I get asked a lot about the optimum lighting for a specific size grow room. The conversation is primarily about the room dimensions with respect to size of the growing area. The issue of hang height is often secondary as PAR at height can be adjusted via dimming or hang height and as such is not considered as important as total wattage and relative efficiency. This narrative considers height in the context of canopy uniformity.
The desirable metric of grams per watt is clearly important when considering LED to HPS for example, led is the current darling of grow lights and its higher upfront cost needs to be considered in the context of optimum utilisation. A single HPS is an excellent performer but is limited by its wall plate efficiency and a relatively fixed beam shape and coverage.
The use of directional point sources of lights provided by LED offers a more flexible option with respect to layout. The ability to move small sources around to gain better coverage is an under appreciated benefit to using led’s. I will demonstrate some performance results here with respect to hang height and the canopy coverage. This is based on the Cree CXB3590 at 56.1 watts per COB and is using 9 pcs of the CXB3590 in a 3×3 Space.
Lets start with PAR figures from a Gavita 1000 Watt DE. I should note this is not a shootout or one light source preference over another. Its is to certainly help optimise an investment in the more expensive Led replacement
The basic metric used will be the percentage difference from the centre hotspot to the corner of the 3ft space. So if we look at the Gavita at 3×3 at 36inches, we can solve as Gavita centre PAR is 949 and the best corner at 3ft is 441, based on that we can say that the difference is 53% reduction in light from the centre to the best 3ft corner. At 48 inches the ratio improves to 705/410 = 41%. This is inherently the case with most light sources but is certainly helped by the reflectors used in HPS systems.
Lets now consider the COB solution
CXB3590 in 3000k
We will use 9 COB’s in a 3 x 3 arrangement. These are running at 56.1 watts per COB for a total 504.9 watts. No secondary optics or reflectors were used in this design and results rely purely on the point source COB light.
The difference from 36 inch and 18inch using COB and using ISOlines again highlights that height has a big impact on uniformity. The 36 inch map really shows this with a more circular ISOline and while it seems counter intuitive the corner PAR actually improves over the 18 inch height at the greater height
As with the HPS performance if we calculate at 36 inches we get 524/322 = 38.5% difference from centre to the best 3 ft corner.
This difference in point source(COB) vs HPS can then be summarised as a nearly 30% improvement in coverage at the 3ft edge for COB over HPS. This makes no allowance for actual better layouts of the COB, another benefit of the COB footprints being the ability to move them around to further reduce that centre to edge uniformity. The use of COB in grow lights has been huge in the last few years and while there is a move to mid power led boards and strips, COB remains a potent solution for higher fixture mount applications