Following from the previous post.

For indoor plants, when it comes to light, two things are generally most important: intensity and duration. To a lesser extent, ambience and hue have a part to play as well. The room or location you put your plants in can affect all these factors. I’ll consider them separately but they do not work in isolation; everything affects everything else wihen it comes to plants (and living organisms in general).

Intensity: basically, how bright is it? Intensity of light is measured in lumens (the old unit was foot-candles) and if you want to be really precise about it you can buy a photometer from any photography shop. Different plants need different intensities; some thrive in shade and some need “full sun” to thrive. Notice I said “thrive”; many, even most, plants will survive in lower than optimal light because they have little choice about where they land as seeds (in nature) and they must make do with what they can get or die. Having said that, bare survival is different from thriving. Bear in mind when assessing light for your indoor plants that plants perceive light differently from humans. After all, they need light. In short, a room that looks brightly lit to humans is often the equivalent of shade (outdoors) to a plant.

The brightest sort of light is direct light: direct sunshine or full exposure to an artificial light source. With artificial light, the closer the better; the strength of artificial light falls off very quickly as it moves further from the source – at least, as far as plants are concerned.

Reflected light, diffused light, or ambient light also contribute to the brightness provided by direct light. A room painted in light colours will reflect more light than a room painted in dark colours. Do not discount the effect this can have on a plant placed close to a wall. And remember that green wavelengths are not too useful to plants. Some plants do better when they’re in bright ambient light rather than in direct sun. A lot of orchids, for example, should be placed a few feet away from the window rather than right in it.

Duration: this should be self-explanatory. Sometimes a longer duration can make up for less-than-optimal intensity. The reverse isn’t necessarily true; while a plant might survive on sixteen daily hours of low light, it might not survive on four hours under a blazing halogen lamp. It’s not always as straightforward as that, however; some plants have specific reactions that depend on day length, night length, or relative day-to-night length, a phenomenon called photoperiodism. Getting poinsettias to turn red is a good example.