If you want to really know the light levels in your growing areas, you should consider purchasing a decent light meter. However, most orchid growers love to take photographs of their flowers, and that may provide a reasonable alternative. If you’re one of those, and have a camera with a built-in light meter – most single-lens reflex (SLR) cameras do – then you can use it for measuring light intensity, too.
You only need three things: your camera, a piece of white paper, and the calculator below.
Simply put the paper in the location you wish to measure, facing it as close to perpendicular to the light source as possible, and point the camera at it so it fills the frame completely – trying to shoot as close to perpendicular to the paper as possible without creating a shadow. Adjust the aperture so that you get a decent shutter speed, and then use the ISO setting and those values in the calculator to determine the foot-candles of light intensity. (Note that these numbers are not exact, but certainly give a “ballpark” figure to work with.)