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Lumens vs PAR

The most common light measurement to most humans is lumens; however lumens have absolutely nothing to do with the intensity of a light source to a plant. Instead lumen values tell you how bright a light source is to the human eye.

Unlike plants that are sensitive to red and blue light, our eyes are most sensitive to green/yellow. A high lumen reading means a light has high output in these wavelengths, which are very inefficiently converted by plants. Just because one light has higher lumens than another, does not mean it produces more PAR.

PAR is the term for the region of light between 400-700nm that plants are able to absorb for photosynthesis. The PAR curve is a measurement system based on the photochemical efficiency of common plants.

PAR measures the intensity of light available for photosynthesis, and is expressed in micromoles per meter squared per second (µmol/m²/sec). This tells you the photosynthetic photon flux density or PPFD. The higher the density, the faster photosynthesis can occur.

Light Energy by Wavelength

Light travels in waves measured in nanometers (nm). One light wave is equal to one photon. The length of these waves determines the color of light and how much energy it can transfer per second. As you move from blue to green to red in the visible spectrum, the wave length gets longer. The shorter the wave, the more energy each photon contains. The longer the wave, the less energy each photon contains. This is why UV light can be harmful to your skin, when blue light does no harm at all.

The frequency of a wave is measured in hertz (Hz), with 1Hz being equal to one wave per second. Light is the fastest wave in the universe, traveling at almost 3 million meters per second (186,000 miles/sec). The frequency of light is typically measured in Terahertz (THz), which represents 1 trillion hertz.

The energy carried by light wavelengths is expressed in electron volts (eV). In physics, the electron volt is the amount of energy gained by an electron when the electric potential increases by one volt (V). As you learned in Photosynthesis 101, the porphyrin ring in chlorophyll captures electrons from light to undergo photosynthesis. So the eV value tells you how much electron energy is available to be be passed along through Chlorophyll.

Photosynthetic Conversion Efficiency

Chloroplast absorbing and reflecting light

Just because a light wavelength carries eV energy, does not mean a plant can convert it. Chlorophyll acts like a light filter, absorbing some colors and reflecting others. The wavelength must be within the absorption range of Chlorophyll A, B or Carotenoids for the electron energy to be captured and used for growth.

There are two main types of Chlorophyll designated A and B. Each absorbs different wavelengths with varying efficiency as shown to the right. The wavelengths with the highest absorption rate are referred to as photosynthetic absorption peaks. These vary from one plant species to the next. For the plants we targeted they are located at 439nm & 667nm for Chlorphyll A, 469nm & 642nm for Chlorophyll B, and 525nmm for Carotenoids.

Absorbance is only one part of the photosynthetic equation. Electrons from photons must be converted into energy, which likewise varies in efficiency based on spectrum (and species).

Chlorophyll absorption by wavelength
Spectral asorption vs Photochemical Efficiency

Often referred to as the "Action Spectrum", the pink area in the graph to the left represents the photochemical efficiency of each wavelength for common plants. This tells you how efficient the chloroplasts can convert the absorbed light energy into growing energy.

To calculate how much potential energy any wavelength has you multiply the eV by the absorptive efficiency of chlorophyll, then by the overall photochemical efficiency. This gives you the net eV available for photosynthesis as shown in the graph below. Dividing the net eV from the total available eV gives you the net efficiency.

Net Efficiency after spectral absorption and photochemical efficiency

Values vary from one species of plant to another.

LEDs are the most efficient light sources on earth for creating high quality PAR light. Hydro Grow tests LEDs every year from manufacturers around the world, to always ensure we have the highest PAR output per watt for every wavelength we use.

The data to the right represents the µmol output of our individual 3W LEDs @ 700mA. Notice how the Volts data resembles the eV energy data above for each wavelength? The small difference is due to electrical inefficiency of the light source.

PAR Output byLED wavelength

Following the net efficiency trend, it is easier to create higher µmol intensity with red or blue light than it is with green. You will notice that the watts consumed varies from one color to the next. This is because volts multipled by amps (700mA or 0.7A) equals watts, and each LED color has a different voltage. Hydro Grow LED lights have the highest proportion of energy concentrated in wavelengths with maximum absorption/efficiency, resulting in the highest rate of photosynthesis.

Supercharging Photosynthesis through Science

Balancing Spectral Ratios

Knowing which colors of light have the highest net efficiency is one half of the spectral equation. The other equally important half is balancing the ratios of red, green and blue light. These ratios (based on the µmol output of each region of PAR) have a dramatic impact on growth rates and yield.

The graph to the right shows that when your spectral ratios are not calibrated properly, it can reduce your yield as much as 40% while lengthening your growth cycle up to 30%! Hydro Grow uses NASA proven ratios of 10% blue, 15% green and 75% red light to supercharge growth rates and yields.

Hydro Grow G4 Spectral Graph Optimal tested ratios of red, green and blue light for photosynthesis

NASA Spectral Variance Testing on Starfire Tomatoes

red, blue and purple lights

The Veg vs Bloom Lighting Debate

Plants evolved under the sun where they get a set spectrum year-round. As the data above shows, you get the highest yields in the shortest amount of time when plants are exposed to a single, properly-calibrated spectrum of red, green and blue light through all stages of growth.

It is a common misconception in the indoor growing community that blue-dominant lights are for vegetative growth, while red-dominant lights are for flowering. The data proves both of these misconceptions are false. This misconception began due to limitations in HID grow lights when indoor growing began, where HPS created a red-dominant spectrum and MH created a blue-dominant spectrum. LED companies with zero knowledge of plant science have continued perpetuating this misconception.

The Emerson Enhancement Effect

Back in 1957 a pioneering scientist named Robert Emerson, discovered that when far red light (700-750nm) is shown simultaneously with deep red light (660-680nm), the rate of photosynthesis increases by 30%!

Hydro Grow was the first company to incorporate 740nm far red in LED Grow Lights to boost your growth rates by 30%. To this day we use more 740nm than any other competitor ensure you get the fastest growth possible, despite the high cost involved.

graphic demonstrating the Emerson Enhanement Effect
Red and Blue light penetrating deeper into chloroplast wit green light

Red and Blue Light Penetrate through More Layers of Chloroplasts in Presence of Green Light, allowing for greater absorption and a higher rate of photosynthesis.

The Penetrating Power of Green Light

While the chlorophyll and carotenoid absorption of green light may be lower than red or blue, green plays a vital role that is often overlooked.

When green light is blended as Hydro Grow does with red and blue light, the result is a strong "white" light. According to the University of Oxford: "green light drives photosynthesis more efficiently than red light in the presence of strong white light."

Green light penetrates deeper through plant tissues than red or blue, stimulating more of the lower chloroplasts. When shown with red and blue light, green acts as a penetrant allowing them to deliver energy to more layers of leaves. The more layers are stimulated, the more energy is absorbed, resulting in higher rates of photosynthesis.

Light Saturation - Sometimes Less is More

Picture a photon as a grain of sugar and a chloroplast as a glass of water. As you pour sugar into the water it dissolves until the point of saturation, after which it collects on the bottom. Chlorophyll like water, can become saturated with light, at which point additional light does not equal increased growth. Instead if you give too much light it can damage chloroplasts leading to stunted growth or even death for sensitive plants like orchids.

An easy way to imagine this is to think of photosynthesis as a car engine, and light intensity as Nitrous oxide. No matter how much NOS you throw at the engine, the top speed of the engine (photosynthesis) will always be limited by the transmission (light saturation point). But if you give the engine too much NOS (light) it suffers catastrophic failure.

sugar being poured into water
Optimal PAR range based on plant variety

The light saturation point varies across varieties of plants. Shade plants like orchids or mushrooms still require light to grow, but need very little (only 50-100µmol) whereas large fruiting plants like watermelon or tomatoes require much higher intensity (800-1500µmol).

When choosing a grow light it is important to consider the optimal growing range for your plant variety. Make sure the light does not supply more intensity than your plant needs, otherwise you will simply be wasting electricity and potentially damaging your chloroplasts.

PAR is NOT Created Equal

There is a reason why Hydro Grow lights often outperform competing LEDs by 2-4x the yield per watt. Or why our lights have never done less than double the grams per watt of HID in independent side-by-side tests. It's simply that PAR is not created equal.

Even if a competing fixture has the same PAR output, they aren't using the highest output LEDs in the proper wavelengths or ratios to ensure optimal conversion of power (watts) into growing energy. Some competitors even want to give you the controls to tune the spectrum on your own. This is the easiest way to tell they have no idea what they're doing. No amateur, professional or PhD has ever bested Hydro Grow's technology in a side-by-side grow test, so don't be their guinea pig!

The lettuce on the right was grown by SuperGrow, a commercial producer of leafy greens. Both sets of plants received the same 300-400µmol intensity, the only difference being the light spectrum. One set was under full-spectrum white CFL, the under other Hydro Grow LEDs. Just because a competitor advertises similar PAR numbers to Hydro Grow, does not mean they can compete with our spectral technology.

LED vs T5 Lettuce Grow Results LED vs T5 Lettuce Grow Results


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