on a 20-cm diameter optical receiver lens:
http://ka7oei.com/optical_comms/laser_p ... _2a1b1.mpg
At visual optical frequencies, it is impossible to maintain temporally and spatially coherent wave fronts over any appreciable distance through even moderately turbulent air. There is therefore virtually nothing to be gained by using visual lasers with links in excess of a few kms. Coherence and heterodyne detection are possible with IR beams, but the sources, detectors, and the optical heterodyne system is difficult of alignment and VERY expensive. Better - and cheaper - to use noncoherent Luxeon sources with cheap large-aperture molded Fresnel collimators.
I hope this expands on the subject - but be assured that we've found the Luxeons very tolerant of high-current abuse. The only one I ever blew was a result of me placing my multimeter across the Luxeon's current-limiting resistor. Usually I read the voltage across this resistor to calculate the Luxeon current. However I forgot, and connected the multimeter across the resistor while it was switched to its 10A range(! - duuhhhhh!) The red Luxeon III probably withstood a 4 Amp surge before it blew its internal gold connecting wire - it was quite a
spectacular flash, though also a depressingly expensive one!
Incidentally, we have found that the RED Luxeon III's can be linearly modulated around a standing current of 1 Amp, so they can be intermittently pushed upwards to about 1.7 Amp with impunity. The WHITE Luxeons are less sturdy, and slower in response.
Best wishes,
Chris Long VK3AML.