What Is Light Spectrum?

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The light spectrum is the range of waves within the electromagnetic spectrum that is visible to us as humans. Measured in nanometres (nm), the visible spectrum sits in between 380nm to 750nm on the scale. This means the blue side (bottom) of the visible spectrum has a slightly shorter wavelength than the red side (top).

The sun emits solar energy, of which a really small portion winds up getting to Earth. This is because the ozone layer absorbs and reflects back the majority of the damaging waves, allowing life to flourish. Waves that infiltrate the ozone layer are in between 300nm - 1100nm, so the spectrum coming into the atmosphere is wider than we have the ability to see.

Outside of the visible light spectrum we have gamma waves, x rays, ultraviolet waves, infrared, microwaves and radio waves. Ultraviolet waves and infrared waves sit at either end of the visible spectrum meaning they are included in the wavelengths that penetrate the ozone. Both are beneficial for plants in some ways.

• UVC (180 - 280nm) - Includes damaging ultraviolet waves that are blocked by the ozone layer. Causes major burns.
• UVB (280 - 315nm) - Can burn skin however only small amounts reach earth. Higher elevation areas will have higher levels of UVB.
• UVA (315 - 400nm) - Always present in the atmosphere but not visible. 95% of ultraviolet waves getting through the ozone are UVA.
• Visible Light (380 - 750nm) - The range of the electromagnetic spectrum that we can see as colour.
• Infrared (700 - 1100nm) - Invisible to the human eye but can be felt as heat.

Why Light Spectrum Is Very Important

Cannabis plants need the right kind of light to grow to their full potential. Without the right spectrums of light, plants can underperform, growing small and generating reduced yields. Fortunately, indoor growing methods have enabled us to mimic the most important elements of the sunlight received by plants outdoors.

Outdoor plants receive the full spectrum of light supplied by the sun. In the spring, the spectrum of light that infiltrates the atmosphere creates a bluish tint. The blue spectra is what allows plants to develop strong foliage during the early stages of growth.

As the summer hits, the sun is higher in the sky and more light penetrates the atmosphere. By the time autumn comes the sun starts to dip, when the red spectra is most pronounced. Red wavelengths indicate the end of summer so plants grow flowers in an effort to pollinate before winter.

How Cannabis Plants React to Light

Marijuana plants can flourish well under blue and red lights provided they receive the right spectra within the colour band. By singling out these wavelengths, plants have better access to the kind of light they need. This is among the reasons why indoor plants can be grown smaller than outdoor plants, since they do not have to expand in search for the necessary spectra.

• Blue (400 - 500nm) - Blue light is necessary for plants to grow strong leaves and branches throughout its vegetative cycle. Providing high amounts of blue results in short, bushy plants, which can be beneficial for cultivators trying to prevent tall plants with long branches. Recommended for seedlings, especially if you are growing indoors.
• Red/Infrared (620 - 750nm) - Red is essential for expanding plant structure and promoting bud production. Naturally, plants utilize red light to identify when it is day or night. Small amounts of infrared make its way through the atmosphere during sunrise and sunset when wavelengths are longer. Cannabis plants grow taller under infrared until visible red becomes more direct.

Photoreceptors in a cannabis plant's leaves are constantly trying to comprehend the light spectrum surrounding them. Leaves will turn to face the light during the day. When there is no light marijuana plants save their energy by 'relaxing' their leaves, making them sag a little. Photoreceptors do not really turn light into energy, which is more to do with photosynthesis.

How a marijuana plant reacts to light will also depend on the strain and where it stems from. Some genetics have been brought from high elevation areas where the light is stronger and there is a higher presence of UVB rays. It has been argued that exposure to more UVB can increase THC production, although this theory is inconclusive and needs to be investigated further.

Finest Spectrum to Grow Cannabis

It is tough to say precisely which part of the colour spectrum is best but a big part of it has to do with the type of light your cannabis plants are grown under. Not all lights produce the same spectrum so choosing will depend on the outcomes you are trying to accomplish. The main thing is that your garden has sufficient blue light throughout the vegetative cycle and enough red light for blooming.

You might be intending to grow inside where you can have full control over the light. Or perhaps you're thinking of trying some outdoor plants, a lot of which will be determined by the sun. Whichever you choose, be sure your plants receive plenty of light.

• CFL (compact fluorescent light) lights are normally measured in Kelvin (K), which tells us the colour temperature of a bulb. Cooler, blueish lights have a higher Kelvin reading (6000 - 6400k), whereas a CFL that looks red will have a lower reading (2800k).
• LED (light emitting diode) - LEDs produce a narrow spectrum of light, either in a small band of blue or red. Red, blue and white LEDs are often combined to produce a 'full spectrum' emission that plants can comprehend for proper growth.
• HPS (high pressure sodium) - Yellow/red light can be used throughout the grow cycle, however plants will grow taller with more spacing between internodes. Cannabis plants benefit from HPS lighting mostly in the flowering stage.

If you are growing outdoors, you can anticipate plants to get larger than they would indoors since the spectrum is broader. Depending on where you live and the time of year, plants will respond to the light in different ways.

The colour spectrum of your grow light is necessary but if you wish to produce good harvests, the intensity is also necessary. The intensity of your light plays a big part in plant development and has to be powerful enough for plants to photosynthesise properly. If marijuana plants do not get sufficient light, it doesn't matter what spectrum they get, because they won't be able to function.