Friday, February 28, 2014

What Unit of Measure is the Most Important When Discussing Plant Lighting?

People always ask us, in terms of plant lighting, what is the most important unit of measure?  Lumens, lux, footcandles, PAR, etc.?  Well, the answer is PAR (photosynthetically active radiation) and here is a great article from Gavita that explains why:

Lux vs.  PAR

What is the best unit for light, used for plant growth?

Most lighting products are designed for the use of people. These products will use the term lumen and Lux. The term Lux comes from the photometry based on the average sensitivity of the human eye. The maximum sensitivity is around 555 nm (yellow light) and decreases with longer (red) and shorter (blue) wavelengths.

In practice, often still the term Lux is used.But does this apply to today's greenhouse? Plants have a different sensitivity to light of different wavelengths. In green plants, the sensitivity to red light is highest and green the lowest. Therefore, the term Lux is not right for the exposure of green plants. Light sources can be considered as sources of energy particles, which are called photons or light quants.

Why is the term micromoles (μmol) used in the case to specify lamps?
Research shows that the growth of plants is determined by the number of photons between 400 and 700 nm that they absorb. This is called the Photo Synthetic Photon Flux (PPF). The higher the PPF value per watt, the more efficient the light source is for plant growth.

Grow light is an important factor for many breeding plants. Photosynthesis and ... growth must be balanced to achieve optimum production.

When investing in assimilation lighting consider the 3 following factors; added grow light, the quality of light and the lighting installation heating temperature.

Since the end of 2004 lamps have been specified in μmol. All guarantees of lamps are issued in μmol values.

This is marked with the short term PAR and is the only reliable measure to indicate whether a lamp suitable for photosynthesis.

The higher the PAR values of the lamp, the more efficient the lamp. Therefore we specify for all grow lamps the PAR value, expressed in micromole photons per second (μmol / s).
What is a micromole (μmol)?

"Micromole" is a collective term for a certain number of light particles called photons.
Micro (μ) = 10-6 and mol = 6.02214 x 10 ² ³ photons (Avogadro number).

Micromole is 6.02214 × 10 ² ³ x 1017 photons. One micromole = 602,300,000,000,000,000 photons or light particles.
Which light spectrum gives optimal growth?

This is very dependent on the application and the crop that is grown. The greatest efficiency of photosynthesis is in the wavelength between 600 and 630 nm (orange area). For some crops it is important with blue light spectre to avoid the plant stretching. Overall, the proportion of blue in natural day light is enough to this kind of stretching. That is the reason to why grow light manufacturers strive to imitate the natural sun light. When growing without day light (e.g. when it comes to climate rooms or multiple layers cultivation) a light source selected with a higher proportion of blue, such as a metal halide lamp (HPI-T or HQI-BT) is highly recommendable.

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