It follows from the definition that the value for the frequency 540⋅10 12 Hz is 683 lm / W = 683 cd sr / W exactly.

The selected frequency corresponds to a wavelength of 555.016 nm in air under standard conditions and is close to the maximum sensitivity of the human eye, located at a wavelength of 555 nm. If the radiation has a different wavelength, then a greater energy intensity of light is required to achieve the same luminous intensity.

Detailed consideration[ | ]

All light quantities are reduced photometric quantities. This means that they are formed from the corresponding energy photometric quantity by means of a function representing the dependence of the spectral luminous efficiency of monochromatic radiation for daytime vision on wavelength. This function is usually represented as K m ⋅ V (λ) (\displaystyle K_(m)\cdot V(\lambda)), where is a function normalized so that it is equal to unity at the maximum, and is the maximum value of the spectral luminous efficiency of monochromatic radiation. Sometimes K m (\displaystyle K_(m)) also called the photometric equivalent of radiation.

Light value calculation X v , (\displaystyle X_(v),) the corresponding energy quantity is produced using the formula

X v = K m ∫ 380 nm 780 nm X e , λ (λ) V (λ) d λ , (\displaystyle X_(v)=K_(m)\int \limits _(380~(\text(nm) ))^(780~(\text(nm)))X_(e,\lambda )(\lambda)V(\lambda)\,d\lambda ,)

where X e , λ (\displaystyle X_(e,\lambda ))- spectral density of quantity X e , (\displaystyle X_(e),) defined as the ratio of the magnitude d X e (λ) , (\displaystyle dX_(e)(\lambda),) falling on a small spectral interval between and λ + d λ , (\displaystyle \lambda +d\lambda ,) to the width of this interval:

X e , λ (λ) = d X e (λ) d λ . (\displaystyle X_(e,\lambda )(\lambda)=(\frac (dX_(e)(\lambda))(d\lambda )).)

It may be noted that under X e (λ) (\displaystyle X_(e)(\lambda)) here we mean the flux of that part of the radiation whose wavelength is less than the current value λ (\displaystyle \lambda ).

Function V (λ) (\displaystyle V(\lambda)) determined empirically and given in tabular form. Its values ​​do not depend on the choice of light units used.

Contrary to what was said about V (λ) (\displaystyle V(\lambda)) meaning K m (\displaystyle K_(m)) is entirely determined by the choice of the main light unit. Therefore, to establish a connection between light and energy quantities in the SI system, it is required to determine the value K m (\displaystyle K_(m)) corresponding to the SI unit of luminous intensity, the candela. With a strict approach to the definition K m (\displaystyle K_(m)) it must be taken into account that the spectral point 540⋅10 12 Hz, which is referred to in the definition of the candela, does not coincide with the position of the maximum of the function V (λ) (\displaystyle V(\lambda)).

Luminous efficiency of radiation with a frequency of 540⋅10 12 Hz[ | ]

In general, the intensity of light is related to the intensity of radiation I e (\displaystyle I_(e)) ratio

I v = K m ⋅ ∫ 380 nm 780 nm I e , λ (λ) V (λ) d λ , (\displaystyle I_(v)=K_(m)\cdot \int \limits _(380~(\text (nm)))^(780~(\text(nm)))I_(e,\lambda )(\lambda)V(\lambda)\,d\lambda ,)

where I e , λ (\displaystyle I_(e,\lambda ))- spectral density of the radiation force, equal to d I e (λ) d λ (\displaystyle (\frac (dI_(e)(\lambda))(d\lambda ))).

For monochromatic radiation with a wavelength λ (\displaystyle \lambda ) formula relating the intensity of light I v (λ) (\displaystyle I_(v)(\lambda)) with radiant power I e (λ) (\displaystyle I_(e)(\lambda)), simplifies by taking the form

I v (λ) = K m ⋅ I e (λ) V (λ) (\displaystyle I_(v)(\lambda)=K_(m)\cdot I_(e)(\lambda)V(\lambda)), or, after going from wavelengths to frequencies, I v (ν) = K m ⋅ I e (ν) V (ν) . (\displaystyle I_(v)(\nu)=K_(m)\cdot I_(e)(\nu)V(\nu).)

From the last relation for ν 0 = 540⋅10 12 Hz follows

K m ⋅ V (ν 0) = I v (ν 0) I e (ν 0) . (\displaystyle K_(m)\cdot V(\nu _(0))=(\frac (I_(v)(\nu _(0)))(I_(e)(\nu _(0))) ).)

Given the definition of a candela, we get

K m ⋅ V (ν 0) = 683 c d ⋅ s r W (\displaystyle K_(m)\cdot V(\nu _(0))=683~\mathrm (\frac (cd\cdot sr)(W)) ), or, which is the same 683 l m W . (\displaystyle 683~\mathrm (\frac (lm)(W)) .)

Work K m ⋅ V (ν 0) (\displaystyle K_(m)\cdot V(\nu _(0))) is the value of the spectral luminous efficiency of monochromatic radiation for a frequency of 540⋅10 12 Hz. As follows from the production method, this value is 683 cd sr / W = 683 lm / W exactly.

Maximum luminous efficiency K m (\displaystyle (\boldsymbol (K))_(m))[ | ]

For determining K m (\displaystyle K_(m)) It should be noted that, as mentioned above, the frequency 540⋅10 12 Hz corresponds to a wavelength of ≈555.016 nm. Therefore, the last equality implies

K m = 683 V (555.016) l m W . (\displaystyle K_(m)=(\frac (683)(V(555(,)016)))~\mathrm (\frac (lm)(W)) .)

Normalized function V (λ) (\displaystyle V(\lambda)) given in tabular form with an interval of 1 nm, it has a maximum equal to unity at a wavelength of 555 nm. Interpolation of its values ​​for a wavelength of 555.016 nm gives a value of 0.999997. Using this value, we get

K m = 683.002 l m W . (\displaystyle K_(m)=683(,)002~\mathrm (\frac (lm)(W)) .)

In practice, with sufficient accuracy for all cases, a rounded value is used K m = 683 l m W . (\displaystyle K_(m)=683~\mathrm (\frac (lm)(W)) .)

Thus, the relationship between an arbitrary light quantity X v (\displaystyle X_(v)) and the corresponding energy quantity X e (\displaystyle X_(e)) in the SI system is expressed by the general formula

X v = 683 ∫ 380 nm 780 nm X e , λ (λ) V (λ) d λ . (\displaystyle X_(v)=683\int \limits _(380~(\text(nm)))^(780~(\text(nm)))X_(e,\lambda )(\lambda)V( \lambda)\,d\lambda .)

History and prospects[ | ]

Hefner lamp - standard "Hefner candle"

Examples [ | ]

The intensity of light emitted by a candle is approximately equal to one candela, so this unit of measurement used to be called "candle", now this name is obsolete and is not used.

For household incandescent lamps, the luminous intensity in candela is approximately equal to their power in watts.

Light intensity of various sources

Source	Power, W	Approximate light intensity, cd
Candle		1
Modern (2010) incandescent lamp	100	100
Ordinary LED	0,015..0,1	0,005..3
Super bright LED	1	25…500
Super bright LED with collimator	1	1500
Modern (2010) fluorescent lamp	22	120
Sun	3,83⋅10 26	2,8⋅10 27

Light quantities[ | ]

Information about the main light photometric quantities is given in the table.

Light photometric quantities SI

Name	Value designation	Definition	SI unit notation	Energy analogue
light energy	Q v (\displaystyle Q_(v))	K m ∫ 380 nm 780 nm Q e , λ (λ) V (λ) d λ (\displaystyle K_(m)\int _(380~(\text(nm)))^(780~(\text(nm )))Q_(e,\lambda )(\lambda)V(\lambda)\,d\lambda )	lm	Radiation energy
Light flow	Φ v (\displaystyle \Phi _(v))	d Q v d t (\displaystyle (\frac (dQ_(v))(dt)))	lm	radiation flux
The power of light	I v (\displaystyle I_(v))	d Φ v d Ω (\displaystyle (\frac (d\Phi _(v))(d\Omega )))	cd	Radiation strength (energy strength of light)
	U v (\displaystyle U_(v))	d Q v d V (\displaystyle (\frac (dQ_(v))(dV)))	lm s −3
Luminosity	M v (\displaystyle M_(v))	d Φ v d S 1 (\displaystyle (\frac (d\Phi _(v))(dS_(1))))	lm m −2	Energy luminosity
Brightness	L v (\displaystyle L_(v))	d 2 Φ v d Ω d S 1 cos ⁡ ε (\displaystyle (\frac (d^(2)\Phi _(v))(d\Omega \,dS_(1)\,\cos \varepsilon )))	cd m −2

Sveta. This article will reveal to readers the properties of photons, which will allow them to determine why light comes in different brightnesses.

Particle or wave?

At the beginning of the twentieth century, scientists were puzzled by the behavior of light quanta - photons. On the one hand, interference and diffraction spoke of their wave nature. Therefore, light was characterized by properties such as frequency, wavelength, and amplitude. On the other hand, they convinced the scientific community that photons transfer momentum to surfaces. This would be impossible if the particles did not have mass. Thus, physicists had to admit: electromagnetic radiation is both a wave and a material object.

Photon energy

As Einstein proved, mass is energy. This fact proves our central luminary, the Sun. A thermonuclear reaction turns a mass of highly compressed gas into pure energy. But how to determine the power of the emitted radiation? Why in the morning, for example, is the luminous intensity of the sun lower than at noon? The characteristics described in the previous paragraph are interconnected by specific relationships. And they all point to the energy that electromagnetic radiation carries. This value changes upwards when:

• decrease in wavelength;
• increasing frequency.

What is the energy of electromagnetic radiation?

A photon is different from other particles. Its mass, and therefore its energy, exists only as long as it moves through space. When colliding with an obstacle, a quantum of light increases its internal energy or gives it a kinetic moment. But the photon itself ceases to exist. Depending on what exactly acts as an obstacle, various changes occur.

1. If the obstacle is a solid body, then most often the light heats it up. The following scenarios are also possible: a photon changes direction, stimulates a chemical reaction, or causes one of the electrons to leave its orbit and go to another state (photoelectric effect).
2. If the obstacle is a single molecule, for example, from a rarefied cloud of gas in outer space, then a photon makes all its bonds vibrate more strongly.
3. If the obstacle is a massive body (for example, a star or even a galaxy), then the light is distorted and changes the direction of motion. This effect is based on the ability to "look" into the distant past of the cosmos.

Science and Humanity

Scientific data often seem to be something abstract, inapplicable to life. This also happens with the characteristics of light. When it comes to experimenting or measuring the radiation of stars, scientists need to know the absolute values ​​(they are called photometric). These concepts are usually expressed in terms of energy and power. Recall that power refers to the rate of change of energy per unit of time, and in general it shows the amount of work that the system can produce. But man is limited in his ability to perceive reality. For example, the skin feels heat, but the eye does not see the photon of infrared radiation. The same problem with units of luminous intensity: the power that radiation actually shows is different from the power that the human eye can perceive.

Spectral sensitivity of the human eye

We remind you that the discussion below will focus on average indicators. All people are different. Some do not perceive individual colors at all (colorblind). For others, the culture of color does not coincide with the accepted scientific point of view. For example, the Japanese do not distinguish between green and blue, and the British - blue and blue. In these languages, different colors are denoted by one word.

The unit of luminous intensity depends on the spectral sensitivity of the average human eye. The maximum daylight falls on a photon with a wavelength of 555 nanometers. This means that in the light of the sun, a person sees the green color best. Night vision maximum is a photon with a wavelength of 507 nanometers. Therefore, under the moon, people see blue objects better. At dusk, everything depends on the lighting: the better it is, the more “green” the maximum color that a person perceives becomes.

The structure of the human eye

Almost always, when it comes to vision, we say what the eye sees. This is an incorrect statement, because the brain perceives first of all. The eye is only an instrument that transmits information about the light output to the main computer. And, like any tool, the entire color perception system has its limitations.

There are two different types of cells in the human retina - cones and rods. The former are responsible for daytime vision and perceive colors better. The latter provide night vision, thanks to the sticks, a person distinguishes between light and shadow. But they do not perceive colors well. The sticks are also more sensitive to movement. That is why, if a person walks through a moonlit park or forest, he notices every swaying of the branches, every breath of the wind.

The evolutionary reason for this separation is simple: we have one sun. The moon shines by reflected light, which means that its spectrum does not differ much from the spectrum of the central luminary. Therefore, the day is divided into two parts - illuminated and dark. If people lived in a system of two or three stars, then our vision would probably have more components, each of which was adapted to the spectrum of one luminary.

I must say, on our planet there are creatures whose eyesight is different from human. Desert dwellers, for example, detect infrared light with their eyes. Some fish can see near ultraviolet, as this radiation penetrates the deepest into the water column. Our pet cats and dogs perceive colors differently, and their spectrum is reduced: they are better adapted to chiaroscuro.

But people are all different, as we mentioned above. Some representatives of mankind see near infrared light. This is not to say that they would not need thermal cameras, but they are able to perceive slightly redder shades than most. Others have developed the ultraviolet part of the spectrum. Such a case is described, for example, in the film "Planet Ka-Pax". The protagonist claims that he came from another star system. The examination revealed that he had the ability to see ultraviolet radiation.

Does this prove that Prot is an alien? No. Some people can do it. In addition, the near ultraviolet is closely adjacent to the visible spectrum. No wonder some people take a little more. But Superman is definitely not from Earth: the X-ray spectrum is too far from the visible for such vision to be explained from a human point of view.

Absolute and relative units for determining the luminous flux

A quantity independent of the spectral sensitivity, which indicates the flow of light in a known direction, is called a "candela". The power unit, already with a more "human" attitude, is pronounced the same way. The difference is only in the mathematical designation of these concepts: the absolute value has a subscript "e", relative to the human eye - "υ". But do not forget that the sizes of these categories will vary greatly. This must be taken into account when solving real problems.

Enumeration and comparison of absolute and relative values

To understand what the power of light is measured in, it is necessary to compare the "absolute" and "human" values. On the right are purely physical concepts. On the left are the values ​​into which they turn when passing through the system of the human eye.

1. The power of radiation becomes the power of light. Concepts are measured in candela.
2. Energy brightness turns into brightness. The values ​​are expressed in candela per square metre.

Surely the reader saw familiar words here. Many times in their lives, people say: "Very bright sun, let's go into the shade" or "Make the monitor brighter, the movie is too gloomy and dark." We hope the article will slightly clarify where this concept came from, as well as what the unit of luminous intensity is called.

Features of the concept of "candela"

We have already mentioned this term above. We also explained why the same word is used to refer to completely different concepts of physics related to the power of electromagnetic radiation. So, the unit of measure for the intensity of light is called the candela. But what is it equal to? One candela is the intensity of light in a known direction from a source that emits strictly monochromatic radiation with a frequency of 5.4 * 10 14, and the energy force of the source in this direction is 1/683 watts per unit solid angle. The reader can easily convert frequency into wavelength, the formula is very easy. We will prompt: the result lies in visible area.

The unit of measurement for the intensity of light is called the "candela" for a reason. Those who know English remember that a candle is a candle. Previously, many areas of human activity were measured in natural parameters, for example, horsepower, millimeters of mercury. So it is not surprising that the unit of measurement for the intensity of light is the candela, one candle. Only a candle is very peculiar: with a strictly specified wavelength, and producing a specific number of photons per second.

In order to quickly and efficiently perform any production task, the lighting of the specialist’s workplace must be properly organized. For this, lamps with certain photometric indicators are selected.

Lighting in the workplace is determined by various physical quantities, the main of which is illumination. Its indicators are calculated for the workplace of any specialist and are regulated by the relevant SNiPs.

Illumination is a characteristic that is defined as the luminous flux per unit area.

Luminous flux (F)

This physical parameter is defined as the power of the visible radiation of the source or the light energy that is emitted by the luminaire per unit of time.

At the same time, light energy is an energy that spreads in all directions and causes visual sensations. Each person has different visual sensations for the same radiation sources, therefore, averaged indicators are taken for calculations.

In physics, the formula is used to calculate:

F \u003d W / t, where:

• W is the energy emitted by the source, measured in watts,
• t is the device operation time in seconds.

It is also a value that characterizes the amount of light emitted by a lighting device in all directions.

Thus, the second calculation formula looks like:

Ф = I w, where:

• I - light intensity, measured in candela,
• w is the solid angle, calculated in steradians.

Lumen

The unit of measure for luminous flux is lumen.

In order to determine which source is more profitable to purchase, we first consider what a lumen is.

The word lumen in Latin means light.

A lumen is defined as the luminous flux that is emitted by a point source having a luminous intensity of 1 candela into a solid angle of 1 steradian:

1lm = 1W / 1s.

On the other hand,the unit of measure lumen (lm) can be found as:

1 lm \u003d 1 cd 1 sr.

If the solid angle is 4π radians, and the luminous intensity is 1 cd, then in this case they talk about the total luminous flux, which is 4π lm or 4 3.14 lm.

It was calculated that this indicator for solar radiation corresponds to 8 lm, and for the starry sky - only 0.000000001 lm.

For any artificial light source, there are tables for calculating this photometric parameter.

In lighting engineering, derivative quantities are used, which are formed using standard prefixes of the international SI system, for example:

• 1 klm = 103 lm or 1 klm = 103 lm;
• 1 Mlm = 106 lm;
• 1 slm = 10-3 lm;
• 1 µlm = 10-6 lm.

Measuring instruments

To measure photometric quantities in industry, special devices are used, which are called spherical photometers and goniophotometers. They allow you to determine both the luminous flux and the intensity of light from various lamps.

Photometers are visual and objective.

The principle of operation of visual instruments is based on the ability of the eye to determine the same brightness of the illumination of two compared surfaces illuminated with the same color.

Currently, objective electric photometers are popular, which allow measuring light parameters not only in the visible zone, but also beyond it.

Goniophotometers allow obtaining data on the magnitude of the luminous flux, luminous intensity, as well as indicators of other photometric quantities, such as brightness, illumination distribution, etc.

Recommendations for organizing the right workplace lighting

When lighting workplaces, two types of sources are used: artificial and natural.

Artificial are devices with lamps of various types: fluorescent, incandescent, LED, etc.

For each type of lamp there are tables indicating the number of lumens emitted by this lamp.

This value is indicated on the packaging of the product, so when buying, be sure to select a light bulb, guided by the information placed by the manufacturer on the box. The packaging of the luminaire indicates the total luminous flux, which includes diffused light.

Attention! When purchasing a lamp, it is important to remember that this indicator does not fully reflect its brightness, since it can be increased through the use of a system of reflectors, lenses and mirrors placed in the device.

Selection of electric lamps

Before purchasing light bulbs, you must first choose which fixtures you need to create the right workplace lighting. If the room is rectangular, then the calculation of the required number of lumens is carried out as follows: you need to multiply the indicators of the illumination norm of the object (determined according to SNiP), the area of ​​\u200b\u200bthe room and the coefficient depending on the height of the ceiling of the room.

One of the most interesting and controversial phenomenon of our world is light. For physics, this is one of the fundamental parameters of numerous calculations. With the help of light, scientists hope to find a clue to the existence of our universe, as well as open up new opportunities for humanity. In everyday life, light is also of great importance, especially when creating high-quality lighting in various rooms.

One of the important parameters of light is its strength, which characterizes the power of this phenomenon. It is the strength of light and the calculation of this parameter that this article will be devoted to.

General information about the concept

In physics, luminous intensity (Iv) means the power of the luminous flux, determined within a specific solid angle. It follows from this concept that this parameter does not mean all the light available in space, but only that part of it that is radiated in a certain direction.

Depending on the available radiation source, this parameter will increase or decrease. Its changes will be directly affected by the value of the solid angle.

Note! In some situations, the light intensity will be the same for any angle. This is possible in situations where the source of light radiation creates a uniform illumination of the space.

This parameter reflects the physical property of light, which makes it different from measurements such as brightness, which reflect subjective sensations. In addition, the strength of light in physics is considered as power. To be more precise, it is estimated as a unit of power. At the same time, power here differs from its usual concept. Here, the power depends not only on the energy that the lighting installation emits, but also on such a thing as wavelength.
It should be noted that the sensitivity of people to light radiation directly depends on the wavelength. This dependence is reflected in a function of the spectral luminous efficiency. In this case, the luminous intensity itself is a quantity dependent on the luminous efficiency. At a wavelength of 550 nanometers (green), this parameter will take its maximum value. As a result, the human eye will be more or less sensitive to the light flux at different wavelengths.
The unit of measure for this indicator is the candela (cd).

Note! The strength of the radiation that comes from one candle will be approximately equal to one candela. The international candlestick previously used for the calculation formula was 1.005 cd.

The glow of one candle

In rare cases, an outdated unit of measurement is used - the international candle. But in the modern world, the unit of measurement for this quantity, the candela, is already used almost everywhere.

Photometric parameter diagram

Iv is the most important photometric parameter. In addition to this value, the most important photometric parameters include brightness, as well as illumination. All these four values ​​are actively used when creating a lighting system in a wide variety of rooms. Without them, it is impossible to estimate the required level of illumination for each individual situation.

The four most important lighting characteristics

For ease of understanding of this physical phenomenon, it is necessary to consider a diagram that depicts a plane reflecting the propagation of light.

Chart for light intensity

The diagram shows that Iv depends on the direction towards the radiation source. This means that for an LED bulb, for which the direction of maximum radiation will be taken as 0 °, then when measuring the value we need in the direction of 180 °, a smaller value will be obtained than for the direction of 0 °.
As you can see, in the diagram, the radiation that is propagated by two sources (yellow and red) will cover an equal area. In this case, the yellow radiation will be scattered, by analogy with the light of a candle. Its power will be approximately equal to 100 cd. Moreover, the value of this value will be the same in all directions. At the same time, red will be directional. In the 0° position, it will have a maximum value of 225 cd. In this case, this value will decrease in case of deviation from 0°.

SI parameter designation

Since Iv is a physical quantity, it can be calculated. For this, a special formula is used. But before reaching the formula, it is necessary to understand how the desired value is written in the SI system. In this system, our value will be displayed as J (sometimes it is denoted as I), the unit of which will be the candela (cd). The unit of measurement reflects that Iv emitted by a full radiator over a cross-sectional area of ​​1/600,000 m2. will be directed in a direction perpendicular to the given section. In this case, the temperature of the emitter will be close to the level at which, at a pressure of 101325 Pa, solidification of platinum will be observed.

Note! Through the candela, you can determine the rest of the photometric units.

Since the light flux in space is distributed unevenly, it is necessary to introduce such a concept as a solid angle. It is usually denoted by the symbol .
Light intensity is used for calculations when the dimension formula is applied. In this case, this value is related to the luminous flux through formulas. In such a situation, the luminous flux will be the product of Iv and the solid angle, to which the radiation will propagate.
The luminous flux (Фv) is the product of the luminous intensity and the solid angle in which the flux propagates. Ф=I .

Luminous flux formula

It follows from this formula that Fv is the internal flux propagated within a specific solid angle (one steradian) in the presence of Iv in one candela.

Note! A steradian is a solid angle that cuts out a section on the surface of a sphere that is equal to the square of the radius of this sphere.

In this case, Iv and power can be related through light radiation. After all, Fv is also understood as a value that characterizes the power of the emission of light radiation when it is perceived by the average human eye, which has sensitivity to radiation of a certain frequency. As a result, the following equation can be derived from the above formula:

Formula for the intensity of light

This is clearly seen in the example of LEDs. In such sources of light radiation, its strength is usually equal to the power consumed. As a result, the higher the electricity consumption, the higher the radiation level.
As you can see, the formula for calculating the value we need is not so complicated.

Additional calculation options

Since the distribution of radiation coming from a real source into space will be uneven, then Фv will no longer be able to act as an exhaustive characteristic of the source. But only with the exception of the situation when, at the same time, the distribution of the emitted radiation in various directions will not be determined.
To characterize the distribution Фv in physics, such a concept as the spatial density of the radiation of the light flux for different directions of space is used. In this case, for Iv, it is necessary to use the already familiar formula, but in a slightly supplemented form:

The second formula for calculating

This formula will allow you to estimate the desired value in various directions.

Conclusion

The power of light occupies an important place not only in physics, but also in more mundane, everyday moments. This parameter is especially important for lighting, without which the existence of the world familiar to us is impossible. At the same time, this value is used not only in the development of new lighting devices with more favorable technical characteristics, but also in certain calculations related to the organization of the lighting system.

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The question of what the luminous flux is measured in began to matter to users of lighting devices only when types of lamps appeared, the brightness of which did not equal the power consumption, measured in watts.

Let's figure out how the concept of brightness is connected with the concept of illumination, as well as how you can imagine the distribution of the flow of light throughout the room and choose the right lighting device.

What is luminous flux?

The flux of light is the power of light radiation visible to the human eye; light energy emitted by a surface (luminous or reflective). The energy of the light flux is measured in lumen-seconds and corresponds to a flux of 1 lumen, emitted or perceived in 1 second. This figure describes the total flow, not taking into account the concentrating efficiency of the entire device. This estimate also includes scattered, useless light, so the same number of lumens can be found in sources of different designs.

It is necessary to distinguish between the light value and the energy value - the latter characterizes light, regardless of its property to cause visual sensations. Each photometric light quantity has an analogue that can be quantified in units of energy or power. For light energy, such an analogue is the radiation energy (radiant energy), measured in joules.

Luminous flux unit

1 lumen is the light emitted by a source with a luminous intensity of 1 candela within a solid angle of 1 steradian. A 100-watt incandescent light bulb generates approximately 1,000 lumens of light. The brighter the light source, the more lumens it emits.

In addition to lumens, there are other units of measurement that allow you to characterize light. It is possible to measure the spatial and surface flux density - this is how we find out the strength of light and illumination. Light intensity is measured in candela, illuminance is measured in lux. But it is more important for the consumer to figure out in what units the brightness of light bulbs and other lighting fixtures is indicated in the sale. Some manufacturers report the number of lumens per watt. This is how the luminous efficiency (light output) is measured: how much light a lamp gives out, spending 1 watt.

Defining Formulas

Since any light source emits it unevenly, the number of lumens does not fully characterize the lighting fixture. You can calculate the intensity of light in candela by dividing its flux, expressed in lumens, by the solid angle, measured in steradians. Using this formula, it will be possible to take into account the totality of rays coming from the source when they cross the surface of an imaginary sphere, forming a circle on it.

But the question arises, what gives in practice the number of candela that we find; it is impossible to find a suitable LED or flashlight only by the luminous intensity parameter, you also need to take into account the ratio of the scattering angle, which depends on the design of the device. When choosing lamps that shine evenly in all directions, it is important to understand whether they are suitable for the buyer's goals.

If earlier light bulbs in different rooms were selected based on the number of watts, then before buying LED lamps, you will have to calculate their total brightness in lumens, and then divide this figure by the area of ​​​​the room. This is how illumination is calculated, which is measured in lux: 1 lux is 1 lumen per 1 m². There are lighting standards for rooms for various purposes.

Luminous flux measurement

Before releasing products to the market, the manufacturer makes in the laboratory the definition and measurement of the characteristics of the lighting device. At home, without special equipment, this is unrealistic to do. But you can check the numbers indicated by the manufacturer using the above formulas using a compact light meter.

The difficulty of accurately measuring the parameters of light lies in the fact that it comes in all possible directions of propagation. Therefore, laboratories use spheres with an inner surface that has a high reflectivity - spherical photometers; they are also used to measure the dynamic range of cameras, i.e. photosensitivity of their matrices.

In everyday life, it makes more sense to measure such important light parameters as room illumination and pulsation coefficient. High ripple and dim lighting cause people to strain their eyes too much, which causes fatigue more quickly.

The pulsation coefficient of the light flux is an indicator that characterizes the degree of its unevenness. Permissible levels of these coefficients are regulated by SanPiN.

It is not always possible to see with the naked eye that the light bulb is flickering. Nevertheless, even a slight excess of the pulsation coefficient affects the central nervous system of a person negatively, and also reduces performance. Light that can pulsate unevenly is emitted by all screens: computer and laptop monitors, tablet and mobile phone displays, and a TV screen. Pulsation is measured with a luxmeter-pulsemeter.

What is a candela?

Another important characteristic of the light source is the candela, which is included in the 7 units of the International System of Units (SI) adopted by the General Conference on Weights and Measures. Initially, 1 candela was equal to the radiation of 1 candle, taken as a standard. Hence the name of this unit of measurement. Now it is determined by a special formula.

Candela is the intensity of light, measured exclusively in a given direction. The spread of rays on the part of the sphere outlined by a solid angle allows us to calculate a value equal to the ratio of the luminous flux to this angle. Unlike lumens, this value is used to determine the intensity of the rays. This does not take into account useless, scattered light.

A flashlight and a ceiling lamp will have a different cone of light, as the rays fall at different angles. Candelas (more precisely, millicandelas) are used to indicate the luminous intensity of sources with a directional glow: indicator LEDs, flashlights.

Lumens and Lux

In lumens, the amount of light flux is measured, this is a characteristic of its source. The number of rays that reached any surface (reflecting or absorbing) will already depend on the distance between the source and this surface.

The level of illumination is measured in lux (lx) with a special device - a luxmeter. The simplest luxmeter consists of a selenium photocell that converts light into electric current energy, and a pointer microammeter that measures this current.

The spectral sensitivity of the selenium photocell differs from the sensitivity of the human eye, so in different conditions it is necessary to use correction factors. The simplest light meters are designed to measure one type of illumination, such as daylight. Without the use of coefficients, the error can be more than 10%.

High-class luxmeters are equipped with light filters, special spherical or cylindrical nozzles (for measuring spatial illumination), fixtures for measuring brightness and checking the sensitivity of the device. Their level of error is about 1%.

Poor illumination of the premises contributes to the development of myopia, has a bad effect on performance, causes fatigue, and a decrease in mood.

The minimum illumination of the computer table surface according to SanPiN is 400 lux. School desks must have at least 500 lux illumination.

Lumen and watt

Energy-saving lamps with the same light output consume 5-6 times less electrical energy than incandescent lamps. LED - 10-12 times less. The power of the light flux no longer depends on the number of watts. But manufacturers always indicate watts, since the use of too powerful light bulbs in cartridges not designed for such a load leads to damage to electrical appliances or a short circuit.

If you arrange the most common types of light bulbs in ascending order of light output, you can get the following list:

1. Incandescent lamp - 10 lumens / watt.
2. Halogen - 20 lumens / watt.
3. Mercury - 60 lumens / watt.
4. Energy saving - 65 lumens/watt.
5. Compact fluorescent lamp - 80 lumens/watt.
6. Metal halide - 90 lumens / watt.
7. Light-emitting diode (LED) - 120 lumens / watt.

But most people are accustomed to looking at the number of watts indicated by the manufacturer when buying light bulbs. To calculate how many watts per square meter you need, you first need to decide how bright the light in the room should be. 20 watt incandescent lamps per 1 m² - such lighting is suitable for a workplace or living room; for a bedroom, 10-12 watts per 1 m² will be enough. When buying energy-saving lamps, these figures are divided by 5. It is important to take into account the height of the ceiling: if it is higher than 3 m, the total number of watts should be multiplied by 1.5.