When outfitting a growing area, we typically think about plant placement, temperature control, humidity, air movement and lighting. If using artificial lighting, we focus on details like color temperature, wattage, lumens or photon flux. We often forget, however, to maximize our lighting efficiency by using the growing area walls as reflectors.
You will hear folks recommend highly reflective surfaces like silvered mylar “space blankets”, or foil-faced, foam insulation board, but in reality, pure flat-, or eggshell white surfaces are the best option. To understand that better, let’s consider room geometry and how surfaces reflect.
Typically, we place our plants on benches or stands that are elevated. Some of the light entering the area hits the plants directly, but a lot is projected elsewhere. Getting the other surfaces to reflect it back to the plants can be a big advantage.
With a mirrored, or “specular” reflective surface, shown on the left in the graphic above, the incident light rays (green) are reflected (blue) directly to the floor, at the same angle at which they hit the surface. As the plants are elevated, those rays never reach them. If we use a mirror with a “crinkled” surface, we have in effect, turned a flat mirror into a multi-faceted one (middle), providing small mirrors facing in many directions, so it is an improvement. “Silvered” paint is an extreme case of multi-faceting, but does not reflect 100% of the light that hits it, so is a step backwards from our goal. Flat, white paint, on the other hand, scatters all wavelengths in all directions (image on the right), ensuring that the maximum amount of light fills the volume and reaches the plants.
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