Houseplants that require direct sunlight should be set back 6 inches from the windowpane. North windows are also the coolest. Trees and shrubs, buildings, awnings, curtains and dirt on the window affect the quantity and quality of light that houseplants receive. Once out of the direct rays of sun, the level of light available to a plant drops drastically. Light-colored walls reflect light onto plants.
Dark walls absorb the light.
Houseplants may be classified by their light requirements. Follow this test to determine level of light in a particular location. Hold your hand 12 inches above a sheet of paper. If there is a definite shadow, the level is classified as high light. If there is a fuzzy shadow, the light level is medium.