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What's the Difference Between Lumen, Candela and Lux?

Light radiates as a wave in free space. It reflects off surfaces and builds a human vision of a subject. But how is light actually measured? What factors are affecting the measurement result? It is essential to understand the differences of light measurement parameters and how it applies in actual context.

Basics: Light measurement and photometry

The objective of light measurement applications is widespread across various industry applications, mainly research and production. Back in the day, people commonly used the term watt to measure the brightness of a light, but it was improper since watt is the unit of power, and in the high-efficiency light era, it does not really describe brightness. In modern engineering, more accurate terms are used for a better approach and to meet the measurement needs for every aspect.

Light Measurement Units: Lumen, Candela and Lux

Light sources radiate electromagnetic energy in all wavelengths to the surroundings, but only light with a 400-700nm wavelength is visible. The variousness of wavelength is related to the color perceived. In light measurement, it is vital to detect the distribution of the visible light waves in the portion of the electromagnetic spectrum. Since human eyes are only sensitive to the visible ones, the light measurement only focuses on light radiated within this range of radiation. This is known as photometry.

Photometry is widely used in the industries developing devices with displays or panels. It consists of three basic measurement parameters. Therefore, it is crucial to understand their differences to determine which one suits the requirement and what products are relevant for measurement.

What are the common light measurement terms and units?

Below are the photometry parameters and units that are commonly used to measure light:

Luminous Flux (measured in lumens/lm)

Luminous Flux represents the total power of visible light emitted in all directions per unit of time. This power—indicated as light output—might vary depending on the light source's energy efficiency.

Lumen
Lumen (lm) is a measurement unit of a luminous flux or luminous power. One lumen equals the amount of light emitted by a light source (radiating equal amount of light in all directions) through a solid angle of one steradian with an intensity of 1 candela.

Luminous Intensity (measured in candela/cd)

Luminous Intensity is defined as the amount of visible light emitted at a specific angle. It directly affects the visibility of light and is mostly used on devices that produce focused light.

Candela
Candela (cd) is a measurement unit in SI of luminous intensity. Candela replaced the older unit that was used to express luminous intensity—candlepower. One regular candle emits approximately 1 candela of luminous intensity, this is why candela was also called candle in older times.

Illuminance

Illuminance is the luminous flux per unit area. The common measurement term is Lux (lm/m2) for a standardized unit or footcandle (lm/ft2). This parameter is independent of surface geometry where the light falls on but is strongly related to how much area was illuminated.

Lux
Lux is the unit of illuminance, equal to one lumen per square meter.
Common Light Measurement Terms and Units (Photometry)
Credit: SILVERSUN

Luminous flux indicates measurement at a distance in all directions from a light source, while luminous intensity focuses on brightness in a single direction. In other words, lumens is better used when comparing the omnidirectional light sources such as ceiling lights, where candela is better for the unidirectional ones.

Meanwhile, to measure how bright the light is going only on a specific area, illuminance (Lux) would be a better fit to describe the properties.

What factors can affect actual luminous intensity and illuminance?

Luminous flux is the measurement of total perceived power emitted in all directions by a light source. This is a constant value. However, the actual luminous intensity and illuminance values can be affected by distance, tilt angle of the fixture, and beam angle.

Distance

Luminous intensity is inversely proportional to the square of the distance. This is because the intensity decreases from the center of the beam angle towards the edge as photons are spread over a wider area. Then, if the distance of the light source is to the subject changes, the illuminated surface area will be proportional to the square of the distance. This means the illuminance is inversely proportional to the square of the distance as well.

The Relationship Between Distance and Luminous Intensity
Credit: BBC

Tilt Angle

The tilt of the surface could affect the amount of light received by the subject at the same distance. The tilted surface would receive less light due to reduced illumination by a factor equal to the cosine of the angle between a light source and the normal surface. The smaller the illuminated area, the bigger the illuminance will be.

Beam Angle

The beam angle is the angle between the two opposed directions on the downward directions for which the luminous intensity is half that of the maximum luminous intensity.

The Relationship Between Beam Angle and Luminous Intensity
Credit: USAI Lighting

A smaller beam angle means more focused light. The same light output would result in a higher maximum luminous intensity and brighter light, even though the luminous flux remains unchanged. Besides, a larger beam angle will provide a bigger illuminated area and smaller illuminance for the same luminous flux value. Hence, both luminous intensity and beam angle affect the luminous flux proportionally.

Conclusions

Luminous flux is the energy emitted by visible light within all directions, measured in lumens. The luminous intensity only counts in a direction and specific angle, measured in candela. Illuminance is the luminous flux per unit area measured in lux.

These photometry parameters represent different characteristics. For a relevant comparison, the correct parameter should be used. Lumens should describe the light power in a wide coverage such as indoor lightings or outdoor spotlight. Candela should determine the luminous flux related to a specific angle. It’s also useful to measure a directional light source such as a flashlight. Lux should be used to determine how much light is required in an exact area to build the desired environment.

The main factors that could affect the luminous intensity and illuminance are distance, tilt angle, and beam angle. This is related to the characteristics of lightwave, where those factors affect the area of illumination.

A lot of things have to be considered to decide the light products that meet the needs. Besides the specification listed, we should also ensure the appropriate installation methods (angles, distance, etc.) and the existing lighting environment.

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