Suzanne,

This is a complex question with many variables as even the type of wood and how a fire is stacked and built can have a profound effect on how much light is emitted. And translating the foot candles into watts is tricky. Because the watt in a light bulb is a measure of electrical energy and does not translate well for luminance. I checked on this and 1 foot candle equals 0.0000015759751708 watts per Cm squared. Already too much math for me!

Your best bet is to build a fire, ask a staff member or friend who has a photo meter to take a reading off of a surface in the room that you want to illuminate (and yes, that can be a face!) - this should give you a semi-quantitative read on how much light energy you are looking at, based on photographic exposure parameters.

It depends on what sort of fixtures you are thinking of to replicate this period lighting effect -  but I would simply get a fixture and try several different bulbs until I approximate the "mood" - you will probably have to use incandescent bulbs if not more on the red side of the color temperature to approximate illumination by fire. Then once you have that dialed in then double check it with the same photo light meter and that should give you an approximate value to compare.

Cheers!
Dave

David Harvey
Conservator
Los Angeles, CA

PS - I once did a whole run of installing digital candle systems in 18th century chandeliers - a great resource for lighting in historic houses is still the 1999 APT Journal on the topic.