The degree of diffusion of a ceiling panel affects several aspects of design, including illumination level. Typically, lower diffusion yields greater light transmission, but with more tendency to produce hotspots. Higher diffusion may have significantly lower light transmission, but will be able to eliminate hotspots more effectively at closer distances between light sources and ceiling plane.
Available height above ceiling plane should also be considered. While LEDs tend to produce directional lighting, their light does spread somewhat, increasing at a greater distance from the source. As with all light sources except lasers, LED light intensity varies as the inverse square of distance from the source. Placing LED strips or tubes further from the ceiling eliminates or reduces hotspots and can influence the needed degree of diffusion. If above-ceiling height is limited, higher diffusion levels may be necessary to produce even lighting without hotspots.
In situations with minimal above-ceiling height, LEDs can be placed close to the ceiling plane and inverted, where they shine onto the above structural ceiling or underside of the roof. Covering that upper surface with high light-reflectance material, such as white or aluminized insulating panels, reflects LED-generated light back downward onto the luminous ceiling while spreading it more evenly. To avoid shadowing, inverted LED strips can be placed over or adjacent to grid lines.
The ability of a luminous ceiling product to diffuse light and eliminate hotspots at a given distance can be tested relatively easily. One enterprising lighting designer obtained ceiling panel samples and built a “down and dirty” test chamber using a 609 x 609 mm (24 x 24 in.) cardboard box. He discovered the translucent panel he had obtained was able to diffuse LED light evenly with no visible hotspots, at as little as 152 mm (6 in.) distance. He had been expecting to use inverted LEDs reflecting off roof insulation panels, but his home-made test changed the plans. He had greater above-ceiling height available and was able to place the LEDs facing downward 508 mm (20 in.) from the ceiling. Compared to pointing LEDs upward, this increased illumination levels with a given density of LED sources.
Aesthetic effects of different diffusions are significant. Clearer materials reveal more of the above-ceiling area and its installed structures. Higher diffusion creates the effect of a surface that emanates light evenly all over the area and may mimic a skylight more effectively.
Light-transmitting panels vary in the degree of acoustic control they offer. Heavy panels will tend to reflect sound, whereas lightweight materials, such as thermoformed panels, can provide significant acoustic control up to a noise reduction coefficient (NRC) of 0.85, greater than many opaque mineral fiber acoustic ceilings.
In occupancies that require fire suppression sprinkler systems, plumbing installed closer to the ceiling than LEDs, including drops for penetrating sprinkler heads, can throw shadows onto panels. To eliminate penetrating sprinkler heads and plumbing shadows intruding into the luminous effect, translucent drop-out panels may be used. Drop-out ceilings were invented in the 1960’s for this specific purpose.
