Energy refers to the work that is done by a physical object, in this case the light bulb. The energy is called light bulb equivalents (LEEs) and they include the time (t) that light takes to travel from an isolated lamp to the eye and between the eye and the outside world. The number of light bulbs (l) is given by l = 4/5*t*λ. The total energy (E) is given by E = 4h+w, where h and w are the distance between the eye and the outside world, and h and λ are the frequency (Hz), wavelength (λ) and power (W).
The energy density (e) of light bulbs is given by the equation: E = 4hxhxw, where x refers to the distance between the lamp and the light from within the lamp (for the sun) and h and w refer to the distance from the outside to the eye and the power (W). For a typical lamp, the e is about 5 W/m and the luminous intensity (LU) is about 1000 J/m2.
The term energy efficiency is often given as l e = 8/5*t*λ, with the latter being the number used for the energy efficiency of a lighting system. For a standard house light bulb, with an efficiency for energy used of about 15%, l e = 8/5*t*λ. It’s not clear which formula should be used at this point, but it should be the number at which energy efficiency decreases in proportion to the square of the power (W).
How much light do we need?
As the lamp is working over a short period of time, the amount of energy used for light is negligible, so the question of how much light is required does not involve any energy efficiency concerns. As explained before, the light bulb is a passive device, and the amount of energy lost during the bulb turns out to be negligible for the light bulb. The time it takes for the light to reach the eye and to hit the surface of the eye is just the time that the lamp is switched on and off. This is called the bulb power (P) or the period of use (POS) of a light bulb.
The energy required for the lamp to light the room is the amount of energy spent in switching on the lamp to begin with. For example, a lamp with a POS of 1 hour has a power/time of about
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