HID vs LED – How they compare for Horticulture

High Intensity Discharge (HID) Lamps include High Pressure Sodium(HPS) and Metal Halide(MH) and are a class of gas discharge lamps that have dominated commercial horticultural as well as outdoor and warehouse lighting for decades.  HID lights have very high efficiencies, (DE HPS approaches 1.7 mmols/joule — a rating of output in photons to input energy)  and this new double-ended (DE) design effectively fixed all the weaknesses of the earlier single-ended system, so the new class of HID lamps are pretty much as good as they are ever going to get.

With the advent of full spectrum LED lighting, HID is rapidly reaching the end of their dominance for several reasons which we shall discuss here.

  • Fixture inefficiencies

HID lamps produce light that emits in all directions, and must be controlled to be effective.  When we say DE HPS has 17.mmol/j efficiency, we are referring to a bare lamp, but a bare lamp in any grow room will waste most of its light on ceilings, walls and floors, so they require a fixture to direct the light where it will be used.   The very best reflective materials run around 95% — that means every time light bounces off that surface it absorbs 5%.   It is almost impossible to design a fixture that emits all the light from an HID lamp one bounce or less, and often it can be many bounces, and with each bounce you lose 5%.  So you can see why even the best HID fixtures have a fixture efficiency of no better than 80%.  That means our 1.7 mmol/joule rating has effectively gone to 1.4mmol/joules actually directed to the plant.

  • Lamp degradation and Spectral instability

HID lamps can degrade as much as 10-15% after only one year of operation and then they continue to slowly diminish to end of life. But more importantly, as they degrade, the spectrum shifts toward the Yellow/Green region which is region of spectrum most poorly utilized by the plant (more on this later).  They same occurs with dimming and so dimming HID lamps in horticultural applications is discouraged.  So as the lamps dim over time, they also produce less light in the critical red and blue regions best utilized by the plants. So most professional growers replace their HID lamps every year with significant on-going lamp ($70) and labor (how much do you pay yourself) expense.

  • Hot Lamps and Infra-Red (IR)

HID Lamps have an interior wall temperature from 300-400C, so they require significant cooling, and can be potential fire hazards. Many city safety codes are beginning to prohibit HID in residential grows due to the dramatic increase in “closet fires”.  Roughly 75% of all the energy consumed by an HID lamp is emitted as heat, i.e.  Infra-Red radiation.  Not only does this require significant air-conditioning in HID grow rooms, but the IR also heats the plants, potentially causing heat stress, fox-tailing, and other changes.

  • HID’s fixed spectrum output – not ideal for plants

Plants absorb light, using the photons energy to strip the hydrogen out of water and combine it with the carbon from the Carbon Dioxide in the air (plus relatively small amounts of soil minerals) to create sugars and plant matter.  They have many pigments that can absorb light and fuel plant processes, but far and away the most important and efficient pigment for photosynthesis is Chlorophyll.


1024px-Chlorophyll_ab_spectra-en.svgAs you can see, Chlorophyll only absorbs RED and BLUE spectrums and reflects yellow and green, (giving plants their green color,) so while the green/yellow bands can be absorbed by other  pigments like the Carotinoids,  most of this spectrum range is reflected away and/or poorly utilized.  HID lamps are very efficient in producing light in general, but they produce as much as 40-50% of their light in this poorly utilized green and yellow spectrum, and HPS produces NO blue and must be supplemented in HPS Horti lamps with Mercury vapor to provide at least a little essential blue.

HortiLux Super HPS Spectrum curve



LED vs. HID – Performance

So this brings us to the primary reason why LEDs are rapidly replacing HID as the ideal horticultural light source, and why HID is effectively at the end of its effective technological life, at least in the Horticultural world.

Individual LEDs come in specific and very narrow spectrum bands, as well as relatively new white LEDs and so LED grow-lights can be effectively blended to deliver a precise spectrum optimized for maximum plant absorption and utilization and at efficiencies of 1.7+ mmols/joule for the best designs.  And it is this ability to deliver the plants exactly what they use optimally that allows the best of the current generation of LED grow-lights to deliver equal yields, and even superior quality to that produced by HID lamps for up to a 40% reduction in electrical input.

But another advantage of this precise spectrum control of LEDs more specific to Cannabis Cultivation is the opportunity to effectively influence the plant and flower structure by controlling the spectrum.

Designing a plant with light

A far less well known aspect of plant photosynthesis is that different spectrum’s not only have different absorption and utilization efficiencies, they also influence different aspects of plant growth.  It’s is the ratio of Deep Red to Red that triggers the shade-stretch response.   It is the hours of red absorption vs. hours of darkness that triggers flowering.  But less known is aspects of plant growth various spectrum’s promote.

Red spectrum’s promote root, stem, and flower mass production in Cannabis and most plants.  Blue light promotes leaf, resin, and terpene (fragrance) production, as well as stimulating other pigments which can bring out more pronounced colors in the flowers.   Also, UV(B) (not to be confused with the “UV” claimed by many other LED mfg’s) supplementation as provided only by the California LightWorks  products can increase THC production by 2-3 %.  STUDY Page

So for years, many expert cannabis growers — seeking to not only produce maximum yield but also optimum cosmetic shelf appeal and fragrance—have used the more blue Metal Halide lamps not only for Vegetation phase, but also right through the first week or two of pre-flower to reduce the stretching that commonly happens when cannabis transitions to flower and support strong leaf growth. Next, they would switch to the heavy red spectrum HPS for the peak flower period to stimulate flower production,( typically the middle 4-5 weeks). Then for the critical ripening period when the majority of resin and fragrance production occurs, they switch BACK to Metal Halide for the last 2 weeks of ripening. This technique not only optimizes photosynthesis, but effectively designs the plant.

LEDs are uniquely suited for this process, and LEDs grow-lights that provide independent control of red and blue spectrum’s like the California LightWorks 440/550/880 products, can accomplish this with just the flip of a switch or program on the controller.

LED safety, simplicity, and ease of use

Finally, LEDs have no lamps to replace, require 40-50% less AC than unventilated HID,  create absolutely NO fire hazard in the close quarters of grow tents,  and generate very little infra-red which heats the plant more the higher up you go creating an uneven metabolic response in the plant, and the potential for sporadic zones of heat stress, and in general just a much simpler, more reliable grow-light platform.

And it is for all these reasons that the current generation of LEDs grow-lights are finally poised to permanently replace HID as the first choice for Horticultural lighting.