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If you're looking for lighting systems, you've probably seen these lighting-related terms when you're looking at LED light specifications, and these include lumens, lux, candela, PAR, PPF, PPFD, and PPE. We first understand the meaning of candela, lux, and lumens.

Candela, Lux, and Lumen are all units of luminosity. These units are related to people and are based on human vision. Because people have different sensitivities to different wavelengths of light and change in different brightness environments, our definition of these units applies only to humans and not to other creatures.

Candela

Candela(symbol: cd) is a unit of luminous intensity, which is the luminous intensity of a light source in a specific direction. An ordinary candle emits light with about 1 cd luminous intensity. The candela measures the amount of light emitted in the range of a (three-dimensional) angular span. Since the luminous intensity is described in terms of an angle, the distance at which you measure this intensity is irrelevant. It tells you how bright the light source is which shows how far away from an object you can be while still being able to see it. For example, laser pointers have extremely low lumens, but very high candela ratings because laser pointers don't emit a lot of light, but can be seen from a great distance.

Lux

Lux(symbol: lx) is a unit of illuminance. An object that is uniformly illuminated by light has a luminous flux of 1 lumen on an area of 1 square meter, and its illuminance is 1 lux. Lux measures the plane illuminance and how much light is illuminating an area at a given distance.Illumination can reach 10,000–25,000lux in full daylight; In an office requires 320-500lx to provide good illumination.

Lumen

Lumen(symbol: lm) is a unit of luminous flux. It refers to a point light source with a luminous intensity of 1 candela (cd), and the luminous flux emitted within a unit solid angle (1 steradian) is “1 lumen”.The lumen answers a simple question: how much light does this source produce? Are lamps with higher lumens, brighter?Yes, there is a proportional relationship between lumens and brightness, all other things being equal. But lumens aren't the only determinant of brightness.

Lumens vs. Candela

The lumen (unit lm) gives the total luminous flux of a light source by multiplying the intensity (in candela) by the angular span over which the light is emitted. With the symbol Φ for lumen, I for candela and Ω for the angular span in steradian, the relation is:
Φ=I⋅Ω
A light source with a luminous intensity of 1 candela (total luminous flux) is 4π (about 12.56) lumens.

Lumens vs. Lux

The relationship between illuminance and lumens is Lux = luminous flux / illuminated area.
With the same lumen, that is, the same light flux, with the different illumination areas, the illuminance value is also different. The larger the radiation divergence area, the lower the illuminance. And vice versa, the more focused the light source, the higher the illuminance on the focal area.

Candela vs. Lux

The difference between lux and candela is that lux measures the illuminance of a surface, not the illuminance of an angle. The end result is that the distance of the surface from the light source becomes an important factor: the farther the surface is from the light source, the less it will illuminate

Terms	Short name	Unit	Unit symbol	Definition
Luminous flux	F	lumen	lm	It’s a physical quantity that represents the optical power and is an indicator that represents the overall brightness of the light source
Illuminance	E	lux	lx	It is the total luminous flux incident on a surface per unit area
Luminous intensity	I	candela	cd	It’s the quantity of visible light that is emitted in unit time per unit solid angle.

A simplified way to remember the differences between the three terms is ...
Lumens - how much light is produced.
Lux - how well illuminated your surface will be.
Candela - visible intensity of the light source from one direction.

Lux, lumens, and candela can tell you how bright the lights are in your room. But these measurements don't tell you anything about what this light can do for your plants. If you want to know more about the role of plant lights, you can pay attention to these parameters: PPF, PPFD, PPE, PAR.