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.
Color Temperature (CCT)
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.
Daily Light Integral (DLI)
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.
Efficacy is used to describe the grow light's ability to turn electricity into light for photosynthesis. It is measured using photosynthetic photon efficacy (PPE) in µmol/J or, more generally, in lumen per watts (lm/W). In general, the higher the efficacy, the better.
Extended photosynthetically active radiation (ePAR) values radiation in the form of light in wavelengths from 400 nm to 750 nm. This extension also includes infrared (IR) radiation in addition to the common PAR spectrum which is light from 400 nm to 700 nm.
Foot-candle measures illuminance in lumen per square foot (lm/ft²). This measurement is most commonly used in countries using the imperial system, such as the United States. The corresponding measure in the metric system is called lux. To learn more, we recommend our detailed article.
Illuminance is a measure for light that hits a surface and is measured in lumen per square meter (lm/m²). Light intensity is valued as it is perceived by the human eye. The corresponding measure for plants is called photosynthetically active radiation (PAR). To learn more, we recommend our detailed article.
Infrared (IR) light describes electromagnetic radiation between 700 nm and 1000 nm. IR radiation can be felt as heat (e.g. when you're exposed to sunlight). Some plants may also use IR radiation to grow.
The total light output or luminous flux of a light source is measured in lumen (lm). While the lumens of a lamp stay the same, the illuminance (lux or foot-candle) 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.
Lux measures illuminance in lumen per square meter (lm/m²). The corresponding measure in the imperial system is called foot-candle (fc). 1 lux is equal to 0.0929 fc or 1 fc is equal to 10.76 lux respectively. Lux 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 PPFD. To learn more, we recommend our detailed article on the difference between Lux and PPFD.
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.
mol/m²/d is the unit of the daily light integral (DLI). It is used to measure PAR per area per day.
Light as it is perceived by plants is referred to as photosynthetically active radiation (PAR). The PAR spectrum contains all the light with 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.
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 describes the electrical power consumption in watts. Your electricity is generally paid in kilowatts per hour (kW/h) meaning that a 1000 W (one kilowatt) grow light turned on for an hour consumes one kilowatt-hour.
Photosynthetic photon efficacy (PPE) describes a grow light's PAR output per Watt of electricity used in the unit of µmol/J. The higher the PPE, the higher is the efficacy of a grow light.
The photosynthetic photon flux (PPF) 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 synonym in lighting for humans would be lumen. To learn more, we recommend our detailed article.
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 that cannot be seen by the human eye but is used effectively by some plants. It is defined as the range of wavelength 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).
µmol/m²/s is the unit of the photosynthetic photon flux density (PPFD). It specifies PAR per area per second.
µmol/J is the unit of the photosynthetic photon efficacy (PPE). It specifies PAR per joule (= PAR per watt per second).
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.
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 PPF which weighs light equally at every wavelength.
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.
We hope this article helps you to understand more of the grow light science – and maybe even impress your friends at parties :)