Other esential parameter is often forgot when designing a LED array: what current we are going to use to run them.

Light emision of a led is directly linked to the current flowing across it. But any led emits at the higher efficiency as lower the current, and it drops as current increases.

Depending to the thermal design, there is a point when increasing further the current dont achive any increase of light. Increasing current from it raises strongly the electrical consuption without any increase of light emited. This is extreme, but what happen very often is using currents so high that efficiency is very low. As i explained on the last post, enough energy efficiency is key in order to achive electric savings.

So current used (or range of current used) may be carefully considered.

Some examples (thermal resistance solder point to ambient temp supposed to be 10 K/W, for a very good mounting with heatsink):

Cree XR-E coolwhite Q4 (luminous bin, 100-107lm@350mA): 28.5%@350mA, 22%@700mA

Cree XR-C red (R3:625-630nm dom wavelenght), bin M (39.8-51.7 lm@350): 29%@350mA, 25%@500mA, 20%@700mA

Cree XR-E Royal Blue, bin group 14 (350-425mW@350mA): 33%@350mA, 29%@500mA, 26%@700mA

Luxeon K2 red, bin S (typ 60lm@350mA): 26%@350mA, 23%@700mA

Luxeon K2 coolwhite, 200lm min group: 26%@350mA, 21%@700mA, 19%@1A, 13%@1.5A.

(Note: there is more unaccuracy on Luxeon products, as i have not digitalized their SPDs at operating conditions).

A LED cost the same independent of at what current you use it. So if you run 2 leds at 350mA instead of one at 1A, you get almost same light but using just a little more than half watts, but it initially cost you double.

But check that except for the Royal Blue, at 700mA or over all those leds that are state of the art currently gives efficiencies too low to be able to save watts against a large HID.

The Procyon, for example, probably use those red leds and lower bins of the whites (i believe P4). If they run them at 350mA (0.8 and 1.2W for red and whites/blues, respectively), and use double of leds, it may offer a good perfomance. Far for sustituting a 600W HID, but maybe able to get close to a 250w. But by driving them at 700mA to reduce costs, the perfomance is too low.

At the end, buyers only think on: "120watt for xx dollars, wow, its nice". If instead of watts, they say how many micromols of photons it deliver, we could compare perfomance. But they dont say anything about it, or how many PAR watts it emit. The only honest LED grow lights manufacturer who says the emission of the lamp is what makes the TI Smartlamp. And unfortunatelly its output (265uE if i remember well, similar to a reflectorized 250w HPS) is too low to do it cost effective.

Always there is a trade off between initial costs and efficiency. Always. So choose carefully the current or current range for your setup, in the way its the best compromise between both ends for your situation.