Other factors which can influence the intensity of light penetrating a window are the presence of curtains, trees outside the window, weather, seasons of the year, shade from other buildings and the cleanliness of the window. Dark surfaces will decrease light intensity. Day-length or duration of light received by plants is also of some importance, but generally only to those house plants which are photosensitive. Most flowering house plants are indifferent to day-length. However, plants require some period of darkness to develop properly and thus should be illuminated for no more than 16 hours.
Excessive light is as harmful as too little light.
When a plant gets too much direct light, the leaves become pale, sometimes sunburn, turn brown, and die. Therefore, during the summer months, protect plants from too much direct sunlight. Additional lighting may be supplied by either incandescent or fluorescent lights. Incandescent lights produce a great deal of heat and are not very efficient users of electricity. For photosynthesis, plants require mostly blues and reds but for flowering, infrared light is also needed.