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What Is Luminous Flux?

Luminous flux refers to the radiation power.  

It can let the human eye feel, which is equal to the product of the radiation energy of a certain wavelength band and the relative visual rate of that wavelength band in a unit of time.

Because the relative visibility of human eyes to different wavelengths of light is different, when the radiation power of different wavelengths of light is equal, the light flux is not equal.


Luminous flux refers to the amount of radiation flux derived from the evaluation of human visual characteristics according to the international standards, which is indicated by Φ (or Φ r) Express.

The relationship between luminous flux and radiation flux is 

Km is the maximum value of spectral luminous efficiency, equal to 683lm / W. 

V ( λ) It is the standard spectral luminous efficiency function specified by the International Commission on Illumination (CIE).

λ: Wavelength, in fact, the human eye only reacts to visible light with a wavelength between 380nm and 780nm. Traditionally, we call light waves below 380nm ultraviolet (UV) and light waves above 780nm infrared (IR).

V( λ): It is called the relative spectral sensitivity curve of the human eye, also known as the visual function curve.

It is obtained by summing up many test experiences for human eyes. It describes the response of the human eyes to the light of different wavelengths.

Φ e λ Is the spectral density of the radiation flux.

The unit of luminous flux is LM (lumen); λ Is the spectral light apparent efficiency.

Lumens (LM) are luminous flux units of the international system of units (SI) and the American system of units (as). 

If you think of light as particles travelling through space (i.e., photons)

The luminous flux of the light beam reaching the surface is proportional to the number of particles hitting the surface in a time interval of 1 second.

1lm is equal to the luminous flux emitted by a point light source with a uniform luminous intensity of 1CD (candela) within a unit solid angle of 1sr (sphericity), i.e., 1 LM = 1 CD · SR

Luminous efficiency is the quotient of luminous flux and corresponding radiation flux, and the unit is LM / W.

What Is Luminous Efficiency(Lm/w)?

For compound radiation, the symbol is k, K= Φ/Φ e. 

Wavelength: λ 

The light efficiency of monochromatic radiation is K( λ).

It is called spectral optical efficiency.

The formula is  

Km is k( λ) The maximum value is called the maximum spectral optical efficiency.

According to the average results measured by national metrology laboratories in various countries, KM is located at a frequency of 540 × At 101hz( λ= 555 nm)

In 1977, the frequency adopted by the International Committee of Metrology was 540 × Maximum spectral optical efficiency of monochromatic radiation at 10Hz (km = 683 LM / W)

Under the dark vision, K ′ M = 1754 lm/ w

The spectral luminous efficiency function is also known as the visual function.

It refers to the wavelength at which the visual brightness feels equal under specific photometric conditions λ M) and λ The ratio of the two radiation fluxes is expressed by the symbol V( λ) express.

Luminous Flux Description 

Luminous flux is the radiant energy flux of a light source. It is also a physical quantity of vision caused by the human eye.

That is, the product of the radiated energy in a certain frequency band and the relative visibility of the frequency band in unit time.

The visual rate of human eyes to different wavelengths of light is different.

Therefore, the optical radiation power of different wavebands is equal, but the luminous flux is different.

The sensitivity of human eyes to brightness is related to color and is not uniform in the whole visible light range.

When the environment is bright, human eyes are most sensitive to light with wavelength x = 555nm (507nm when the place is dark). 

We define the relative visual acuity VS (555) = 1

When X is other values, VX (x) is less than 1

If the radiant power of monochromatic light with a specific wavelength x is p(x), and the relative visual acuity function is vs(x), the luminous flux can be defined as y(x)=p(x)*vs(x)

When p (x) is in watts, y (x) is in watts. Only when x = 555nm does one watt of optical radiation power produces 683lm (lumens) of luminous flux.

Since the visual function of human eyes to the light of different wavelengths is different, even if the radiation power of the light of different wavelengths is equal, the light flux is not equal.

For example, When the radiation power of green light with a wavelength of 5500 is the same as that of red light with a wavelength of 6500 Å. The luminous flux of the former is ten times that of the latter.

In photometry, it is derived from the unit CD of luminous intensity and the relationship between luminous intensity and luminous flux.

1lm is numerically equal to the luminous flux transmitted by a point light source with a luminous intensity of 1CD in a certain direction within a unit solid angle in that direction.

What Is Light Intensity?

Light intensity is the abbreviation of luminous intensity. 

Represents the light flux of the light source in the unit solid angle

Luminous intensity refers to the luminous flux emitted by the light source at a unit solid angle in a specified direction, that is, the luminous flux density radiated by the light source in a certain direction in space.

The luminous intensity symbol is I. The international unit is the candela (candela) abbreviation CD.

The light intensity represents the radiation ability of the light source in different directions. Generally speaking, the luminous intensity is the intensity of the light emitted by the light source.

What Is Brightness?

Brightness refers to the physical quantity of the intensity of light emission (reflection) on the surface of a light emitter (reflector).

When the human eye observes the light source from one direction, the ratio of the light intensity in this direction to the light source area “seen” by the human eye is defined as the unit brightness of the light source, that is, the luminous intensity on the unit projection area.

The luminance symbol is L.

And the unit of luminance is candela / m2 (cd/m2)

The brightness of the light source is related to the surface area of the light emitter. Under the same light intensity, if the light emitting area is large, it will be dark. Otherwise, it will be bright.

The brightness is also related to the direction of the light-emitting surface. The brightness value of the same light-emitting facial is different in different directions. 

For example, in common lighting, if you want to reduce the brightness of the subject, especially the face of the person, the usual way is to extend the distance of the lamp or add soft paper in front of the lamp to reduce the light intensity.

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