If we want to check how is the light density drop with distance due the spreading of light, its a must to do it on a black painted area to eliminate reflection, and of course, with nothing blocking light.
On the other hand, measuring with the reflective walls we use is the best to have a good idea of actual atenuation with distance, due the loss at reflection.
But always be aware than measuring just on a line, directly below the light source may be tricky. We are measuring local irradiance here, only valid for that single point at which we measure it. If we measure just along the optical axis, for sure a narrow optic will get a lower drop with distance, because its intensity is higher, but coverage, less. The higher reading on the optical axis is compensated for the lower reading off axis: lighting is more uneven.
But as I explained, this happen because you are measuring results from just a single panel. So light spreads from it, but on a conical pattern, so light density on the axis falls at lower rate than off it. But if you put 4 panels as that on the 4 corners, you realize that past the distance at which beams joins, drop of light density is way lower, as light going out the beam is compensated from light from the other panels going into it, producing an even lighting. It happen the same if instead of panels, we place LEDs along all the top area lighting downwards.
So, what are the benefits of narrow optics (beam angle) and wide ones?
Narrow optic:
Pros:
-Lower light losses (less light goes to side walls)
-Light more focused, being able to send more light through a hole in canopy of a given size
-Work very well when placed at some distance of plants (typically, more than 1ft)
Cons:
-More uneven lighting. In the case of using LEDs separated to the next instead of crowded panels, difficulties mixing light spectrum.
-Dont work fine at short distances. Need some distance to get light distributed evenly, and, more as narrower optics used, exist the risk of excessive local irradiances when plants are too close. But in general, they can be used at short distances without deleterious effects, apart of reduced efficacy.
-Shadowing may be an issue.
In general, narrow optics is a good idea when k and/or LAI are low (low k:leaves mostly in angle, not horizontal; low LAI: leaves not too abundant, there is enough holes in the canopy) as it allows to use just top lighting.
So light losses are lower (good), but lighting obtained, less even (bad).
Wide optics:
Pros:
-Very even lighting with excelent spectrum mix still at short distances. Increased efficacy of the light, especially at short distance of the lamp.
-Produces a kind of diffuse lighting: light comes from almost all directions, being able to enter any hole in the canopy, independent of leave's angle
-Shadowing is never a problem.
Cons:
-More light loss due more photons goes to the side walls. Worse as larger is the distance from lamp to plants and walls less reflective.
So this option only have a drawback, but its an important one. I choosed to go this way and use very reflective walls to overcome the problem, as apart of it, IMHO, wide optics are way more versatile. I tend to use side lighting always that is possible, and for side lighting for sure that narrow optics arnt a good idea.
Wide beam angles are very useful on side lighting and where k is high, as there is little shadowing and light enters for all the holes in the canopy, "looking" for them on a 3D fashion, and not by brute force vertically. Two leaves separated 1" vertically may block fully space looking from top, but have a wide hole between them. Wide optics works better here.
But I admit than 90Âş beam angles are generally enough to get the advantages of "wide optics". I use larger ones simply because LEDs I like for efficiency/price arnt avalaible on it. If I could get good LEDs with 90Âş optics, I would use them. Using secondary lens on larger beam angle LEDs is not a good solution because they have their own losses.
My ideal LED setup would use 90-120Âş beam angles on the center, and 30-45Âş ones on the edges, in order to get the lowest light losses but the more even lighting and more diffuse light. And with LEDs distributed along all the grow area, not all in a central panel.
On the other hand, on side lighting, 120-170Âş beam angles are ideal. The wider the better in this case.
I am working with optical films, that may overcome all the cons for a relativelly low cost in light. Just changing the curvature of the film, it is possible to get a very wide beam angle and a colimated column of light with high intensity from the same LED string.
The debate between brick panels and LEDs evenly distributed along all the grow area is very similar to narrow/wide optics, but both pros and cons are very extreme. While narrow optics on LEDs result on a more uneven light distribution a short distance, if LEDs are not together in the top center of the grow area, at 1ft from the lamp or less there is already an even lighting. But when all LEDs are in the center, it happen very similar to HIDs, that need relatively long distance to lit effectively all the space, and always producing a very uneven lighting that seriosly drop efficacy of the light.
Using multiples sources of light is all advantages, both if considered for single LEDs or full panels: better 2 panels of 50W instead of 1 of 100, and 4 of 25 better than 2 of 50. There is disminishing results here, past a number of light sources there is little gain, but from 1 to 2 or 4, there is a very noticiable difference. Light distribution is always way evenly this way, and evenly lighting result on increased efficacy. On the other hand below a brick panel penetration is increased because intensity is higher, but it only happen below the lamp, while at the sides, penetration is lower, because the light intensity does.
Hempyguy, I dont like rules at all. They only exist to be broken
And I have noticed along the years on the growing boards that there is many different types of growers with different priorities. Its almost impossible to find rules that fits all on almost any thing related to growing. If we narrow the conditions of our "rules", as for example, efficacy of light being target nÂş1, I would rephrase rule as:
"Use wide angle LED on the center, and narrow on the sides (except if your lamps is going to work always at more of 1 1/2ft of canopy, in which case use narrow optics, and very narrow on sides)."
Or better yet, change the emphasis to:
"If your plants are taller than 2 ft, use side or intracanopy lighting"