Watt for watt, modern LEDs outperform High-Intensity Discharge (HID) lamps such as HPS, Metal Halide and Ceramic Metal Halide because they convert more energy to light and less energy to heat.

LED technology also provides other advantages traditional HID lights can’t match:

* Better efficiency (reduced power costs)
* Less heat (less plant stress, cheaper climate control)
* Wider light spread (more even canopy)
* Better spectrum (broader, smoother, targeted at photosynthesis)
* Reduced flowering times (higher percentage of red spectra)
* Thinner profile (suited to low ceilings or tall plants)
* Less light degradation (HPS lamps lose 10% or more output after a few months)
* Longer life (50,000 hours vs 3,000-5,000 hours for HPS)
* Lower voltage (safer to handle)
* No replacement bulbs (no ongoing costs)

Currently, 350-400W of top-efficiency LED lighting will produce enough light to replace a 600W HPS lamp, while around 650W of LED will replace 1,000W of HPS.

These figures are fairly conservative, as LED technology is improving every year – unlike HID technology that has improved only incrementally in the past 25 years since the introduction of the Ceramic Metal Halide bulb by Philips in 1994.

Not all light is equal, either. While Photosynthetically Active Radiation (PAR) refers to the visible light spectrum between 400nm and 700nm, plants are more responsive to some wavelengths than others. They also respond to wavelengths outside the PAR range.

It is here that LEDs – literally – shine. They can be tuned towards certain spectra that enhance photosynthesis and reduce flowering times (from a few days to a week for most 8-12 week species), as well as control plant hormones that increase fruiting or flowering, or leaf thickness in the case of leafy greens.

This becomes apparent when you compare the spectra of different lamps.

The first graph depicts an Eye Hortilux HPS spectrum showing most of its light output in the yellow and green range, with a smaller amount of red and blue. What can’t be seen in this graph is the large spike to the right of the graph representing infra-red light, or heat. The most efficient double-ended HPS lamps have an efficiency of 1.8-2.0 umol/j (micromoles per joule – more of which later), while the least efficient single-ended HPS lamps are 1.0 umol/j or less. These numbers tell us how much light is produced in the PAR region (photons, or umol) for the amount of energy consumed (joules).

The second graph depicts an Eye Hortilux CMH bulb spectrum. This has a nice spectrum, characterised by a broad weighting and a decent amount of red and blue light, as well as some UVA – which can be beneficial for essential oil production (UV stresses the plant into producing more oils to protect itself from UV damaging radiation). However, this lamp is also weighted towards the green and yellow spectra, and the most efficient CMH lamps are still only around 1.8-2.0 umol/j.

The last graph is a High Light UV LED board spectrum. Note the smoother, fuller spectrum of the LED, as well as the red and blue weightings. High Lights have a minimum 49% red (which promotes flowering and shortens flowering times for the same yield), with a second peak in the blue range. These peaks both coincide with Chlorophyll A and B, phytochrome, phototropin, chryptochrome and caratenoid absorption peaks (see below). This particular High Light also has a small amount of UVA and near-UV to promote essential oil production.

The High Light has a very high Colour Rendering Index (CRI) of 95. Sunlight has a CRI of 100; CMH lamps are typically in the low-mid 90s; and HPS lamps are in the low 20s. This can be important to indoor gardeners who require “true light” to spot leaf discolourations from nutrient, pest and other issues. The sooner gardeners recognise problems, the sooner they can fix them.

Even though High Lights sacrifice some efficiency for their higher red weighting and CRI, they still have a measured efficiency of 2.5 umol/j. Some LED manufacturers claim similar or higher numbers, but the first question is: have they actually tested their LEDs?

Those that are higher typically use CRI80 LEDs that are slightly more efficient, but do not have the same high CRI or red spectrum weighting, which is specifically tuned for fruiting and flowering plants.

The graph above shows the typical plant response curve. These curves represent the spectra plants most respond to in terms of photosynthesis (chlorophylls and phytochromes) and terpene production (essential oils).

This last graph is the McCree curve. The McCree curve takes all the peaks in the previous graph and weighs them against the amount of energy in each photon to determine which wavelengths are the most efficient in terms of overall photosynthesis. As you can see, the most efficient spectra are between 600nm and 680nm – hence the value of red light for growing plants, but especially for flowering.

It is important to note that plant species are not all the same, and different species may respond to light in different ways. But the above curves are typical of many measured plants and are widely used in horticulture.

Most importantly, though, the curves do not always represent what different plants respond to best during different stages of growth. Most plants, for example, yield more under heavier red spectra throughout all stages of growth with the bonus of shorter flowering times.

However, whilst red may produce bigger flowers (fruiting and flowering, incidentally, occur in early spring and late autumn when red light levels are higher), it can also cause additional stretch and lead to bigger, thinner leaves. Bigger leaves have more surface area for photosynthesis, but they are not necessarily desirable in edible food crops, such as leafy greens.

It just goes to show that not all spectra are equal – and neither are all horticultural lights.