1. Introduction to Luminous Flux and Its Units

What is Luminous Flux?

Luminous flux is the total amount of light emitted by a light source in unit time. It is measured in lumens (lm) and is an artificial quantity based on the response of the human eye to light.

The Unit of Luminous Flux: Lumens

A point light source with a luminous intensity of 1 candela (cd) emits a luminous flux of 1 lumen per unit solid angle (1 steradian). Lumens refer to the brightness of a candle at a distance of one meter.

The Relationship between Luminous Flux and Luminous Intensity

Luminous intensity measures the amount of light emitted in a particular direction by a light source, whereas luminous flux measures the total amount of light emitted by the light source. The two units are related because luminous intensity is measured in candelas per steradian, and the total luminous flux of a light source is equal to the luminous intensity in candelas multiplied by the solid angle in steradians.

ANSI Lumens: Measuring Projector Luminous Flux

The unit of a projector’s luminous flux is ANSI lumens, which is a standard formulated by the American National Standards Institute. ANSI lumens measure the illuminance of each point on the nine intersections of the screen’s “field” shape, multiplied by the area, and then find the average value of the points. The higher the lumen value, the brighter the projector, and the more the need to turn off the light while projecting.

2. Human Perception of Light and Luminous Efficiency

The Influence of Color Perception on Luminous Flux Conversion

The human eye perceives light of different colors differently, which determines the conversion relationship between luminous flux and optical power. For example, for the most sensitive yellow-green light of 555nm, 1W=683lm, which means the power of 1W is all converted into light with a wavelength of 555nm, resulting in 683 lumens. However, for other colors of light, such as red at 650nm, the light of 1W is only equivalent to 73 lumens because the human eye is not sensitive to red light.

Maximum Conversion Efficiency of Yellow-Green Light

The maximum light conversion efficiency and calibration value is 683 lumens per watt for yellow-green light of 555nm, because the human eye is most sensitive to this light.

Conversion Efficiency of Other Colors and White Light

For white light, the conversion efficiency depends on the spectral structure, which is different for different light sources like LEDs, TVs, and sunlight. Measuring the luminous flux of an irregular luminous body requires an integrating sphere, which is a professional and complicated tool.

Luminous Efficiency of Electric Light Sources

Luminous efficiency measures how much luminous flux can be converted into 1 watt of electric power. For example, incandescent lamps can reach 1W=20 lm, but the rest of the energy is lost as heat or infrared.

3.Applications and Measurement of Luminous Flux

Convergence and Brightness of Light Sources

To make an irradiated point look brighter, not only must the luminous flux be increased, but also the means of convergence, which reduces the area and obtains greater intensity.Luminous Flux Measurement of Irregular Light Sources

Measuring the luminous flux of irregular luminous bodies requires an integrating sphere, which is a specialized and complex tool.

Importance of Luminous Flux in Lighting Design

Luminous flux is an important factor in lighting design as it helps designers determine the right light source and its location in the space. The desired brightness level in a space depends on its intended use and the amount of natural light available.

In recent years, the use of energy-saving lighting sources such as LEDs has become more and more common because they have higher luminous efficiency and longer service life. The luminous flux of these light sources can be further optimized by using reflectors, diffusers, and lenses to direct or scatter light in the desired direction.

4.In conclusion

Understanding the luminous flux and its units is very important for choosing the right lighting source and designing the lighting system. The conversion efficiency of different colors and white light depends on the spectral structure of the light source and the perception of color by the human eye. The measurement of luminous flux requires specialized equipment like integrating spheres, and its importance in lighting design cannot be overstated.

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