When researching lighting and especially grow lights, you will inevitably stumble across some common metrics and abbreviations. Some of them are more advanced while others are pretty common. Assuming you aren’t an engineer or a horticulture scientist, we’ll briefly explain the most common terms used in lighting and plant grow lights.
CCT: Correlated Color Temperature (Kelvin)
The color temperature or correlated color temperature (CCT) quantifies a white light's perceived warmth and is measured in kelvin (K). The CCT ranges from around 1700 K (redish / warm) to 12000 K (blueish / cold) in practice.
Coverage defines the grow light’s ability to illuminate an area effectively. It is often compared using PAR maps (also called PPFD footprint charts or PPFD maps) that contain different measurements across an area at a certain height of the grow light.
DLI: Daily Light Integral (mol/m²/d)
The daily light integral (DLI) combines light intensity (PPFD) with the lighting duration (photoperiod) over a 24 hour window and is measured in the unit of mol/m²/d (i.e. light per area per day). To learn more, we recommend our detailed article.
ePAR: Extended Photosynthetically Active Radiation
Extended photosynthetically active radiation (ePAR) values radiation in the form of light in wavelengths from 400 to 750 nm. This extension also includes some infrared (IR) radiation in addition to the common PAR spectrum which is light from 400 to 700 nm.
Illuminance (Lux or Foot-candle)
Illuminance is a measure for light intensity, meaning the total amount of light hitting a surface area. The standard unit to measure illuminance is lux (lx) which is defined as lumen per square meter (lx = lm/m²). The imperial unit for illuminance is foot-candle (fc). It is defined by lumen per square foot (fc = lm/ft²). 1 lx is equal to 0.0929 fc or 1 fc is equal to 10.76 lx respectively.
Illuminance is valued as it is perceived by the human eye. The corresponding measure for plants is called photosynthetic photon flux density (PPFD). To learn more, we recommend our detailed article.
Infrared (IR) light describes electromagnetic radiation between 700 nm and 1 mm. IR radiation can be felt as heat (e.g. when you're exposed to sunlight). Some plants may also use near infrared (NIR) in the range of 700 to 1400 nm to grow. IR light cannot be seen by the human eye.
Luminous Flux (Lumen)
The total light output of a light source is described as luminous flux or luminous power. It is measured in lumen (lm). While the luminous flux of a lamp stays the same, the illuminance increases the closer a surface is in respect to the light source.
Lumen is a measurement for humans and light is valued as it is perceived by the human eye. The synonym in lighting for plants would be PPF. To learn more, we recommend our detailed article on the difference between Lumen and PPF.
The McCree Curve represents the average photosynthetic response of plants to light energy and was researched and published by Dr. Keith McCree in the 1970s. The usage of the McCree curve can be ambiguous because the shape of the curve can vary depending on the species of plant, the stage of growth, and the environment.
PAR: Photosynthetically Active Radiation
The range of wavelenths of light which is used by plants to grow is referred to as photosynthetically active radiation (PAR). The PAR spectrum contains wavelengths from 400 nm to 700 nm. The amount of PAR reaching a surface, e.g. the plant's leaves, is measured in PPFD, which stands for photosynthetic photon flux density. To learn more, we recommend our detailed article.
The photoperiod describes the duration of daily illumination received by a plant. It is commonly used to describe grow light schedules, meaning how many hours a day a grow light is turned on. This is why we measure the photoperiod in hours (h).
Photosynthesis is the process in which plants transform light, water and carbon dioxide into oxygen and energy in the form of sugar. This is the key to how plants actually grow.
Power Draw (kW/h)
Power Draw describes the electrical power consumption in watt (W). Your electricity is generally paid in kilowatt per hour (kW/h) meaning that a 1000 W (one kilowatt) grow light turned on for an hour consumes one kilowatt-hour.
PPE: Photosynthetic Photon Efficacy (µmol/J)
Photosynthetic photon efficacy (PPE) is measured in µmol/J. It describes a grow light's output in the PAR spectrum (µmol/s) per watt (W = J/s) of electricity used. The higher the PPE, the less you spend on electricity for the same yield.
PPF: Photosynthetic Photon Flux (µmol/s)
Photosynthetic photon flux (PPF) is measured in µmol/s. It describes the total output of a light source within the PAR spectrum. While the PPF of a lamp stays the same, the PPFD increases the closer a surface is in respect to the light source. The PPF changes only, if its power intake is modified (e.g. by dimming) or the light's efficiency changes (e.g. due to aging).
The synonym in lighting for humans would be lumen. To learn more, we recommend our detailed article.
PPFD: Photosynthetic Photon Flux Density (μmol/m²/s)
Photosynthetic photon flux density (PPFD) measures all photons (light particles) within the PAR spectrum in micromoles per square meter per second (μmol/m²/s) and is the most relevant measure for plant lighting. The synonym in lighting for humans would be lux. To learn more, we recommend our detailed article.
A grow light’s spectrum describes the ratio of the different wavelengths (or colors) of light it emits. Plants “see” light differently than the human eye and require more of some wavelengths for biochemical processes such as photosynthesis. In order to quantify the amount of light within the right spectrum correctly, photosynthetically active radiation (PAR) – meaning “light for plants” – is measured as PPFD.
Ultraviolet (UV) light describes electromagnetic radiation in the range of wavelengths between 100 nm and 400 nm and is divided into three bands: UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm). Most UV light cannot be seen by a human eye.
Light is electromagnetic radiation that can be seen by humans. Light, as it also behaves like a particle (photon) travels in waves. The wavelength describes the distance between the repetition of these waves and ultimately defines its color. The wavelengths is a unit of distance and measured in meters. Since visible light has a relatively short wavelength we use nanometers (nm), which is one billionth of a meter, as a common unit of measurement. Sometimes also micrometers (μm) or millimeters (mm) are being used.
YPF: Yield Photon Flux (μmol/s)
Yield photon flux (YPF) measures the total light output of a light source weighted by the plant's light response defined by the McCree curve. The more common metric is photosynthetic photon flux (PPF) which weighs light equally at every wavelength.
YPFD: Yield Photon Flux Density (μmol/m²/s)
Yield photon flux density (YPF) measures light in micromoles per square meter per second (μmol/m²/s) weighted by the plant's light response defined by the McCree curve. The more common metric is PPFD which weighs light equally at every wavelength.
If you want to measure any of the above (or at least, what's most important: PPFD and DLI), we highly recommend our Grow Light Meter app. It allows you to quickly and easily measure PAR as PPFD, the daily light integral as well as calculate your plant's ideal lighting – and all of this on your phone or tablet.
Still unclear? Or did we miss something? Simply reach out to us via the contact option. We hope this article helps you to understand more of the grow light science – and maybe even impress your friends at parties :)