600 watt distance

Available Light
Available light is the lumens available from your lamp to be distributed across your plant's canopy. It is not the light striking your canopy, which is measured in foot-candles (fc) at the canopy. Available light is a measure of lumens at the source, while foot-candles is a measure of lumens at the target. Between these two points light is lost to empty space or objects that don't reflect light, for the production minded indoor gardener that translates into lost growth and yield. Lamp-to-canopy distance and your choice of reflectivity preferences will ultimately determine the light your plants receive.

Because cannabis is a high energy sun plant most indoor cannabis gardens have the available light very tightly focused, more so than gardens used for conventional indoor produce (vegetables, herbs, etc). As a result, indoor fc levels approaching that of the most intense sunlight can be reached or exceeded by growers, but can damage plants if heat from the lamp doesn't do more damage first. The most light intensity plants are likely to see in nature is around 10,000 fc, and for only a couple of hours during the day while the sun is directly overhead. For artificially lit plants, above 9,000 fc is considered shaky ground by many indoor growers, either due to the potential for damaged growth or for the more watchful eye needed to prevent it.

The YOR uses Available Lumens per Square Foot as a measure for the average available light distributed across a space. While this does address the space's average lighting, it does not reflect the closest safe-point when dealing with real lamp-to-canopy distances, nor does it reflect the furthest distance at which light can be used effectively.

Distance and the Sweet Spot
Every top-lit indoor garden using artificial HID lighting has a sweet spot located directly under the lamp, where it's closest to the canopy. If not accounted for in your plans to distribute available light, fc in the sweet spot can far exceed that of the sun, especially when lamp cooling technologies are used to reduce normal distances limited by heat. Finding the limits of your coverage area implies a compromise between the minimum light levels at the outside edges of your area (furthest from the lamp) and the maximum light level at the sweet spot (closest safe-point to the lamp). The happy medium is not to overdose the sweet spot with too much light (or heat) and to distribute enough light to the outside edges of your grow space for acceptable growth. The controlling factor for both of these limits is the height of the lamp above the canopy. Reflective surfaces surrounding your garden will help reduce light losses at the outside edges, but light at and below the sweet spot is controlled by distance between it and the lamp. The table below gives an idea of the distances involved, fixtures used during the meter readings are common to indoor gardens.

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