Posted on by Alex Russell

Colour Temperature and Colour Spectrums

This guide is a continuation from Part 1, which discussed the different varieties of grow lights and the forms they can be commonly found in. Part 2 of the horticultural lighting guide will discuss Colour Temperatures and Colour Spectrum, how it effects the growth of plants and how you can use it to maximize your plants potential.

What is Colour Temperature:

By definition, Colour Temperature is the method of measuring a light source of a black-body radiator which radiates light of a colour comparable to that of the light source. Colour Temperature is measured in the form of Kelvins, using the symbol K, a unit of measure for absolute temperature.

Colour Temperature or Correlated Colour Temperature (CCT) will dictate the visible range of colours that will be produced by the light source, which can be seen and measured on a colour spectrum. The colour temperature will only determine the range of colours that will be produced across the spectrum, but not the intensity or the quantities of each colour of light produced.

To simplify this, colour temperatures when used for light bulbs and lamps are generally are found to be between 1000 K and 10,000 K. A Lamp in the low end of the colour temperature scale, 1000 K up to around 3000 K, will produce a warm, red to yellow light, such as a candle flame or sunset / sunrise at around 1850 K, up to a warm “soft white” light around 3000 K. This lower end of the scale is generally thought of as being a “warm” lighting source, producing a calm, cozy and intimate colour setting if used in a household environment.

A light source producing light in the range of 5000 K to 6500K will provide a clear cool white lighting, like that of daylight, creating a bright and vibrant light source. An example of this would be the Sun when producing vertical daylight, which is approximately 5800 K.

Light producing a colour temperature of 7000 K to 10,000 K will be projected as a bright blue light, just like that of a clear blue sky. Light produced by the sun throughout a regular day will be seen to move from the lower Kelvin range as the sun rises, through to a higher Kelvin rating during clear blue skies and finally back down to the lower ratings as the sun sets in the evening. While the Sun produces a colour temperate of around 5800 K, this range of colour temperatures changes seen from earth throughout the day is due to various external factors, such as Rayleigh Scattering and the Tyndall Effect.

Colour Temperature Scale - A-Grade Hydroponics

Contradictious to the cultural understanding of “warm light” as being red/orange/yellow in colour, this is the coolest colour temperature and “cold, white or blue” light is the hottest colour temperature, just to confuse you.

Colour Temperature and Plants:

Now that we have a basic understanding of what colour temperature is, how does it affect plants?

From Seedling to harvest, throughout the life cycle of a plant, different colour temperatures or colour spectrums can be utilized for optimized growth during certain phases of a plants life. For example, growers have found greater success growing a young plant under cool white 6K lamps all the way through a growth (vegetative) cycle, then switching to a 2K lamp to flower or fruit, and finally finishing the growth cycle using a 10K lamp for the final 2-3 weeks. This has shown to generate better results compared to growing under a single colour spectrum / colour temperature.

When using different lamps with different colour temperatures for plant growth, it is important to keep in mind that the colour temperature rating should be used as a general guide for the approximate colour spectrum that will be produced by the lamp, the actual colour spectrum is what you should look for when choosing your light. The Colour Temperature will tell you the range of colours the lamp will produce, but not the exact amount of each colour produced within that range.

You should always be able to find a representation of the colour spectrum produced by a light from the manufacturer, as good lamp manufacturers will supply this information about their product, steer clear of brands that do not provide this information.

Growing using a single Colour Temperature Lamp:

Using a variety of different lights and colour temperatures are not required to achieve great results, but better results can be achieved with interchangeable spectrums if you have the option to do so, granted this will require the right equipment or lighting in the first place.

Many of the industry standard horticulture lighting options, such as HID (High Intensity Discharge) HPS Lamps commonly produce an orange to yellow colour light at a colour temperature of about 3000-4000~ K, which is perfectly suitable to produce healthy, large plants with just a single lamp, from seed to harvest.

The latest trends in horticultural lighting are moving towards the use of LED lighting systems, which can produce a strong full colour spectrum and even allow the grower to select the colour spectrum they want to use, helping you to grow plants more efficiently with just a single lighting system.

For those who grow outdoors, using the Sun as their light source, are technically using a variety of different light spectrums throughout the day, which will differ from day to day, depending on a variety of conditions.

The Sun Colour spectrum and temperature - A-Grade Hydroponics

So, there you have it, hopefully this guide has given you a basic understanding of what Colour Temperature and Colour Spectrum are and how this relates to plant growth.