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1 candela [cd] = 1 lumen/steradian [lm/sr]

Initial value

Converted value

candela candle (German) candle (British) decimal candle pentane candle pentane candle (10 light output) Hefner candle Carcel unit candle decimal (French) lumen/steradian candle (international)

More about the power of light

General information

Luminous intensity is the power of the luminous flux within a certain solid angle. That is, the intensity of light does not determine all the light in space, but only the light emitted in a certain direction. Depending on the light source, the luminous intensity decreases or increases as the solid angle changes, although sometimes this value is the same for any angle if the source distributes the light evenly. Light power - physical property Sveta. In this way, it differs from brightness, since in many cases, when they talk about brightness, they mean a subjective sensation, and not a physical quantity. Also, brightness does not depend on the solid angle, but is perceived in the general space. The same source with a constant luminous intensity can be perceived by people as light of different brightness, since this perception depends on environmental conditions and on the individual perception of each person. Also, the brightness of two sources with the same luminous intensity may be perceived differently, especially if one produces diffuse light and the other directed light. In this case, the directional source will appear brighter, even though the luminous intensity of both sources is the same.

Luminous intensity is considered as a unit of power, although it differs from the usual concept of power in that it depends not only on the energy emitted by the light source, but also on the wavelength of the light. The sensitivity of people to light depends on the wavelength and is expressed by the function of relative spectral luminous efficiency. The luminous intensity depends on the luminous efficiency, which reaches a maximum for light with a wavelength of 550 nanometers. This - green. The eye is less sensitive to light of longer or shorter wavelengths.

In the SI system, luminous intensity is measured in candela(kd). One candela is approximately equal to the intensity of light emitted by one candle. Sometimes the obsolete unit is also used, candle(or international candle), although in most cases this unit is replaced by candelas. One candle is approximately equal to one candela.

If you measure the luminous intensity using a plane that shows the spread of light, as in the illustration, you can see that the magnitude of the luminous intensity depends on the direction towards the light source. For example, if we take the direction of maximum radiation LED lamp beyond 0°, the measured luminous intensity in the 180° direction will be much lower than for 0°. For diffuse sources, the luminous intensity for 0° and 180° will not be much different, and may be the same.

In the illustration, light emitted by two sources, red and yellow, covers an equal area. Yellow light is diffused, like candle light. Its strength is approximately 100 cd, regardless of direction. Red is the opposite, directional. In the direction of 0°, where the radiation is maximum, its strength is 225 cd, but this value quickly decreases with deviations from 0°. For example, the luminous intensity is 125 cd when directed at a source of 30° and only 50 cd when directed at 80°.

The power of light in museums

Museum staff measure the light intensity in museum spaces to determine the optimal conditions for visitors to view the works on display, while at the same time providing gentle light that causes as little damage as possible to museum exhibits. Museum exhibits containing cellulose and dyes, especially from natural materials, deteriorate from prolonged exposure to light. Cellulose provides strength to fabric, paper and wood products; Often in museums there are many exhibits made from these materials, so the light in the exhibition halls poses a great danger. The stronger the light intensity, the more museum exhibits deteriorate. In addition to destruction, light also discolors or yellows cellulose-containing materials such as paper and fabrics. Sometimes the paper or canvas on which paintings are painted deteriorates and breaks down faster than paint. This is especially problematic since the paint on a painting is easier to restore than the base.

The damage caused to museum exhibits depends on the wavelength of light. For example, light in the orange spectrum is the least harmful, and blue light is the most dangerous. That is, light with longer wavelengths is safer than light with shorter wavelengths. Many museums use this information and control not only the total amount of light, but also limit blue light using light orange filters. At the same time, they try to choose filters that are so light that, although they filter out blue light, they allow visitors to fully enjoy the works exhibited in the exhibition hall.

It is important not to forget that exhibits deteriorate not only from light. Therefore, it is difficult to predict, based only on the intensity of light, how quickly the materials from which they are made will degrade. For long-term storage in museum premises, it is necessary not only to use low lighting, but also to maintain low humidity, as well as low amounts of oxygen in the air, at least inside display cases.

In museums where flash photography is prohibited, they often cite the harmful effects of light on museum exhibits, especially ultraviolet light. This is practically unfounded. Just as limiting the entire spectrum of visible light is much less effective than limiting blue light, banning flash has little effect on the extent of light damage to exhibits. During the experiments, the researchers noticed slight damage to the watercolors caused by professional studio flash only after more than a million flashes. A flash every four seconds at a distance of 120 centimeters from the exhibit is almost equivalent to the light that is usually found in exhibition halls, where the amount of light is controlled and blue light is filtered. Those who take photographs in museums rarely use such powerful flashes, since most visitors are not professional photographers and take photos with phones and compact cameras. Flashes in the halls rarely work every four seconds. The damage from the ultraviolet rays emitted by the flash is also in most cases small.

Luminous intensity of lamps

The properties of lamps are usually described using luminous intensity, which differs from the luminous flux - a value that determines the total amount of light and shows how bright this source is in general. It is convenient to use luminous intensity to determine the luminous properties of lamps, for example, LED lamps. When purchasing them, information about the light intensity helps determine with what strength and in what direction the light will spread, and whether such a lamp is suitable for the buyer.

Light intensity distribution

In addition to the luminous intensity itself, luminous intensity distribution curves help to understand how the lamp will behave. Such diagrams of the angular distribution of luminous intensity are closed curves on a plane or in space, depending on the symmetry of the lamp. They cover the entire range of light propagation of this lamp. The diagram shows the magnitude of the light intensity depending on the direction of its measurement. The graph is usually plotted in either a polar or rectangular coordinate system, depending on the light source for which the graph is being plotted. It is often placed on lamp packaging to help the buyer imagine how the lamp will perform. This information is important for designers and lighting engineers, especially those who work in the field of cinema, theater, and the organization of exhibitions and performances. Luminous intensity distribution also affects driving safety, which is why engineers designing vehicle lighting use luminous intensity distribution curves. They must comply with strict regulations governing the distribution of light intensity in headlights to ensure maximum safety on the roads.

The example in the figure is in the polar coordinate system. A is the center of the light source, from where the light spreads in different directions, B is the luminous intensity in candelas, and C is the angle of measurement of the direction of the light, with 0° being the direction of the maximum luminous intensity of the source.

Measuring the intensity and distribution of light intensity

Light intensity and its distribution are measured special devices, goniophotometers And goniometers. There are several types of these devices, for example with a movable mirror, which allows you to measure light intensity from different angles. Sometimes, instead of a mirror, the light source itself moves. Typically these devices are large, with a distance of up to 25 meters between the lamp and the sensor that measures light intensity. Some devices consist of a sphere with measuring instrument, mirror and lamp inside. Not all goniophotometers are large; there are also small ones that move around the light source during measurement. When purchasing a goniophotometer, the decisive factors, among other factors, are its price, size, power, and the maximum size of the light source that it can measure.

Half Brightness Angle

Half-brightness angle, sometimes also called glow angle, is one of the quantities that helps describe a light source. This angle indicates how directional or diffuse the light source is. It is defined as the angle of the light cone at which the luminous intensity of the source is equal to half its maximum intensity. In the example in the figure, the maximum luminous intensity of the source is 200 cd. Let's try to determine the half-brightness angle using this graph. Half the luminous intensity of the source is 100 cd. The angle at which the luminous intensity of the beam reaches 100 cd., that is, the angle of half brightness, is equal to 60 + 60 = 120 ° on the graph (half the angle is depicted in yellow). For two light sources with the same total amount of light, a narrower half-brightness angle means that its luminous intensity is greater, compared to the second source, for angles between 0° and the half-brightness angle. That is, directional sources have a narrower half-brightness angle.

There are advantages to both wide and narrow half-brightness angles, and which one should be preferred depends on the application of the light source. For example, for scuba diving, you should choose a flashlight with a narrow angle of half brightness if there is good visibility in the water. If visibility is poor, then there is no point in using such a flashlight, since it only wastes energy. In this case, a flashlight with a wide angle of half brightness, which diffuses the light well, is a better choice. Also, such a flashlight will help during photo and video shooting, because it illuminates a wider area in front of the camera. Some dive lights can be manually adjusted to half brightness, which is useful since divers can't always predict what the visibility will be like where they're diving.

Illuminance is a luminous quantity that determines the amount of light falling on a certain surface area of ​​the body. It depends on the wavelength of light, since the human eye perceives the brightness of light waves of different lengths, that is, different colors, in different ways. Illuminance is calculated separately for different wavelengths, since people perceive light with a wavelength of 550 nanometers (green) and colors that are nearby in the spectrum (yellow and orange) as the brightest. Light produced by longer or shorter wavelengths (violet, blue, red) is perceived as darker. Illumination is often associated with the concept of brightness.

Illumination is inversely proportional to the area on which the light falls. That is, when illuminating a surface with the same lamp, the illumination of a larger area will be less than the illumination of a smaller area.

Difference between brightness and illuminance

Brightness Illumination

In Russian, the word “brightness” has two meanings. Brightness can mean a physical quantity, that is, a characteristic of luminous bodies, equal to the ratio of the intensity of light in a certain direction to the area of ​​​​projection of the luminous surface onto a plane perpendicular to this direction. It can also define a more subjective concept of overall brightness, which depends on many factors, such as the eyes of the person looking at the light, or the amount of light in environment. The less light there is, the brighter the light source appears. In order not to confuse these two concepts with illumination, it is worth remembering that:

brightness characterizes light, reflected from the surface of a luminous body or sent by this surface;

illumination characterizes falling light onto the illuminated surface.

In astronomy, brightness characterizes both the emitting (stars) and reflecting (planets) ability of the surface of celestial bodies and is measured on the photometric scale of stellar brightnesses. Moreover, the brighter the star, the lower the value of its photometric brightness. The brightest stars have a negative stellar brightness value.

Units of measurement

Illuminance is most often measured in SI units suites. One lux is equal to one lumen per square meter. Those who prefer imperial units to metric units use to measure illumination footcandle. It is often used in photography and cinema, as well as in some other areas. The foot in the name is used because one foot-candela refers to the illuminance of one candela on a surface of one square foot, measured at a distance of one foot (just over 30 cm).

Photometer

A photometer is a device that measures illumination. Typically, light is sent to a photodetector, converted into an electrical signal, and measured. Sometimes there are photometers that work on a different principle. Most photometers display illuminance information in lux, although other units are sometimes used. Photometers, called exposure meters, help photographers and cinematographers determine shutter speed and aperture. In addition, photometers are used to determine safe lighting in the workplace, in crop production, in museums, and in many other industries where it is necessary to know and maintain a certain lighting level.

Lighting and safety in the workplace

Working in a dark room threatens vision impairment, depression and other physiological and psychological problems. That is why many labor safety regulations include requirements for minimum safe illumination of the workplace. Measurements are usually carried out with a photometer, which produces the final result depending on the area of ​​light propagation. This is necessary in order to ensure sufficient illumination throughout the room.

Lighting in photography and videography

Most modern cameras have built-in exposure meters, making the work of the photographer or operator easier. An exposure meter is necessary so that the photographer or operator can determine how much light needs to be let into the film or photo matrix, depending on the illumination of the subject being photographed. Illumination in lux is converted by the exposure meter into possible combinations of shutter speed and aperture, which are then selected manually or automatically, depending on how the camera is configured. Typically, the combinations offered depend on the settings in the camera, as well as what the photographer or cinematographer wants to depict. Studios and film sets often use an external or in-camera light meter to determine whether the light sources being used provide sufficient illumination.

To receive good photos or video material in poor lighting conditions, sufficient light must reach the film or sensor. This is not difficult to achieve with a camera - you just need to set the correct exposure. With video cameras the situation is more complicated. For video shooting high quality Usually you need to install additional lighting, otherwise the video will be too dark or with a lot of digital noise. This is not always possible. Some camcorders are specifically designed for shooting in low light conditions.

Cameras designed for shooting in low light conditions

There are two types of low-light cameras: some use higher-end optics, and others use more advanced electronics. The optics let more light into the lens, and the electronics better handle even the little light that gets into the camera. Usually it is with electronics that problems are associated and side effects described below. High-aperture optics allow you to shoot higher-quality video, but its disadvantages are additional weight due to the large amount of glass and a significantly higher price.

In addition, the quality of shooting is affected by the single-matrix or three-matrix photomatrix installed in video and photo cameras. In a three-matrix array, all incoming light is divided into three colors by a prism - red, green and blue. Image quality in dark conditions is better in three-array cameras than in single-array cameras, since less light is scattered when passing through the prism than when it is processed by the filter in a single-array camera.

There are two main types of photomatrices - charge-coupled device (CCD) and those based on CMOS (complementary metal oxide semiconductor) technology. The first usually contains a sensor that receives light and a processor that processes the image. In CMOS sensors, the sensor and processor are usually combined. In low-light conditions, CCD cameras generally produce better images, while CMOS cameras have the advantage of being cheaper and consuming less power.

The size of the photo matrix also affects image quality. If shooting takes place with a small amount of light, then the larger the matrix, the better quality images, and the smaller the matrix, the more problems with the image - digital noise appears on it. Large matrices are installed in more expensive cameras, and they require more powerful (and, as a result, heavier) optics. Cameras with such matrices allow you to shoot professional video. For example, in lately A number of films have appeared that were shot entirely on cameras such as the Canon 5D Mark II or Mark III, which have a matrix size of 24 x 36 mm.

Manufacturers usually indicate the minimum conditions in which the camera can operate, for example, with illumination of 2 lux or more. This information is not standardized, that is, the manufacturer decides for himself which video is considered high-quality. Sometimes two cameras with the same minimum illumination level produce different shooting quality. The Electronic Industries Association (EIA) in the United States has proposed a standardized system for determining the light sensitivity of cameras, but so far it is used only by some manufacturers and is not universally accepted. Therefore, in order to compare two cameras with the same light characteristics, you often need to try them in action.

At the moment, any camera, even one designed for low-light conditions, can produce low-quality images with high grain and afterglow. To solve some of these problems, you can take the following steps:

• Shoot on a tripod;
• Work in manual mode;
• Do not use zoom mode, but instead move the camera as close to the subject as possible;
• Do not use auto focus and automatic selection ISO - higher ISO increases noise;
• Shoot at a shutter speed of 1/30;
• Use diffused light;
• If it is not possible to install additional lighting, then use all possible light around, such as street lamps and moonlight.

Although there is no standardization about cameras' sensitivity to light, for night photography it is still best to choose a camera that says it operates at 2 lux or lower. Another thing to remember is that even if a camera is really good at shooting in dark conditions, its light sensitivity, stated in lux, is sensitivity to light directed at the subject, but the camera is actually receiving light reflected from the subject. When reflected, part of the light is scattered, and the further the camera is from the object, the less light enters the lens, which deteriorates the quality of shooting.

Exposure number

Exposure number(eng. Exposure Value, EV) - an integer characterizing possible combinations excerpts And aperture in a photo, film or video camera. All combinations of shutter speed and aperture that expose the same amount of light to the film or sensor have the same exposure number.

Several combinations of shutter speed and aperture in the camera at the same exposure number allow you to obtain an image of approximately the same density. However, the images will be different. This is due to the fact that when different meanings aperture, the depth of field of the imaged space will be different; at different shutter speeds, the image on the film or matrix will be different times, resulting in it being smeared to varying degrees or not at all. For example, the combinations f/22 - 1/30 and f/2.8 - 1/2000 are characterized by the same exposure number, but the first image will have a large depth of field and may be blurry, and the second will have a shallow depth of field and, quite possibly, , will not be blurry at all.

Higher EV values ​​are used when the subject is better lit. For example, an exposure value (at ISO 100) of EV100 = 13 can be used when shooting landscapes if the sky is cloudy, and EV100 = –4 is suitable for shooting bright aurora.

By definition,

EV = log 2 ( N 2 /t)

2 EV = N 2 /t, (1)

Where
• N - aperture number(for example: 2; 2.8; 4; 5.6, etc.)
• t- shutter speed in seconds (for example: 30, 4, 2, 1, 1/2, 1/4, 1/30, 1/100, etc.)

For example, for a combination of f/2 and 1/30, the exposure number

EV = log 2 (2 2 /(1/30)) = log 2 (2 2 × 30) = 6.9 ≈ 7.

This number can be used for night scenes and illuminated storefronts. The combination of f/5.6 with a shutter speed of 1/250 gives the exposure number

EV = log 2 (5.6 2 /(1/250)) = log 2 (5.6 2 × 250) = log 2 (7840) = 12.93 ≈ 13,

which can be used to shoot a landscape with a cloudy sky and no shadows.

It should be noted that the argument of the logarithmic function must be dimensionless. In determining the exposure number EV, the dimension of the denominator in formula (1) is ignored and only the numerical value of the shutter speed in seconds is used.

The relationship between the exposure number and the brightness and illumination of the subject

Determining exposure by the brightness of light reflected from the subject

When using exposure meters or lux meters that measure light reflected from the subject, shutter speed and aperture are related to the brightness of the subject as follows:

N 2 /t = L.S./K (2)

• N- aperture number;
• t- shutter speed in seconds;
• L- average scene brightness in candelas per square meter (cd/m²);
• S- arithmetic value of photosensitivity (100, 200, 400, etc.);
• K- exposure meter or lux meter calibration factor for reflected light; Canon and Nikon use K=12.5.

From equations (1) and (2) we obtain the exposure number

EV = log 2 ( L.S./K)

2 EV = L.S./K

At K= 12.5 and ISO 100, we have the following equation for brightness:

2 EV = 100 L/12.5 = 8L

L= 2 EV /8 = 2 EV /2 3 = 2 EV–3 .

Lighting and museum exhibits

The rate at which museum exhibits deteriorate, fade, and otherwise deteriorate depends on their illumination and the strength of the light sources. Museum staff measure the illumination of exhibits to ensure that a safe amount of light is reaching the exhibits, but also to ensure that there is enough light for visitors to see the exhibit well. Illumination can be measured with a photometer, but in many cases this is not easy, since it must be located as close to the exhibit as possible, and for this it is often necessary to remove safety glass and turn off the alarm, and also obtain permission to do so. To make things easier, museum workers often use cameras as photometers. Of course, this is not a substitute for accurate measurements in a situation where a problem is found with the amount of light that falls on the exhibit. But in order to check whether a more serious check with a photometer is needed, a camera is quite enough.

The exposure is determined by the camera based on the illumination readings, and, knowing the exposure, you can find the illumination by doing a series of simple calculations. In this case, museum staff use either a formula or a table that converts the exposure into illumination units. During calculations, do not forget that the camera absorbs part of the light, and take this into account in the final result.

Lighting in other areas of activity

Gardeners and growers know that plants need light for photosynthesis, and they know how much light each plant needs. They measure light levels in greenhouses, orchards and vegetable gardens to ensure that each plant is receiving enough light. Some people use photometers for this.

What is LED? (LED)

Light-emitting diode (LED, LED, LED Light-emitting diode) - a semiconductor device that creates optical radiation when passed through it electric current. The emitted light lies in a narrow range of the spectrum, its spectral characteristics depend, among other things, on chemical composition semiconductors used in it.

What is luminous flux? (lumen, lm, mml)

Luminous flux - physical quantity, characterizing the “amount” of light energy in the corresponding radiation flux. In other words, this is the power of such radiation that is perceptible to the normal human eye.

What is a lumen? (lm, mlm)

Lumen (symbol: lm, lm) is a unit of measurement of luminous flux in SI.

What is luminous intensity? (kd, mkd)

Luminous intensity is the quantitative value of the radiation flux at a certain radiation angle. (The luminous intensity is the luminous flux related to the infinitesimal angle within which it propagates.)

What is Candela? (kd, mkd)

A unit of measurement for luminous intensity. Application for measuring the light intensity of directional sources. This is why for 5mm LEDs the values ​​are given in candelas or millicandelas (1 cd=1000 mcd).

What is illumination? (lux, lx, lx)

Illumination is measured in lux (lx). The unit is the illumination of a surface with an area of ​​one square meter at a normal incidence of a light flux of one lumen on it.

What is brightness and luminosity? (cd per square meter and lm per square meter)

The unit of measurement of brightness, as follows from the definition of this value, is candella per square meter (cd/m2), and luminosity is correspondingly measured in lumens per square meter (lm/m2).

Luminosity is expressed as the ratio of the luminous flux to the surface area of ​​the source.

Brightness is characterized by the glow of the surface, which is given in the direction of the observer by each unit of its visible area. Self-emitting sources can be called primary, and those reflecting the light of others can be called secondary. The brightness of luminous surfaces is determined by the ratio of the luminous intensity in the direction under consideration to the area of ​​projection of this surface onto a plane perpendicular to this direction.

What is half brightness angle?

The LED gives maximum light in the center and at the top, that is, the angle is zero. The farther from the center, the less light. The half-brightness angle is when at “0” degrees the LED gives 100 conventional units of light, and, for example, at 30 degrees (relative to the “0” axis) - 50. half-brightness angle In the figure, I is the luminous intensity, Imax is the maximum luminous intensity . ImaxCos is half the luminous intensity. Since the LED emits symmetrical light, the degrees must be multiplied by two, so the angle is “double”. The result is an isosceles triangle of light. There is also light outside this triangle, but the reference point for the LED characteristic is a half angle. (excerpt from an article by Yuri Ruban)