Light flux is a physical term that allows one to estimate the power transferred by radiation in a given direction. The value is used to calculate the illumination, the latter is set by state standards for objects that provide human life.
The position of the luminous flux in the hierarchy of physical quantities of optics
The uninitiated man does not speak much of the labels of the photographs given labels. Given the characteristics of the luminous flux in lumens. Knowing the luminous flux, one can calculate the surface illumination. Requirements are given GOST, more precisely, BCH-1-73 for sports facilities. Conditions are not taken from the ceiling, determined by human physiology. If we are talking about a school class, the health of the eyes of students, teachers, and the gym is taken into account - the interests of the students are taken into account.
Negative, unpleasant consequences are possible when departing from the norm. Each case is a document. Sanitary regulations, construction regulations. Here is the position of the light flux in the hierarchy of optical quantities. ..
The primary characteristic of the optical range emitter( light bulb) is the light intensity. Shows how much power is transferred in a given direction bounded by a solid angle. Consequently, the luminous intensity on the graph is not a straight line. The maximum power of the diode array is transferred in the direction perpendicular to the aperture of the eyepiece( a small lens that is built in constructively to improve the focusing of the radiation).As for the incandescent lamp, it is close to the parameters of an isotropic source.
For a complete understanding of the question let's make an idea about steradians. The unit of measurement of the solid angle with the top in the center of the sphere. Formally, a steradian is equal to the ratio of the area of an imaginary ball and the square of the radius. Consequently, the value is dimensionless, like an angle. Therefore, in order to avoid confusion, physicists have introduced steradians. Clearly, the value is closely related to the parameters of the sphere. It follows from the definition: 12.56( 4 Pi) steradians fit on the area of the sphere.
The solid angle is volumetric, looks like a cone with a vertex at the center of an imaginary ball. The base of a three-dimensional shape is not a plane. It is incorrect to compare the solid angle and the cone. The base is the part of the sphere, cut off by the generator( side) surface. The unit of light power is a candle, as they are called in the Western style, candela( the name appeared in 1948, before this unit was called a new candle).Both terms are a translation, the original sound of the Latin word from which the name derives. Candela is one of the six main plates of the international SI system, through which other units are expressed.
According to the peace agreements, most countries adopted meter, second, and kilogram in 1960.The list consists of six parameters. It should be supplemented with candela, ampere and Kelvin degree. Candles were given different definitions. Here are some:
- one-sixtieth of the melting radiation of platinum with an area of one square centimeter when the metal solidifies( 2046 K) in the direction of the perpendicular to the surface( CIPM 1946);
- power emitted perpendicularly to a black-body plot with a temperature of 2046 K, an area of one six hundred thousandth of a square meter at a normal pressure of 101325 Newtons per square meter( CGPM 1967);
- luminous intensity of 540 GHz, power of one six hundred and eighty-third of a watt( the definition given by the CGPM in 1979, taking into account the physiological features of vision).
Until 1946, the intensity of light had other definitions. In fact, for each country is unique. In practical calculations, the value is used little. Engineers found a sufficiently integral parameter. They became the luminous flux present on the labels. With regards to candel, closely related to solid angle, direction, do not give a complete picture of the source. It is clear that it is inconvenient for a person, even an experienced physicist, to operate with the values of a three-story complexity. It would take a table of values depending on the direction. Simply put, the power of light will not allow to solve the tasks set in practice, for example, to calculate the parameters of illumination from the point of view of the requirements of the state standard.
Luminous flux shows how much power falls on the surface. Does not depend on solid angles. Just make sure that the light flux is indicated by the label of any light bulb. The value will allow us to say that LEDs, consuming 9 W, produce more than the filament, taking 60. The label shows an error: the total luminous flux of the light source is given without setting the proper fact accent. There may be confusion. In physics, the following definition of units of measurement of luminous flux, lumens is accepted: the power emitted by an isotropic light source, enclosed within a solid angle equal to one steradian.
From the obvious facts available everywhere, the Internet is clear: with a filament of 60 W, the luminous flux enclosed within a single steradian cannot be equal to 710 lumens. Too big number. In the textbook Course of Physics for secondary specialized educational institutions of authorship Zhdanova LS, Marandzhyan V.A.(Volume II) states: a 100 W light bulb incandescent has a luminous intensity of 100 candles( the modern analogue of the unit is candela).Consequently, the total luminous flux in lumens is 1256. Obviously, the figure is of the same order as 710, it never took place if an amendment of 12.56( 4 Pi) was needed. It can be assumed that since 1971 the production technology has undergone changes, but not qualitative ones.
In the case of an LED lamp, the product cannot be considered an isotropic radiator. Indirectly, the label indicates the scattering angle of 240 degrees. Implied cone, limiting part of the sphere. The cut part will be found using a simple analogy. Obviously, the scattering angle of 180 degrees is similar to 6.28 steradians. Exactly half the scope. Converting other values to solid angles is difficult. However, the parameter indicated on the label is convenient for calculations. Visually makes it clear in which direction the light spreads, which helps the engineers to solve practical problems. Satisfy the requirements of the standards( see above).More is said below, now you need to finally dot the i with the luminous flux.
Figure schematically depicts a light bulb with characteristics taken from the label. The scattering angle indicates: the radiation propagates above the plane of the instrument. Changes imposes a chandelier. The radiation is propagated without changes by the ceiling solution. In other directions, the residual part of the scattering angle( from the label) is approximately uniformly radiated, taking into account the weakening of the glass.
In addition to the above, the luminous flux is used to evaluate the reflective properties. Black letters are clearly visible on light paper. The luminous flux reflected by white objects is larger. Therefore, in light tones are painted cases of refrigerators.
Illumination and requirements of
standards The luminous flux in the current interpretation characterizes the total power emitted by the source of the optical range. Crashed on the surface of the room unevenly. In the standards of various requirements, the norms indicate the illumination of the object. The value is measured in lux( lat. Pronunciation of the word “light”), numerically equal to the ratio of the luminous flux in lumens to the area to which it falls. The interpretation itself is not difficult, the practical application is accompanied by a number of problems.
When calculating, the luminous flux in lumens is inconvenient to use in conjunction with the scattering angle. The area cut off from the sphere is non-linearly dependent on degrees. It goes well with steradians. For specific calculations, it is better to apply the formula of connection given in the figure. Allows you to convert degrees to steradiana.
In the figure at point I, the center of the sphere. The covered solid angle cuts off part of the surface, the resulting shape resembles a cone. To find the illumination, a parallel is drawn between the steradians and degrees using the indicated formula. The first unit will allow a simple way to calculate the power( luminous flux) of the proportion, the second - to measure the relative position of objects in the room. Please note: this formula is applicable for opening angles less than 90 degrees. Therefore, in the case of our LED light bulb, we will have to find the parameters of the cone, which complements 240 degrees to the sphere.
The total aperture will be 120 degrees( 360–240).The formula appears half, 60 degrees. Substituting this value and given that the sine of 30 is 0.5, we get 3.14 steradians. Consequently, the other part of the sphere will be 3 Fisteradians. Apply to a specific room:
Suppose a light bulb is located above a table with an area of 0.5 square meters. The distance to the plane is 2 meters.
We find the total area of the ball, the value is 4 x Pi x 2 x 2 = 16 Pi sq.m. The figure in terms of steradians is 4 x Pi. The lamp illuminates 3 x Piradian( see above), which is expressed in square meters, like 12 Pi. We divide the area by the dimensions of the tabletop, we get 24 Pi. Consequently, the region has an illumination of about 75 lux. According to the SNiP, Sanitary Rules, the value is barely enough for the archive room. In the study you need more than one light source, or change the type of devices. Get more powerful.
Limitations on the calculations of the illumination of the
The above calculations were carried out in a highly simplified form. A case of total darkness was taken. During most of the working day the sun shines through the window, therefore, some of the light is provided by a natural source. It is logical to split the illumination in several parts, each is designed for its time. Much is determined by the design of the lighting device. For example, if the cover unambiguously introduces losses, the flashlight reflector greatly enhances the flow in a given direction. Source specifications are separate from the lamp. An obvious, often overlooked fact.
In practice, the light parameter is measured with a luxmeter. Known: engineering calculation contains an error. In any industry, the value is known. In radio engineering it reaches 30%.Lighting calculations are performed approximately, to meet the requirements of existing standards need exactly. Therefore, the result should be checked. A luxmeter is used, showing the true illumination of the room area. The instrument readings are compared to table values taken from standards.
The luminous flux given by the label is not constant over the area of the sphere. Not always in practice there is a simplified case when the lamp is above the object. It is necessary to take the projections of the investigated surface on the ball, to work in the chosen key.