What is the effective lit area from a 40W light bulb?

Question

What is the effective lit area from a 40W light bulb?

e.g. 40W light bulb effectively lights 1 Square Metre

Answer 1

What is the definition of "effectively lit"? There has to be some level at which an area is considered lit to even begin to answer the question....

Answer 2

The effective lit area from a 40W light bulb is about 8 feet.

Here's a quote from a 1998 article in the "FBI Law Enforcement Bulletin"

"This research study demonstrates that police officers have difficulty differentiating between lethal and nonlethal objects illuminated by less than .50 foot-candles of light, the level frequently encountered during routine police working conditions at night"

The effective lighting level required [cited in above article] is that given off by a 1/2 Candela light source received at 1 foot away.

A GE 40 Watt light bulb is rated at 445 lumens.

"A light source that uniformly radiates one candela in all directions radiates a total of 4π lumens"

445 lumens is about equivalent to 35 Candela (445/(4*pi))

Therefore a 40 Watt light bulb is 35 times brighter than one Candela or 70 times brighter than 1/2 Candela.

The surface area of sphere around a light source increases as R^2.  A light source 70 times brighter than 1/2 Candela can shed the same light as the 1/2 Candela light source a distance of sqrt(70) or about 8.3 feet away.

All of this of course assumes that it's a completely otherwise dark environment and that light is directed uniformly in all directions from the light bulb (i.e. no mirrors, painted walls, moonlight, etc.)

Answer 3

and it depends on the type of bulb and its fitting