Diminishing Returns
When indoor growers think of diminishing returns it's usually in terms of what more light will do for the weight of their yield. To put the above definition in our context.... A point beyond which the application of more light intensity returns less than a proportional increase in yield. For example, when a space's light intensity is increased 20% and returns 20% more yield weight, the point of diminishing return had not yet been reached. But had the return been just 15%, the point would have been reached and returns diminished. This shouldn't be confused with a point of negative return, total yield weight didn't actually decrease (it did rise 15% after all), it's just that the weight increase wasn't proportional to the light increase.

Light intensity can be increased by adding light to an existing space, or by removing space from an existing light. Whether or not returns will diminish depends on what the intensity was before the change. How much returns diminish depends on how close the canopy was to being light-saturated. But when speaking of canopy in terms of light saturation, we have to look seriously at the entire three dimensional canopy space and not just the top of the canopy. This becomes easier to conceptualize if we look at the plant's parts and the thousands of different addresses at which they live and grow within the canopy. Under artificial light, some parts can be light-saturated while at the same time others won't produce acceptable growth without having more light to compensate.

We also have to make a clear distinction between the most light intensity a cannabis plant can tolerate, and the most light intensity it can use. Cannabis is a hardy full sun plant that can tolerate a lot more than it can use, we know it can tolerate up to 10,000 fc at the top of its canopy in the sweet spot, but we also know there will be less light reaching beneath the top.

Sun plants have a high optimum light intensity, meaning that compared to other green plants with lower light requirements sun plants can use higher light intensities before the point is reached where growth energy will no longer increase with more light. They also have a low light intensity point at which growth energy produced by light is just enough to sustain the growth they already have.

Because photosynthesis manufactures food while respiration uses food to provide energy for growth, the light saturation point speaks to the most food a plant can manufacture in order to produce new growth, while the light compensation point speaks to the least food a plant must manufacture in order to sustain old growth. All points in between result in various rates of new growth, from very fast - when light levels are nearer the saturation point, to very slow - when they're nearer the compensation point. These points are species dependent however. Even among sun plants, different types will have different saturation and compensation points.

This brings us back to the point of diminishing return. Knowing that plants have a limit on how much light intensity they can make use of, it stands to reason that a strongly lit garden has less to gain from adding more light than a weakly lit one. The light saturation point for most high energy sun plants is around 4000-5000 fc. Research on the ever so popular cannabis stand-in, the tomato plant, puts its light saturation point at 6510 fc (Tatsumi & Hori, 1969).

I could not find any evidence that cannabis has been the subject of this sort of costly research. In fact, many well known cannabis authors draw analogies from research performed on other plants because cannabis-specific studies are unavailable. Given that many nations classify cannabis as a noxious, poisonous or a culturally insignificant weed, the odds that production related studies on drug cannabis will be funded are low. Nevertheless, and being cautious of possible analogous connections, some cannabis authors have published lighting guidelines (not saturation/compensation points per se) stating that flowering cannabis does well at 3000-5000 fc. However, one does allude to a saturation point when it says cannabis "can use up to 3000 fc efficiently". But given the known saturation points for tomato and most sun plants, and until such information becomes available for flowering cannabis, it seems reasonable to put its saturation point somewhere between 3000-6500 fc, with 5000 fc being a likely candidate.

Maximizing versus Optimizing
When we see references in gardening literature to optimum light intensity they're usually based on light saturation points (when that data is available), with the implied restriction being diminished returns on total yield. Depending on the source, many of these references give a single fc value, and many assume the user will be applying the value to plants grown under natural sunlight where light intensity at the bottom of the plant is virtually the same as that at the top. Under artificial lighting that is not the case. While optimum intensity may be available at the canopy top under artificial lighting, the inverse square law insures that anything below that point will see a dramatic reduction in light intensity as distance from the lamp progressively increases.

Tolerance versus Saturation
In order to saturate as much of the canopy interior as possible and not just the top, indoor growers often maximize light at the top of their canopy by supplying as much light as the plant can tolerate (9000-10000 fc) to insure that areas below the top are optimized (saturated at 5000 fc for example). Plant parts at the top of the canopy will only use up to 5000 fc even though they're receiving more, but some areas under the top are now receiving optimum saturation where they weren't before the canopy top was maximized. This struggle to keep as much of the canopy space optimized to the saturation point is why many growers try to keep their canopy depths to a minimum. And, as mentioned previously, the exact depth will depend on the intensity/wattage of the lamp being used and the growth density dictated by one's growing style.

Diminishing returns, regardless of whether it applies to total yield or yield per square foot, can be offset to some extent by lamp placement and by closely cropping the sides, and even the bottom, of a grow space with reflective materials to improve growth density. The art of growing, as opposed to the mechanics, is to produce acceptable results in both density and total yield. When left with limited lighting resources a grower will play these two points of contention against one another by opting for the better yield per square foot values that come from a smaller but more intensely lit space, or opting for the higher total yields that come from a larger but less intensely lit space. These balancing acts can cause an unwary beginner to spend several crops coming to terms with the yield he needs and the esthetics he wants.