Terms from the world of lighting and horticulture

SPD = Spectral Power Distribution

In simple words:

The lighting fixture gives us light that is composed of several colors.
The SPD shows us what percentage of that light is used by each color.
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Luminous efficacy
(lm/w, µmol/w, µmol/j, PFE, PPE, PPFE)

In simple words:

How many light units are produced for each unit of electricity. The more units of light received from 1 unit of electricity, the more efficient the lighting fixture is. The lighting fixture produces the same light for less electricity.
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CCT = Correlated Color Temperature

In simple words:

Light color. Defined in Kelvin temperature degrees. The higher the number the cooler\"whiter" the light color. 6000 degrees Kelvin resembles the light color of a common florescent bulb, 3000 degrees Kelvin is a warmer white color and resembles the light color of an incandescent bulb. Moonlight is 4100 degrees Kelvin.
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CRI = Color Rendering Index

In simple words:

How the light source shows colors compared to sunlight. An artificial light source will always show colors differently compared to a natural light source. The CRI indicated the percentage of the correctness of colors compared to sunlight. A light source with a CRI of 90 shows colors at a correctness level of 90% compared to sunlight.
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PAR = Photosynthetically Active Radiation

In simple words:

The section of the light spectrum between 400 and 700 nanometer, which plants use for photosynthesis.
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DLI = Daily Light Integral
(mol/m2/d)

 

In simple words:

The amount of light in the PAR range that an area receives in 24 hours.
Consists of the intensity of lighting multiplied by the hours of lighting. Each plant has its optimal DLI, and from it a lighting solution is designed, that will provide sufficient energy levels according to the number of hours of illumination we work with. Values are usually imprecise, because they vary according to the seasons and clouds.
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PPF = Photosynthetic Photon (µmol/sec) Flux

In simple words:

How many units of energy in the PAR spectrum the light source produces in one second. We won’t know from this parameter how much energy reaches the plant, because that is also affected by physical conditions. When we know how many units of energy the lighting fixture produces we can compare with the electricity consumption and see how efficient the lighting fixture is.
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PPFD = Photosynthetic Photon Flux Density (µmol/m2/s)

In simple words:

How many units of energy in the PAR spectrum reached the plant at the measuring point. Using this information we can know how much energy the plant actually receives, and whether or not the lighting fixture can effectively light the facility.
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Circadian rhythm

In simple words:

Our biological clock. The rhythm of mechanisms that operate over 24 hours in humans. Sleep, waking, melatonin production, serotonin production. Our levels of functionality change according to these mechanisms.
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LST

In simple words:

Leaf Surface Temperature


VPD = Vapor Pressure Deficit

In simple words:

Vapor Pressure Deficit is the difference (deficit) between water pressure (saturation) in the leaf and the air surrounding it, it is expressed in pressure units Kilo Pascal. The measurement in both cases make use RH and temperature (leaf or air). A healthy leaf functions in 100% humidity, different than the air around it. In simple works the VPD expresses the deficit in water vapor between a saturated leaf and the air.
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RH

In simple words:

Relative Humidity
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Transpiration

In simple words:

Transpiration is the process of water movement in a plant. From the substrate to roots, stem, leaves and to the air. As opposed to normal evaporation from a regular water body, transpiration is a physiologic processes controlled by the stomata.
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Photoinhibition

In simple words:

A phenomenon that occurs from over lighting (there are more causes) and decreases the plants photosynthesis efficiency. PS2 is more sensitive to the phenomenon.
In general, in many cases it is a term used to describe and damage to photosynthesis while the plants are exposed to light.
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BTU/H = British Thermal Unit

In simple words:

The amount of heat needed to raise one pound of water by one degree Fahrenheit.
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Wavelength

In simple words:

The distance between two identical points on the shape of a wave. The peak, the lower part of the wave or the crossing of the 0 axis are examples of measuring points for a wave length. It is measured in Nano-meters (Nm).
The shorter the wavelength, the more energetic it will be, like x-rays for example that are very short and can penetrate our body's tissue.
In agronomic lighting we mostly use wavelength to describe light colors plants "see".
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Lumen

In simple words:

Light output. The number of light units received by the human eye, that our light source produces.
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Lux

In simple words:

The number of lumens per square meter. The lux measurement already expressed losses, reflections, disturbance, distance etc.
It is the measurement that tells us how many light units that can be received by the human eye reached the point in which we are measuring.
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