Choosing Your Grow Light: how light is measured
Deciding which grow light system to use for your cannabis grow room is a mine of choices. HID? LED? T5? CFL? and apart from these main variations, there seems to be different means of measuring the light outputted by each system – so how do you compare say one LED grow light from one company, with an HID bulb manufactured by another, with a T5 from Quantum? Many thanks to Midnight Toker for this explanation of how light is measured and the different terms used :)
How is Light Measured?
The “color” of light sources comes from a complicated relationship derived from a number of different measurements, including correlated color temperature (CCT) or Kelvin temperature (K), color rendering index (CRI), and spectral distribution (PAR Watts). However, color is most accurately described by a combination of Kelvin temperature and CRI.
Color Rendering Index – CRI
CRI is a numeric indication of a lamp’s ability to render individual colors accurately. The CRI value comes from a comparison of the lamp’s spectral distribution to the standard (e.g. a black body or the daytime sky) at the same color temperature. The higher the CRI the more natural and vibrant the colors will look. A bulb with a CRI of 85 or higher is excellent being that the sun has a CRI of 100. Eye Hortilux makes 90 -92 CRI bulbs that are used in aquarium, horticulture and other applications such as the 400W Eye Hortilux Blue 90CRI and 1000W Eye Hortilux Blue 92CRI. Standard Metal Halide bulbs have a CRI of about 70, so only 70% of colors will be rendered correctly. HPS bulbs have a CRI of 22.
What is the Color Temperature or “K” – Kelvin Rating?
The K rating is a generalized form of addressing the color output of a Light Bulb. Color Temperature is not how hot the lamp is. Color temperature is the relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin. HPS has a warm (red) color temperature of around 2700K as compared to MH at 4200K, which has a cool (blue) color temperature. The higher the kelvin temperature gets, the bluer. 10k lamps seem to be a nice crisp white, while higher kelvin can go from a blue/white to very blue and lower kelvin seem more like that of sunlight (6500k). Metal Halide bulbs go up to 20,000K (commonly used in aquariums) providing the bluest light.
What is Spectral Energy Distribution & PAR Watts?
The total visible spectrum is perceived by us humans as white light, but the “white light” is actually separated into a spectrum of colors from violet to blue, to green, yellow, orange and red made up of different wavelengths. Plants use the blue to red part of the spectrum as their energy source for photosynthesis. The different combinations and the relative intensity of various wavelengths of light determines the CRI of a light source.Only part of solar radiation is used by plants for photosynthesis. This active radiation Photo synthetically Active Radiation (PAR) contains the wavelengths between 400 and 700 nanometers and falls just within the visible spectrum (380 – 770nm). Thelight in this region is called PAR watts when measuring the total amount of energy emitted per second. PAR watts directly indicates how much light energy is available for plants to use in photosynthesis.
What is the Lumen Measurement?
Lumen is a measurement of light output. It refers to the amount of light emitted by one candle that falls on one square foot of surface located at a distance of one foot from the candle. Traditionally, lumens have been the benchmark of a lamps ability to grow plants; meaning the brighter the lamp the better the plant. However, studies have shown that a broader color spectrum lamp will perform much better than a lamp with high lumen output, especially when it comes to plant growth.
Lighting Spectrum and Photosythesis
The most common mistake people make with plants is to not understand photosynthesis and the visible spectrum of lighting that affects plant growth. Most people choose lighting solely based on the Kelvin temperature of a bulb. This tells you very little about what type of light within the spectrum is being emitted and at what strength. Visible light is on a scale in nanometers (radiated wavelength) from 400nm (violet) to 700nm (red). Simple matter of photosynthesis: plants can only utilize light that is absorbed. Bright light is essential yet only a portion of this white light is used for photosynthesis. The blue and red zones of the visible spectrum are the most beneficial to plants. Green plants appear green because it is reflected light. How “bright” a light appears has more to do with how much light is output in a given area visible to the human eye, with “brightness” being at a maximum in the green spectrum (middle of visible spectrum, or around 550nm