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Okay, for many of you novices or inexperienced growers, lighting is probably one of your main problems.
Sun: traveling 93 million miles before getting to earth, the suns lumen output still reaches 5,000 lumens per square foot. That may not seem too fancy, but considering that sunlight can break through canopies and treetops and STILL give that output to all the smaller plants below, its simply amazing. Nothing man-made can come close to this.
Bulbs: Artificial light cannot even come close to the power of the sun, in fact light emitted from HIDs will decrease in lumens exponentially as the distance, both vertical and horizontal, increases between the light source and the plants.
Lumens/Lux: Lumens are a way to measure light density, these units are equal to the number of candles it takes to light a specific area. One lumen actually equals one candles worth of light per square foot of surface area when that candle is exactly one foot away. Similarly, for all you smart people using the metric system, one Lux would be one candles worth of light from one meter away. 1 Lumen = 10 Lux. However, two important distinctions must be made when understanding Lumens/Lux, and that is while lumens are a way to measure light energy leaving the source or bulb (aka flux), lux is a term generally reserved for indicating the amount of light energy actually reaching a given surface or leaf (aka illumination).
Why know this? Because if youre going to buy an HID bulb, its very important to know the lumens and lux of it. Although it may say how many lumens it puts out, this is merely the lumen output at one foot away. This number will decrease exponentially with every foot the bulb is farther away from the plant.
Inverse square law: The intensity of light from a point source is inversely proportional to the square of the distance from the source, so an object twice as far away does not receive 1/2 the energy, it receives 1/4 of the energy.
So for all you English speaking potheads: if you have a light that produces one lumen of illumination over an area of one-square-foot hanging the light from 1 foot above the plants, then you increase the distance to two feet, the light energy reaching it will NOT be 1/2 the intensity, but rather 1/4 the intensity, resulting in less lumens, and hence LESS BUD (because it is not only traveling down, it is traveling side-to-side as well).