Sounds like using the walls and ceiling as a hood. Much of the light would continue to bounce between corners or even directly back to the source and to the ceiling again. And every time it bounces you lose a little depending on the properties of the wall/ceiling.
Not necessarily a bad thought though. Reflector hoods aren't just to simply reflect as much light as possible, but to create a fairly uniform and more rectangular footprint as well. That last thing also helps to get a more even spread in a rectangular grow space with reflective walls.
View attachment 3372626 View attachment 3372629
Second image shows why wide is better.
I use this one:
View attachment 3372633 Trade some additional loss of light by reflection more to create a more uniform spread. I only use one so run it on deep, but when using multiple one side (at the wall) can be set to deep, the other (where there's overlap with another light source) to wide. Leads to more uniformity, light from more different angles, better light penetration, higher yields. Being that led is more directional one could do something similar especially when using more than a couple of light sources, simply by raising/lowering/tilting some of the cobs.
Probably not the best way to go in general but I can see situations where a hood could work. Depending on space, amount of light sources and degrees of spread, foot print per cob, it could actually lead to better results pointing a couple of them upwards into a hood that reflects it downwards more uniformly. An extreme example would be a water cooled HPS bulb shaped (but 4-6 sides instead of round) heat sink with leds on its sides that can be used in HPS hood. Since you lose a little when reflecting, it's obviously less efficient lum/w wise but when it comes to gpw...
It's a trade off again. Uniformity can be taken too far too, especially if there's a cost, which there would be from reflecting, and especially if you don't run a SoG with mini plants. I mean at some point it's uniform enough to divide the light fairly evenly over the entire surface, and then the uniformity on a particular sqft of space isn't that important as long as it falls within the desired range (and no hotspots). The higher the output per cob, and the lower the degrees of spread, the more sense it will become to use a hood. HPS needs a hood to combine the light into a beam, one high intensity LED, could possibly benefit from a hood to spread out that light. Not at that point yet, but when you put multiple cobs (
http://www.ledhorticulture.com/the-end-of-led-grow-lights/ ) close together you end up with a similar situation and hence you lose one of the benefits of a typical led setup (many light sources so easier to get a uniform spread without using hoods that reduce lum efficiency).
