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What’s the difference between the spectrum of the Sun, HPS, CMH, Red/Blue LED and Full Spectrum LED

We have tested the spectrum of all the major grow light technologies and compare them so you can see their makeup in Blue, Green and Red. It is the proportions of these colours that is the key to understanding the differences between the grow light spectrum.

This is a table of the spectrum output of the most common light sources used in indoor growing

The light spectrum of grow light sources: The sun, Full Spectrum, CMH/LEC, RED/Blue LED, HPS dual spectrum

The Sun is the ultimate light source in that it has all of the colours necessary for plant growth at high levels. Therefore the proportions of Blue, Green and Red are balanced.

The Full Spectrum LED and the CMH have similar proportions of Blue, Green and Red to the Sun.

The Red/Blue LED has approximately 4 times more Red than Blue and almost no Green.

The HPS ‘Dual Spectrum’ Bulb emits mostly Red and Green with almost no Blue.

So what is the best grow light spectrum? Let’s see what the plants require first and than see which spectrum is most suitable.

The ideal spectrum for a plant will be different by species. Also the requirements of the plant will vary at different stages of growth. For example: plants perform better in the early stages of growth (known as vegging) with more blue light than in the later flowering stage when they prefer more red.

However McCrees action spectrum is the most used reference for the ideal spectrum for the healthiest and most productive growth.

What is McCree’s action spectrum and relative quantum efficiency?

In the 1970s a scientist called McCree determined the wavelengths of light a plant can absorb. He assessed the relative quantum efficiency of different wavelengths of light on photosynthesis. In other words he recorded the rate of growth of plants exposed to each wavelength or colour of light.

McCree discovered that plants use all parts of the PAR spectrum but at different rates. The quantum efficiency for red is close to 100% efficient whereas blue and green were less efficient, ranging from 65% to 75%. McCrees action spectrum identifies what colors of the visible spectrum are best for photosynthesis.

 

Why More Blue for Vegging and Red for Flowering?

Having a minimum of 15% blue in the light spectrum is known to prevent plant stretching (getting very tall but not necessarily more productive). Having a lot of Blue light in the spectrum (60%+) is known to inhibit growth and productivity. So a grow light used for seed to harvest growth needs somewhere between 15% – 60% Blue light.

Example: High Pressure Sodium (HPS) lights have only 3% blue light in a their grow light spectrum so are not used for the early vegging cycle as this would result in stretched plants and low productivity later on.

Red is the ideal spectrum for stem growth, flowering and fruiting. A grow light used for flowering will typically have a minimum of 40% Red light in it’s spectrum. Example: HPS has 45% Red in it’s spectrum, ideal for flowering.

Green light is also used for productive growth. The addition of Green light is known to improve penetration into the canopy and improves the productivity of the Red and Blue light through the Emerson effect.

Example: HPS has 52% Green in it’s spectrum, ideal for flowering.

The spectrum output of the most common grow light sources represented in % Blue, Green and Red for comparison purposes

A table of the % Blue, Green and Red light in the spectrum output of common grow light sources: The sun, Full Spectrum, CMH/LEC, Red/Blue LED, HPS dual spectrum

What is a Full Spectrum Grow light?

A ‘Full Spectrum’ light contains all of the colours of light in similar proportions to the Sun. True Full Spectrum light is only really available for Grow lights in the last few years due to developments in technology.

The new COB (Chip On Board) LED technology produces ‘Full Spectrum’ light. MIGRO uses COB technology and has balanced the spectrum output of the light to match the natural provider of light for growing, the Sun. It has sufficient Blue light for vegging growth (15%), Red for flowering (40%) and Green (45%) for maximum canopy penetration and healthy growth.

Ceramic Metal Halide (CMH, also known as Light Emitting Ceramic, LEC) also produce a full spectrum light. The CMH or LEC are a High Intensity Discharge (HID) light, similar to High Pressure Sodium (HPS). They are a development of an old technology, more expensive to run and inefficient compared to COB technology. However, they do produce and Full Spectrum light and there are two bulbs available for flowering (Philips 942 detailed in the graph) and a bulb for vegging (Philips 930).

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Watch Grow Light Spectrum Comparison Video Here:

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