This document compares and contrasts LED and HID (high intensity discharge) grow lights. It discusses that while HID lights have traditionally dominated the horticultural lighting market, LEDs are positioned to take over the leading role. Some key advantages of LEDs outlined include longer lifespan, higher energy efficiency savings of 50-70% over HIDs, ability to target specific wavelengths for optimal plant growth, and lack of toxic materials or need for cooling. The document provides detailed information on light spectra, plant absorption, and how different wavelengths impact photomorphogenesis.

1. LED vs. HID
Since their invention in the early 1700s, High Intensity Discharge (HID) lights have become the dominant
technology in horticultural and turf Grow Light systems. These lights put out huge amounts of light and
their spectrum is close enough to the sun to be effective in plant growth. However, LEDs are well
positioned to take over the leading role in this market as we move further into the 21st century.
HID bulbs are categorized based on the gas that is used in them. The two main types used for growing
are high pressure sodium (HPS) and metal halide (MH) lights. The type of gas used in the bulb
determines the color spectrum and wavelength, so if you need a different type of light you have to
change the bulb itself. Typically HPS lights as used in the SGL and MLR Lighting rigs
(http://sglconcept.com; https://mlrsports.com ) used at the MCG, ANZ Stadium and a number of venues
worldwide are more towards the red and orange hue, whereas an MH bulb produces a blueish color,
useful for the vegetating phase.
LED grow lights as used by UK based manufacturer SeeGrow Developments (http://www.seegrow-
grass.com) produce light differently and in conjunction with or without their use of carbon dioxide the
results gained are outstanding (up to 10mm growth in 24 hours leaf and root). The core of an LED bulb
consists of a chip that directly transforms electricity into light, yielding far better results. One of the
main advantages of LED over HPS and MH lights is the adjustable light spectrum. Their comparatively
high efficiencies and long operating life have ensured that these have gained a market foothold being
used at Manchester City, Arsenal FC, Juventus, The San Siro, Tokyo Stadium, Chelsea FC, and Wembley
Stadium to name a few. The introduction of high-power LED systems could well spell the end of their
dominance for a number of reasons:
• LED grow lights last six times as long
• LED directional lighting requires no reflectors.
• LED grow lighting uses no toxic metals.
• Energy savings range from 50%-70% over HID lights.
• No cooling necessary; no plant burn.
• Maximum chlorophyll absorption.
• Less frequent watering.
• Bias on desired spectrum possible by dimming.

2. • Due to the 2-3 times efficiency per m2 in comparison to HPS, less pitch treatment in operational
hours and/or lesser equipment is required
• Savings on carbon footprint - 9 HPS rigs each containing 60 lamps by 2,000 (seasonal)
operational hours produce 800 ton of CO2 (800,000 kg). By using LED, savings are to be obtained
varying from 200 to nearly 400% on CO2 emissions
A ‘’light’’ refresher course!
Daily light integral (DLI). Just to refresh your memory, DLI is the cumulative amount of photosynthetic
light that is received each day. The DLI is measured as the number of moles of light (mol) per square
meter (m-2) per day (d-1), or mol·m-2·d-1. The DLI can have a profound effect on root and shoot
growth.
Variations occur in DLI between C3 and C4 grasses and also between different cultivars
Optimum Solar Radiation
Warm‐Season Turf: Dudeck and Peacock, 1992
812 ‐ 969 Ly/day (AVG = 890 Ly/day
390 ‐ 465 w/m2/day (AVG = 427 w/m2/day)
Cool‐Season Turf:
242 ‐ 485 Ly/day (AVG = 360 Ly/day)
116 ‐ 233 w/m2/day (AVG = 175 w/m2/day)
w/m2/day = 0.48 Ly/day; Ly/day = 2.07 w/m2/day

3. Warm season DLI requirements (mol m-2 d-1) Glenn and Unruh 2014
Turfgrass Cultivar Summer Winter Spring
Tifway hybrid couchgrass 21.0 10.6 17.9
TifGrand hybrid couchgrass 19.9 9.8 14.6
Celebration common couchgrass 19.6 8.8 14.9
SeaDwarf seashore paspalum 13.2 8.0 11.9
Diamond zoysiagrass (matrella) 11.3 7.4 10.9
Palisades zoysiagrass (japonica) 11.2 8.2 11.2
Cool season DLI requirements (mol m-2 d-1) (
Christians, Patton, Law; Cockerham et al, 2002)
Turfgrass Cultivar
Kentucky Bluegrass <11.1 to >24.1
Perennial Ryegrass >20
Tall Fescue >8.8
PAR (Photosynthetically Active Radiation) is a region of the electromagnetic spectrum (400 to 700 nm)
that promotes photosynthesis.
PPF (Photosynthetic Photon Flux) tells us how much PAR a light-source emits. PPF does not measure PAR
at a specific location (e.g. your crop canopy), but it tells you how many photons within the PAR region
are coming out of the light-source every second.
PPFD (Photosynthetic Photon Flux Density) measures the number of photons within the PAR region at a
specific location (e.g. the amount of light delivered to your canopy) every second. If you have a PAR
meter, it is reporting PPFD (μmol/m2/s) measurements.
So what are the key differences between HID and LED lights?

4. 1) LED grow lights last six times as long – frequent bulb changes
The output of HID lamps can degrade as much as 10-15% after only one year of operation, and in the
case of HPS, as they degrade, the spectrum shifts toward the Green/Yellow range, which is the range of
spectrum most poorly utilized by plants. LEDs are rated at for at least 50,000 hours with less than 10%
drop in output and often continue to operate way beyond their rating with little or no shift in spectrum.
2) LED directional lighting requires no reflectors
The process of creating light with HID lamps is extremely efficient, with HPS lamps running as high as
1.7umol/j (#photons out / joules of energy in). However, directing that light down to the area where it
will be used is not so efficient, as it requires what is called a reflector. The very best reflective materials
used in fixtures run around 95% total reflectivity. This means that every time light strikes that surface,
95% reflects off and 5% absorbs and is converted to heat. With a large diffused light source like a HID
lamp, it often it can be many reflections, and with each reflection you lose at least 5%. So even the best
HID fixtures typically have an overall fixture efficiency of 85% or less. That means a 1.7 mmol/joule
rating of a DE HPS lamp is effectively 1.45mmol/joules when actually directed towards the canopy.
Since LEDs are directional they require not reflectors and therefore do not have this limitation.
3) LED grow lighting uses no toxic metals.
LED (Light Emitting Diode) is a solid-state semiconductor device that can convert electrical energy into
visible light; it can convert electrical to light directly.
4) Energy savings range from 50%-70% over HID lights
In terms of lumens per watt, HPS lights are quite efficient producing over a 100 lumens per watt.
However, when we consider LEDs, light output has already exceeded 300 lumens per watt. As the image
below shows LED’s are showing ever increasing efficiencies.

5. 5) No cooling necessary; no plant burn
HID Lamps have an interior wall temperature around 700 degrees Celsius. LEDs typically emit 50% less
heat compared to HIDs.
6) HID’s fixed spectrum output – not ideal for plants
Plants absorb light and use 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
plant matter. They have several pigments that can absorb light and fuel plant processes, but far and
away the most prevalent and efficient pigment for photosynthesis is Chlorophyll.

6. Spectrum and Photosynthesis
How do plants use light? Plants convert incoming light into chemical energy by using chlorophyll in
leaves to absorb only the blue and red wavelength portions of the electromagnetic spectrum.
No other wavelengths have any use to the plant.
As the above shows there are two forms of Chlorophyll (Chlorophyll A and Chlorophyll B). Each has
absorption peaks in both the Red and Blue spectrums, and both reflect yellow and green, (giving plants
their green color,) so while the green/yellow bands can be absorbed by other pigments like the
Carotenoids, over 50% of this spectrum range is reflected away and/or poorly utilized.
Blue light (400 to 500 nm). Blue light helps regulate the opening of stomata and acts to inhibit extension
growth, so plants grown under light that contains blue have smaller leaves and shorter stems. For these
reasons, many LEDs for plant applications emit at least a small amount (such as 10-20 percent) of blue
light.

7. Green light (500 to 600 nm).
Plants appear green
because they reflect and transmit slightly more green light than they do blue or
red light. Chlorophyll also absorbs green light poorly. For these reasons, green
light is sometimes stated as not being useful to plants for photosynthesis. However, green light is still
moderately effective since other pigments absorb the light and make it useful for photosynthesis.
Red light (600 to 700 nm). Most LED arrays emit a high percentage (often 75-90 percent) of red light
because it is absorbed well by chlorophyll, and the electrical efficiency of red LEDs is high. Red light is
considered the most efficient waveband for photosynthesis, but as mentioned previously, plants can be
elongated in the absence of other light wavelengths.
The graph opposite shows plant absorption throughout the PAR spectrum range. Absorption around the
green/yellow boundary is as low as 30% and from 620 (range/red boundary) to 520 (Blue/Green
boundary) is completely under 50%. Other pigments can absorb and utilize this spectrum range, but
they are far less efficient.
The graph below shows a typical HPS spectrum. You can see from these graphs that the majority of the
HPS spectrum actually falls in the region of lowest plant absorption.

8. Metal Halide (MH, CMH) Spectrum
Metal Halide differs from HPS in the fact that the spectrum bands are generated by individual Metal
Halides, for example a green halide, blue halide and red halide, which turn to gas and emit light when
heated. Metal Halide lamps have traditionally favored the blue end of the spectrum, for as the lamp
light gets “warmer” in color — i.e. generates more red — they get less efficient, more expensive, and
degrade faster. And as you can see from the graphs they still produce minimal amounts of far red at the

9. critical 675nm Chlorophyll A peak. LEDs are the only lighting technology that can be designed to meet
specific light spectrums for plants.
LED Performance
Individual LED chips produce very narrow spectrum bands, so LED grow-lights can be precisely mixed to
deliver a spectrum that is optimized for maximum plant absorption and utilization.
And it is this ability to deliver exactly those frequencies the plant uses best, and all at very high
efficiencies, with 30-40% less input power.
Photomorphogenesis
Different spectrum can have a major impact on specific plant morphology (changes in plant
characteristics such as leaf and stem size, plant shape, etc..) and this is referred to as
“Photomorphogenesis.”
The main influencer on this is the ratio of Red: Far red light. Under normal light conditions, the R:FR
ratio is usually constant all year round, averaging about 1.15 (Smith, 1982). However, R:FR ratio could
drop to as low as 0.7 at dusk.
One aspect of Photomorphogenesis that has significant research support is the shade stretch response.
Shade light has a higher proportion of Far Red (FR) than is found in direct sun, so If the ratio of Red (R) to
Far Red(FR) falls below a certain threshold (such as under trees or in shade), turf will begin to stretch
and become etoliated, and exhibit a reduction in chlorophyll content and leaf thickness.

10. References
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financial support from NASA, unpublished preliminary data, N63 16152.
Roberts, L.M. (2010). Spectrum of a typical High-Pressure Sodium (HPS) lamp.
High_Pressure_Sodium_Lamp_Spectrum.jpg.LMRoberts at en.wikipedia.
Bugbee, B. (1994). Effects of Radiation Quality, Intensity, and Duration on Photosynthesis and Growth.
In T.W. Tippets (Ed.).NASA-CP-95-3309, International Lighting in Controlled Environments Workshop,
Kennedy Space Center, FL: National Aeronautics and Space Administration.
Campbell, T. (2012).White Paper: The Right Lighting for Photosynthesis: Which Wavelengths are Most
Important and Why? Prepared by the author for Blu LED.
Christians N, Patton AJ, Law QD, Fundamentals of Turfgrass Management, 2016
Cockerham, S.T., S.B. Reis, G.H. Riechers, and V.A. Gibeault. 2002. Turfgrass growth response under
restricted light: growth chamber studies. California Turfgrass Culture 52(3&4):13-15
Dudeck, A.E., Peacock, C.H. (1992) Shade and turfgrass culture, p. 269–284. In: Waddington, D.V., R.N.
Carrow, and R.C. Shearman (eds.). Turfgrass. ASA Monogr. 32. ASA,CSSA, SSSA, Madison, WI
Geiger, D.R. (1994). General Lighting Requirements for Photosynthesis. InTibbits, T.W., International
Lighting in Controlled Environments Workshop, National Aeronautics and Space Administration, John F.
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Glenn, B., J. Kruse, and J. B. Unruh. 2012. Daily light integral requirements for twelve warm-season
turfgrasses. Int. Ann. Meet. p. 72111.
Harder, S. (2007). White Paper: Metal Halide (MH) vs High Pressure Sodium (HPS). Paper concerns
outdoor lighting such as street lamps. Accessed online at
www.darkskysociety.org/handouts/white_paper--mh_vs_hps.pdf
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481–518.
Tikhomirov, A.A. (1994). Spectral Composition of Light and Growing of Plants in Controlled
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