the HYSAL treatment for 30 days were 199 moles/m^2 , being 60.6% solar and 39.4% artificial. The HPS
reference was made to receive the same daily moles of photons as in the HYSAL treatment throughout
the growth period, resulting in both HPS reference and HYSAL treatments having the same total number
of moles (199 moles/m^2 ) at the end of the growth period. Over the entire growth period, the HPS refer-
ence had an average instantaneous PPF of 194 mol m^2 sec^1 and an average daily photoperiod of 9.5
hr. The resulting average total dry weight per plant for the HYSAL treatment of 1.37 0.38 g exceeded
significantly by 76% (0.05) that for the HPS reference of only 0.78 0.17 g. This significant dis-
crepancy could be explained physiologically by the HPS reference having both a significantly longer dark
period and a higher light compensation point than the HYSAL treatment. Whereas the HYSAL treatment
had no dark period at all, the HPS reference had 14.5 hr of dark period each day or a total of 435 hr (18.1
days) over the 30-day growth period. The resulting average light compensation point for the HYSAL
treatment of 55.3 mol m^2 sec^1 was also significantly lower (0.05) than the average LCP for the
HPS reference of 169.1 mol m^2 sec^1. Further experimentation showed that it was indeed the com-
posite lighting profile of the HYSAL treatment, not the light quality factor, that caused the biomass dis-
crepancy. In the same experiment, Cuello et al. [5] also successfully demonstrated that, with respect to
crop response, a simultaneous composite lighting profile (as that shown in Figure 2A) was significantly
indistinguishable from an alternating composite lighting profile (as that depicted in Figure 2B). Although
needing further study, these results provide strong preliminary evidence that composite lighting could be
employed in growth-chamber settings as a practical strategy to optimize crop performance at a given to-
tal integrated PPF.
V. DESIGN OF COMPOSITE PROFILES
The practical usefulness of composite lighting is that it makes possible the optimization of a given crop’s
performance by making allowance for its daily photoperiod to be extended as much as permissible with
respect to the crop and for its average instantaneous PPF to be maintained at a desired relatively low level
even when the daily integrated PPF is raised significantly. This important flexibility is simply lacking in
conventional lighting. For instance, given a conventional-lighting case where the instantaneous PPF is set
at 100 mol m^2 sec^1 and the daily photoperiod is stretched to the maximum possible value of 24 hr,
the resulting daily integrated PPF is 8.64 mol m^2 , which is the maximum daily integrated PPF corre-
sponding to the set instantaneous PPF of 100 mol m^2 sec^1. This means that conventional lighting can
only be used for implementing an instantaneous PPF of 100 mol m^2 sec^1 if, and only if, the specified
daily integrated PPF does not exceed 8.64 mol m^2. Conversely, this means that conventional lighting
cannot be employed to implement an instantaneous PPF of 100 mol m^2 sec^1 when the specified daily
integrated PPF exceeds 8.64 mol m^2.
Considering a second conventional-lighting case where the instantaneous PPF is set at 100 mol
m^2 sec^1 and the daily photoperiod is set at 16 hr, yielding a daily integrated PPF of 5.76 mol m^2 , there
are only three possible ways by which the daily integrated PPF can be raised significantly above 5.76 mol
m^2. First is by keeping the instantaneous PPF at 100 mol m^2 sec^1 while lengthening the photope-
riod beyond 16 hr, which may not be an issue for some crops but may be for others. Second is by keep-
ing the photoperiod at 16 hr but raising the instantaneous PPF above 100 mol m^2 sec^1 , with the likely
undesirable consequence of raising the crop’s light compensation point. And third is by both lengthening
the photoperiod beyond 16 hr and raising the instantaneous PPF above 100 mol m^2 sec^1. Composite
lighting circumvents this difficulty by providing for the flexibility of raising the daily integrated PPF sig-
nificantly without changing either the effective daily photoperiod or the average instantaneous PPF.
Thus, given the desired values for the daily integrated PPF (Q), average instantaneous PPF (PPFave),
and the long photoperiod (P 1 ), the composite lighting profile is designed as follows. Expressing the long
photoperiod as a multiple of the short photoperiod,
P 1 kP 2 (4)whereP 1 is the long photoperiod, P 2 is the short photoperiod, and kis a constant. Similarly, expressing
the low instantaneous PPF as a fraction of the high instantaneous PPF, then,
PPF 1 mPPF 2 (5)COMPOSITE LIGHTING FOR PLANT FACTORIES 921