Under those few special conditions when we are treated to the sight of a rainbow, we see the best way to explain that light is made of visible and invisible colors.  PAR or Photosynthetically Available Radiation, refers to the light spectrum that is most useful to plants. Window light is the most practical, as it is full spectrum and costs nothing.  Most energy efficient window glazing reduces ultra violet radiation which is OK as plants do not use or can be harmed by too much UV.

Factors influencing window lighting include internal obstructions such as shades or blinds. A blind can be adjusted in such a manner as to allow light for the plant while creating comfort for the people. A horizontal blind with the blades canted at an angle up towards the outside can provide enough light for the plants at the window, while the opposite adjustment may be too little for the plant to survive.

               Author Ensuring Plant Has Sufficient Light

External obstructions such as buildings and awnings are easily recognized; however, deciduous shade trees would only have an influence on interior light during peak foliar density such as summer. Window direction is often missed because you have to be standing in front of it at the right time.

For example, if you chose a plant based upon available window light at 10 a.m. you will not know that the western exposure is hot and sunny in the afternoon. Solve that problem with a simple compass. As distance from the window increases, light intensity diminishes. Twenty feet from a standard western window, otherwise considered bright light, is now medium. Photo period refers to the duration of light and is often even more important than bright light for less time.

Until recently, fluorescent lighting had less output for its size, but today compact and T 5 bulbs can deliver full spectrum value and are suitable for many office plants if placed directly under the fixture. Incandescent bulbs help supplement natural lighting but have a low output when compared to modern fluorescent. Ultimately HID or high intensity discharge lighting which includes Metal Halide (MH) and High Pressure Sodium (HPS) preferably in combination, provide the best of artificial light to plants from a spectral and efficiency standpoint.  For example, it would require ten 4-foot T 5 fluorescent bulbs to equal a single 400 watt HPS HID and eighty-four 60 watt incandescent bulbs to equal the same!  

Because of fixture size and design, HID lights are not as popular for the average office application as compact or T 5 fluorescent and are most often used in building lobbies and atriums. Fortunately there is a plant suited for all but the lowest of building light. Your professional interiorscaper should be able to help you make the right selection.