skunky33
Active Member
This is from cannastats
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.Approximate fc in the sweet spotLamp Distance Foot-Candles 1000 watt HPS 1.50 foot (18 inches/45 cm) 9,000 fc 600 watt HPS 1.00 foot (12 inches/30 cm) 9,000 fc 1.50 foot (18 inches/45 cm) 5,600 fc 400 watt HPS 0.75 foot (9 inches/23 cm) 9,000 fc 1.50 foot (18 inches/45 cm) 4,000 fc 250 watt HPS 0.58 foot (7 inches/18 cm) 9,000 fc 1.50 foot (18 inches/45 cm) 2,800 fc
(metered directly beneath the bulb)Highlighted items approximate the closest distance that particular lamp should be to the sweet spot to avoid over-lighting. (those interested in photosynthetic radiation can use this PPF PAR converter.)
The highlights don't convert it looks like but the first number is the closet you should get a bulb and that's under good environmental conditions, along with heat I bet a lot of people have to keep them higher.
I know a lot of people think a cool tube is made to keep their lights within 2 inches of their canopies but In my experience, the best place for my bulbs is a wee bit higher than most people would think in my gardens.
The reason I believe in 2.5+ feet is through personal experiences and advice. The above calculations are just to simple. It doesn't give ambient heat temperatures and radiant heat fluctuations so if someone is growing in an 85 degree room would it still be okay to have the 1000 wt 1.5 feet away or would it be more advisable to have the light further away to lessen radiant+ambient heat stress on the canopy? what kind of lid?
Cool tubes are made primarily for Co2 inclosed operations and to keep rooms a little cooler by not using a metal lid which raises room temps. Plus there's only so much a plant can absorb. Why have a cool tube inches away sacrificing light spread.