LI-COR LI-6400, Manual

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Light Sensor Considerations
Gallium Arsenide Phosphide (GaAsP) Sensor
Using the LI-6400 8-7
Temperature Effects
The LED sourceÕs efficiency and spectral characteristics depend a bit on tem-
perature. At 50C, the efficiency will be about 75% of what it is at 0C. This
means an increase in the power required for a given output (and a reduced
maximum output) at higher temperatures. The spectral shift is strongest for
the red LEDs. Typically, the peak wavelength is shifted towards longer wave-
lengths by 7 or 8 nm when comparing 0C to 50C performance. The blue peak
shifts up by only 2 nm over those temperatures.
Aging
Like all light sources, the LED source is subject to aging. We have had a
6400-02 running continuously since November 1995, and as of this writing
(25 months later), its output has dropped by 30%: (10% the first 6 months,
10% the second 6 months, and the 10% over the past 12 months).
If you find that your light source cannot achieve high enough light levels, and
you suspect itÕs simply due to the age of the source, then there is a possible
remedy. See
Source IsnÕt Bright Enough on page 20-32.
Gallium Arsenide Phosphide (GaAsP) Sensor
The spectral response of a GaAsP sensor is shown in Figure 8-4 on page 8-8,
along with the spectral response of an ideal quantum sensor for comparison.
The GaAsP sensor spectral response is similar to the ideal quantum sensor,
but it begins to drop dramatically at 650nm, the slope from blue to red is gen-
erally steeper than ideal, and itÕs nonlinear. While this is certainly not an ideal
quantum response, corrections for spectra of sources commonly encountered
in photosynthesis work can be made, and are usually less than ± 15%.
Mixed light sources will cause complications, but if one light source predom-
inates, the appropriate value for that source can be used, or the GaAsP sensor
can be calibrated to the specific lighting conditions using a LI-COR quantum
sensor (
Generating a Calibration Correction on page 18-28).