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For consistence, weights must be less than or equal to one at all
times. When weight calculation is enabled by the likelihood
calculator, photons are prevented from being absorbed, and instead
their weight is decreased to reflect their probability of survival.
Once the photon's weight drops below a given threshold (0.01 for now),
the weighting procedure is stopped and the photon can be extinguished.
With weighting enabled, the detection efficiency of surfaces is also
applied to the weight, and the photon terminated with the DETECT bit
set in the history. This is not completely accurate, as a photon
could pass through the surface, reflect, and reintersect the surface
later (or some other PMT) and be detected. As a result, weighting
will slightly underestimate PMT efficiency compared to the true Monte
Carlo. This is not intrinsic to the weighting procedure, but only
comes about because of the inclusion of detection efficiency into the
weight.
Without the detection efficiency included, weighting cuts in half the
number of evaluations required to achieve a given likelihood
uncertainty (at least for the hit probabilities). Add in the
detection efficiency, and that factor becomes 1/5 or 1/6!
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