LED light source

Richard B. Peterson 72103.765 at CompuServe.COM
Sun May 23 18:07:52 EST 1993

	I am interested in putting together a high intensity light
source comprised of light emitting diodes (LEDs) for use in
photosynthetic measurements.  Why LEDs?  LEDs are supposed to
be highly efficient sources of light and the emission can be
controlled by varying the driving current.  Hewlett-Packard
sells a high intensity LED (15 candelas) with a peak emission
wavelength of 660 nm and very little output above 700 nm, almost
ideal for photosynthesis work.  Most important, the time constant
of response for these LEDs is in the nanosecond range which is
desirable if one wants to examine light-on and light-off 
transients in Chl fluorescence and/or in vivo absorbance in
	The problem arises due to the relatively low power output
of an individual LED.  I would like a maximum intensity (PAR)
at the leaf surface which is equivalent to full sunlight. This
is about 400 mW over a 10 sq cm area. Based on some 
preliminary measurements I estimate that the necessary maximum
flux could be acheived if I could collect light from 50-100
LEDs.  Has anyone attempted anything like this using, perhaps,
integrating sphere physics?  Alternatively, are there any
North American companies which specialize in fiberoptic
coupling to LEDs? The latter idea is based on collecting the
light from an individual LED into an optical fiber then 
bundling all the fibers.

Richard B. Peterson   New Haven, CT USA

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