One of the biggest causes of confusion and disagreement in the world of grow lighting is the word “brightness”. The problem is that we constantly use the word colloquially to describe how things “appear” to us – it’s a bright sunny day, that bulb is a bit bright, etc. It is no way a scientific definition of light intensity – the number of photons hitting a specific area over time – and ignores wavelengths of light that humans are less sensitive to.
Take as an example an X ray light source. Is this bright? Well you can’t see it and you wouldn’t be able to read a book using X rays. But when you consider that a very short burst from an X ray machine can pass straight through you onto a film plate, casting shadows only where your bones got in the way, you have to conclude that maybe brightness and intensity aren’t one and the same thing after all.
Likewise, because LED Grow Lights accurately target the wavelengths preferred by plants they both don’t need to be and in fact don’t seem so bright to us, even though they are actually delivering a greater amount of “useful” light. That perception is further bolstered by the fact that HIDs emit a lot of light in the green/yellow part of the spectrum which increases their perceived intensity to the human eye (unlike plants we favor light around the 550 nm mark). But the fact is that they are just wasting all this additional light since it is of little benefit to plant life.
If we take a 400w HID as an example, about 260 watts of that will actually be completely outside the Photosynthetically Available Range (PAR) so at least 65% of the light (and the energy used to produce it) is completely wasted. So with less than 40% of the light generated by HPS and MH lamps being absorbed by plants, and barely 10% of the electricity you pay to run them being actually converted into light in the first place this is not looking promising for conventional lighting (on these figure about 4% of the input energy ends up doing some good for the plant).
Some LED grow light systems do indeed include additional wavelengths, simply to enhance the visual appearance of the plants since foliage often looks black when lit purely by blue and red light. But there is no getting away from the fact that plants benefit enormously from the capability with LED lamps to smoothly vary the balance between blue and red exactly as needed. Seedlings do better with the balance in favor of blue light, while crop yields can be significantly boosted by switching over to red towards the end of the growing cycle. In this way you can simply replace MH lamps for early growth and HPS lamps used later in the cycle with a single, dynamically variable solution that reduces running costs and maintenance issues at the same time.