Thanks for the confirmation of 12th power.
It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.
At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.
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