Consider the room to be a 15 feet X 15 feet square with 10 feet high ceiling. The four windows are centrally placed in the walls of size 3 feet X 4 feet, and all are exposed to sunlight.
Depends on how the sun-light shines (its direction with respect to the room) and what the interior walls of the room consist of. If the interior walls are perfectly reflecting, with the assumption that the CFL radiates outwards isotropically, by opening the windows a fraction of the radiation energy of the CFL equal to the ratio of the total surface of the windows to the total interior surface of the room (with those of the windows included) will leave the room. If the light shines directly from the sun and perpendicularly to one of the widows, most of the incoming radiation of the sun's light will leave from the opposite window; some of it will be diffracted at the edges of the windows in question and it is the radiation of this diffracted light that will reflect on the walls, and with the windows open, a fraction of it leaves the room through the windows. In short, to give a precise answer to the question, one will have to know more about the details of the surface area of the room, the direction of the incoming sun light, etc. Incidentally, it is also important to know where the room is located; is it suspended from a long rope, far from any external surface, or is it somewhere amongst many buildings? In the latter case, a considerable fraction of the radiation energy of the sun's light is in the diffracted light, as distinct from the light directly arriving from the sun.
Sun light entering the windows will vary from morning to evening in the same day.Moreover, the seasonal variation are in the sun light intensity even on the sunlit day. In addition to these factors, whether the room is isolated from other buidings or not is also important. Therefore, the light intensity entering the windows will depend on various parameters and the intensity of CFL is same under all the situations. Hence, the exact answer can not be obtained for this question.
I appreciate the response from Behnam Farid and A. Kumar.
Reflection will certainly play an important role.
The actual room has off-white (oil-paint) walls and ceiling and off-white floor of tiles. This is a lower floor corner (south-west) room in a house of two floors. The south and west sides are completely open, and the sunlight enters the room directly through the windows. The floor is very reflecting, such that there is strongly reflected sunlight on the walls.
Will interference between sunlight and CFL not play an important role?
I plan to take measurements with a solar cell meter to have some quantitative data.
You are welcome Samares. Since you mention that "there is strongly reflected sunlight on the walls", I think it is certain that opening the windows (assumed to be fully non-transparent) will lead to more light. To appreciate this, consider the attached diagram of the spectrum of the solar radiation. Limiting oneself to the visible part of the spectrum, taking account of the atmospheric absorption, with the assumption that the spectrum were flat in the visible part, one has approximately 1.25 W/m2/nm times 300 nm (the width of the visible part of the spectrum), which is equal to 375 W/m2, making for 375 Wh per 1m of area inside the room covered by direct sunlight. This is some 10 times larger than the total radiated energy of the CFL at hand in 1h. Of course, you do not mention what area of the walls are covered by sunlight. My implicit assumption is that the total area is of the order of 1 m2. As for the interference, the lights of both CFL and the sun as reaches us are incoherent, so that interference cannot be significant.
Incidentally, what is known as the solar constant, equal to 1.362 kW/m2, corresponds to the total electromagnetic spectrum of the sun outside Earth's atmosphere. The number 375 W/m2 that I have given above corresponds to the visible part of the spectrum and on the surface of the Earth (at 'sea level'). My value is reasonable, since solar panels of area 1x2 m2 are quoted as having an output of between 75 and 350 Wh; taking account of the efficiency of around 20% of these panels, which gain energy over a wider spectrum of the sunlight than the visible part, one obtains between 375 and 1750 Wh. The lower bound miraculously coincides with the value that I have presented above.
http://en.wikipedia.org/wiki/Sunlight#mediaviewer/File:Solar_spectrum_en.svg
http://en.wikipedia.org/wiki/Solar_constant
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