How long does a plant need its MDR of light?

[chopster01]

Lighting requirements still mystify me—the more I read, sometimes the more confused I get . I am especially confused about how the requirements listed for particular plants relate to the intensity/duration of a light source. For example, if a plant is listed as requiring 2000-3000 fc, how long must it receive the minimum 2000 fc to thrive? Four hours? Eight hours? Twelve hours? I have plants both in windows and under lights and the question seems pertinent to both, if for slightly different reasons.

I have two east facing windows. Depending on the time of day, I can get high readings of 3000 fc and even 5000 fc for short periods, which then fall off rapidly after noon to below 400 fc. If I take an average of light intensity to determine which plants I can grow, should I take it over four hours, eight hours, or twelve hours (or more) to determine if I can grow medium light varieties as well as Paphs, Phals, and an occasionally Encyclia? In other words, over how long of a period does a plant need to receive its minimum daily requirement of light?

The same question would apply to my western windows, which I am exploring the possibility of filling with plants as well—never to see the outside world again!

Fluorescents put a different spin on the same question. I have a bank of 4 T8 lights that I currently keep on for 16 hours a day. Depending on placement, plants could receive anywhere from 500 fc to around 1000 fc a couple of inches from the tubes. These are just the minimum requirements for Paphs and Phals, yet people on the boards claim they flower higher-light varieties like Tolumnias, Neofinetia, and even Oncidiums under T8s. So duration must have some sort of positive impact on intensity. How do you calculate that?

I am considering ordering a Hydrofarm four-lamp T5 fixture. Though less expensive on ebay, it is still considerably more expensive than the T8 setup. Yet one member claims he only gets 1800 fc two-three inches from his T5 setup. While this is slightly less than double the light put out by the T8 setup, it is hard for me to believe that the expense would be worth it unless the T5 lights enable you to grow higher-light plants than the 1800fc figure might indicate. And if duration does elevate that figure to higher levels, say to 2500fc so your oncidiums thrive, how much distance would be necessary so you can keep your lower light varieties from burning up (an important consideration when building a setup).

I know you are probably bone-tired answering these questions—over and over and over. But I haven’t quite got it yet, so please be patent with me and help me out if you can.

Thanks,
Charlie