I found graphs in a 1978 GE miniature lamp data book, giving life,
candlepower, and current as a function of lamp voltage. It has a bunch
of qualifications:

"Calculations of characteristics shown in Chart I are approximate only
between 95% and 110% of rated voltage for lamp types with 5,000 hours
life or less. Certain lamp types will vary widely from calculated
values. This chart will not apply to lamps with lives in excess of 5,000
hours. This chart does not apply to halogen cycle lamps."

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.

In the other direction, at 140% of rated voltage, you get about 120% of
rated current, about 325% normal brightness, and about 0.016 times
normal life.

"Double life" bulbs are popular. You can make any bulb into a "double
life" bulb (according to the graphs) by running it at 95% of normal
voltage. Current will drop 3 or 4 percent, and you'll get about 85% of
normal light output.

I'm sure more information is available on the web for anyone who's
interested.

Roy Lewallen, W7EL