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oops, part of previous post got removed, sorry, here is the correct text.
hmm, it is not that much work putting together a simple a buck/boost or even variable pulse width multivibrator circuit to drive a LED from as low as 0.8V or so.
with care taken regarding component placement and using surface mount components it is even possible to squeeze inside the socket of a bulb (been there done that).
Among other things I modified several Maglite® penligghts to use red LED this way,
hardest part was setting the "lightlevel" so that it was more or less constant at intensity high enough to read by and yet low enough to save both battery and night vision.
also allowing it to work from 3.2-0.8V without overheating or fading is a challenge. i can not find the circuit i used for tha maglite modification, i remember that it used SMT components, hardest to find where the tiny (6x6x7mm) ferrite transformers
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