What is known about the intensity of light and the formula for calculating it.

Anyone who begins to study the characteristics of lamps and individual species lamps, necessarily encounters such concepts as illumination, luminous flux and luminous intensity. What do they mean and how do they differ from each other?

Let's try to understand these quantities in simple, understandable words. How they are related to each other, their units of measurement and how the whole thing can be measured without special instruments.

What is luminous flux

In the good old days, the main parameter by which a light bulb was chosen for the hallway, kitchen, or living room was its power. No one has ever thought to ask in a store about some lumens or candelas.

Today, with the rapid development of LEDs and other types of lamps, a trip to the store for new copies is accompanied by a bunch of questions not only about the price, but also about their characteristics. One of the most important parameters is luminous flux.

Speaking in simple words, luminous flux is the amount of light that a lamp gives.

However, do not confuse the luminous flux of individual LEDs with the luminous flux of assembled luminaires. They may differ significantly.

It must be understood that luminous flux is just one of many characteristics of a light source. Moreover, its value depends:

from source power

Here is a table of this dependence for LED lamps:

And these are the tables of their comparison with other types of incandescent, fluorescent, DRL, HPS lamps:

Incandescent light bulbFluorescent Lamp Halogen DNA DRL

However, there are also nuances here. LED technologies are still developing and it is quite possible that LED light bulbs of the same power, but from different manufacturers, will have completely different luminous fluxes.

It’s just that some of them have gone further forward and learned to extract more lumens from one watt than others.

Someone will ask what are all these tables for? So that you are not stupidly deceived by sellers and manufacturers.

Beautifully written on the box:

power 9W

light output 1000lm

analogue of incandescent lamp 100W

What will you look at first? That's right, to what is more familiar and understandable - the indicators of an analogue of an incandescent lamp.

But with this power, you won't get anywhere near the light you used to have. You will begin to swear at LEDs and their imperfect technologies. But the problem turns out to be an unscrupulous manufacturer and his product.

on efficiency

That is, how effectively a particular source transforms electrical energy into the light. For example, a regular incandescent lamp has an output of 15 Lm/W, and a sodium lamp high pressure already 150 Lm/W.

It turns out that this is a 10 times more efficient source than a simple light bulb. With the same power, you have 10 times more light!

The luminous flux is measured in Lumens - Lm.

What is 1 Lumen? During the day, in normal light, our eyes are most sensitive to the color green. For example, if you take two lamps with the same power of blue and green, then for all of us the green one will seem brighter.

The green wavelength is 555 Nm. Such radiation is called monochromatic because it contains a very narrow range.

Of course, in reality, green is complemented by other colors so that in the end you can get white.

But since the sensitivity of the human eye is maximum to green, the lumens were tied to it.

So, a luminous flux of one lumen exactly corresponds to a source that emits light with a wavelength of 555 Nm. In this case, the power of such a source is 1/683 W.

Why exactly 1/683, and not 1 W for good measure? The value 1/683 W arose historically. Initially, the main source of light was an ordinary candle, and the radiation of all new lamps and lamps was compared with the light from a candle.

Currently, this value of 1/683 is legalized by many international agreements and accepted everywhere.

Why do we need such a quantity as luminous flux? With its help you can easily calculate the illumination of a room.

This directly affects a person's vision.

The difference between illumination and luminous flux

At the same time, many people confuse the units of measurement Lumens with Luxes. Remember, illumination is measured in lux.

How can you clearly explain their difference? Imagine the pressure and force. With just a small needle and a little force, high specific pressure can be created at a single point.

Also, with the help of a weak luminous flux, it is possible to create high illumination in a single area of the surface.

1 Lux is when 1 Lumen falls on 1 m2 of illuminated area.

Let's say you have a certain lamp with a luminous flux of 1000 lm. Below this lamp is a table.

There must be a certain level of illumination on the surface of this table so that you can work comfortably. The primary source for illumination standards is the requirements of the codes of practice SP 52.13330

For a typical workplace this is 350 Lux. For a place where precise small work is carried out - 500 Lux.

This illumination will depend on many parameters. For example, from the distance to the light source.

From foreign objects nearby. If the table is located near a white wall, then there will be more suites than from a dark one. The reflection will definitely affect the overall outcome.

Any illumination can be measured. If you do not have special lux meters, use the programs in modern smartphones.

However, be prepared for errors in advance. But in order to do an initial analysis offhand, a phone will do just fine.

Calculation of luminous flux

How can you find out the approximate light flux in lumens, without any measuring instruments at all? Here you can use the light output values and their proportional dependence to the flow.

Light is a form of energy that travels through space as electromagnetic waves with frequencies perceived by human eyes. Photometry – These are methods for measuring light energy in the optical range. Luminous flux call the light energy flowing through a certain surface unit of time, estimated by visual sensation, i.e. light flow is the power of light radiation. Visual sensation changes visually and qualitatively. The light source is called Point if its dimensions are negligibly small compared to the distance at which its action is assessed. To describe the luminous flux emitted by a light source in different directions, the concept is used solid angle, i.e. a region of space that is shaped like a cone. Ω=S/R 2 – solid angle. Ω=4П – solid angle of the sphere. By the power of light is the luminous flux created by a light source in a unit solid angle. I c =Ф s /Ω – Luminous intensity (cd(candelah)) I c =Ф s /4П – luminous intensity around a point source (sphere) Ф s =I c * Ω – luminous flux. The light source almost always illuminates the light surface unevenly. Illumination is the ratio of the light current incident on a certain area of a surface to the area of this surface. E = Ф s / S = I c / R 2 – Illumination (LK (lux)). First law of illumination: Illumination is directly proportional to the light intensity of the source and inversely proportional to the square of the distance from the source. E 0 = I c / h 2 – illumination under the light source. Second law of illumination: Surface illumination created parallel rays is proportional to the cosine of the angle of incidence of the beam. E=E 0* cosα=I c /R 2 * cosα

53. lenses. Optical power. Thin lens formula.

Lens is a transparent body bounded by two spherical surfaces. If the middle of Lisa is thinner than its edges, then it is called scattering, and it itself is concave. If the middle of the lens is thinner than the edges, then it is called converging. |O 1 O 2 | - main optical axis. Any straight line passing through the center of the lens is called a secondary axis. The point at which all rays intersect after refraction in a collecting lens incident parallel to the main optical axis is called the principal lens focus. The lens has 2 main focuses. The line on which Lisa's tricks lie is called focal plane. A converging lens produces a real image, while a diverging lens produces a virtual image. A value equal to the inverse focal length called optical lens power. D=1/F – optical power of the lens (diopter). F – Focus. 1/F=1/f+1/d – formula of a thin lens (for collecting) 1/f=1/F+1/d – formula of a thin lens (for diverging). Г=H/h=f/d – lens magnification.

One of the most interesting and controversial phenomena 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 also has 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 a given phenomenon. This article will be devoted to the intensity of light and the calculation of this parameter.

General information about the concept

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

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

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

This parameter reflects physical property light, making it different from measurements such as brightness, which reflect subjective sensations. In addition, the intensity of light in physics is considered as power. To be more precise, it is measured as a unit of power. At the same time, power here differs from its usual concept. Here, power depends not only on the energy emitted by the lighting installation, but also on such a concept as wavelength.
It is worth noting that people's sensitivity to light radiation directly depends on the wavelength. This dependence is reflected in the function of relative spectral luminous efficiency. Moreover, the luminous intensity itself is a quantity dependent on luminous efficiency. At a wavelength of 550 nanometers (green), this parameter will take its maximum value. As a result, human eyes will be more or less sensitive to light flux at different wavelength parameters.
The unit of measurement for this indicator is candelas (cd).

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

Glow of one candle

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

Photometric parameter diagram

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

Four Most Important Light Characteristics

To make this easier to understand physical phenomenon it is necessary to consider a diagram that depicts a plane reflecting the propagation of light.

Diagram for luminous intensity

Thanks to the diagram, it can be seen that Iv depends on the direction to the radiation source. This means that for an LED light bulb, for which the direction of maximum radiation will be taken as 0°, then when we measure the value we need in the 180° direction, the result will be a smaller value than for the 0° direction.
As can be seen in the diagram, radiation that is propagated by two sources (yellow and red) will cover equal area. In this case, the yellow radiation will be scattered, similar to the light of a candle. Its power will be approximately 100 cd. Moreover, the value of this quantity will be the same in all directions. At the same time, red will be directional. At 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°.

Parameter designation in SI

Since Iv is physical quantity, then it can be calculated. A special formula is used for this. But before you get to the formula, you need to understand how the desired quantity is written in the SI system. In this system, our quantity will be displayed as J (sometimes written as I), the unit of which will be the candela (cd). The unit of measurement reflects that Iv emitted by a complete emitter over a cross-sectional area of 1/600000 m2. will be directed in a direction perpendicular to this section. In this case, the temperature of the emitter will be close to the level at which, at a pressure of 101325 Pa, hardening of platinum will be observed.

Note! The candela can be used to define other photometric units.

Since the luminous 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 .
Luminous intensity is used for calculations when the dimensional formula is applied. Moreover, this value is related through formulas to the luminous flux. In such a situation, the luminous flux will be the product of Iv and the solid angle to which the radiation will propagate.
Luminous flux (Фv) is the product of luminous intensity and the solid angle through which the flux propagates. Ф=I .

Luminous flux formula

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

Note! The steradian is understood as a solid angle that cuts out a section on the surface of a sphere that is equal to the square of the radius of the given sphere.

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

Formula for luminous intensity

This can be 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 will be.
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, Fv can no longer act as an exhaustive characteristic of the source. But only with the exception of a situation where at the same time the distribution of emitted radiation in various directions will not be determined.
To characterize the distribution of Фv in physics, they use such a concept as the spatial radiation density of the light flux for different directions of space. In this case, for Iv it is necessary to use the already familiar formula, but in a slightly expanded form:

Second formula for calculation

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

Conclusion

Light intensity takes important place not only in physics, but also in more mundane, everyday moments. This parameter is especially important for lighting, without which the world as we know it would be impossible to exist. Moreover, this value is used not only in the development of new lighting devices with more profitable technical characteristics, but also with certain calculations related to the organization of the lighting system.

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The total luminous flux characterizes the radiation that propagates from the source in all directions. For practical purposes, it is often more important to know not the total luminous flux, but the flux that goes in a certain direction or falls on a certain area. For example, it is important for a motorist to obtain a sufficiently large luminous flux in a relatively narrow solid angle, within which there is a small section of the highway. For someone working at a desk, what is important is the flux that illuminates the table or even part of the table, a notebook or a book, that is, the flux that falls on a certain area. In accordance with this, two auxiliary concepts have been established - luminous intensity and illumination.

Luminous intensity is the luminous flux calculated per solid angle equal to a steradian, i.e. the ratio of the luminous flux enclosed within the solid angle to this angle:

Illumination is the luminous flux calculated per unit area, i.e. the ratio of the luminous flux incident on the area to this area:

It is clear that formulas (70.1) and (70.2) determine average strength light and average illumination. They will be the closer to the true ones, the more uniform the flow or the smaller and.

It is obvious that with the help of a source sending a certain luminous flux, we can achieve very varied luminous intensities and very varied illumination. Indeed, if you direct the entire flow or most of it inside a small solid angle, then in the direction highlighted by this angle you can get very great strength Sveta. For example, in spotlights it is possible to concentrate most flux sent by the electric arc in a very small solid angle and receive enormous luminous intensity in the corresponding direction. To a lesser extent, the same goal is achieved with the help of car headlights. If you concentrate the light flux from any source on a small area using reflectors or lenses, you can achieve high illumination. This is done, for example, when trying to strongly illuminate a specimen viewed through a microscope; A lamp reflector serves a similar purpose, providing good illumination of the workplace.

According to formula (70.1), the luminous flux is equal to the product of the luminous intensity and the solid angle in which it propagates:

If the solid angle is , i.e. the rays are strictly parallel, then the luminous flux is also zero. This means that a strictly parallel beam of light rays does not carry any energy, i.e. it does not have physical meaning, - in no real experiment can a strictly parallel beam be realized. This is a purely geometric concept. Nevertheless, parallel beams of rays are very widely used in optics. The fact is that small deviations from the parallelism of light rays, which are of fundamental importance from an energy point of view, in matters related to the passage of light rays through optical systems, play virtually no role. For example, the angles at which rays from a distant star strike our eye or telescope are so small that they cannot even be measured existing methods; practically these rays do not differ from parallel ones. However, these angles are still not equal to zero, and it is thanks to this that we see the star. IN Lately light beams with very sharp directionality, i.e. with very low divergence of light rays, are obtained using lasers (see § 205). However, in this case, the angles between the rays have a finite value.

State exam questions in the discipline “Electrical lighting”

The energy and flux of radiation by themselves cannot indicate greater or lesser perception of this radiation by a person. Indeed, if the radiation is in the infrared or ultraviolet region, then no matter how powerful it is, it will remain invisible to the human eye. If radiation of the same power belongs to the visible region of the spectrum, a person will perceive them differently: to a greater extent at wavelengths around 555 nm (yellow and green radiation) and much weaker at the boundaries of the visible range (red and violet). Therefore, to assess the perception of radiation by a person, it is necessary to take into account not only the energy of the radiation, but also the relative spectral sensitivity of the eye, which is a function of the wavelength of the radiation.

Luminous flux F– the power of the radiation flux, estimated by the light sensation that it causes in a selective receiver - a standard photometric observer, the curve of the relative spectral sensitivity of the eye is standardized by the CIE. In other words, the luminous flux is the radiation flux effectively transformed by the eye.

Behind unit of luminous flux adopted in accordance with international agreement lumen (lm).

There is no constant conversion factor from Watts (radiant flux) to Lumens (luminous flux). More precisely, such a coefficient exists, but it is different for different wavelengths.

Light intensity I is the spatial density of the light flux in a given direction:

I a = dФ/dw,

Where F- luminous flux, lm;

w ‑ solid (spatial) angle with the vertex at the location of the light source, within which this luminous flux is evenly distributed, cf.

The unit of solid angle - steradian (sr) - is taken to be an angle that, having its vertex at the center of the sphere, cuts out a spherical section on its surface, with an area equal to the square of the radius.

The solid angle of the sphere is 4π..

The unit of luminous intensity, in accordance with the decision adopted by the 13th General Conference on Weights and Measures in 1967, is the candela [cd]. Candela – basic unit in the C system on a par with meter, kilogram, second, ampere, etc.

Illumination E is the surface density of the incident light flux. Illumination of a surface element in given point determined by the luminous flux ratio dФ incident on the surface element under consideration, to the area dS 2(index 2 usually denotes the illuminated surface) of this surface element: E = dФ/dS 2.

The unit of illumination is lux (lx). Lux is equal to the illumination of a surface with an area of 1 m2, over which a luminous flux of 1 lm is evenly distributed:

The illumination of a surface element created by a point source is proportional to the intensity of light and the cosine of the angle of incidence of light on the illuminated surface, and inversely proportional to the square of the distance from the light source to this surface.

Brightness L a is the surface density of light intensity in a given direction, i.e. the ratio of the luminous intensity in a given direction to the area of projection of the luminous surface onto a plane perpendicular to this direction.

The unit of brightness is the candela. square meter(cd/m 2).

The level of perception of light by a person depends on the brightness of the luminous object.