The basic unit of luminous intensity - Candela

Luminous intensity candela is the basic unit of the International System of Units one of the basic unit. In this paper, a more detailed description of its history and its definition of evolution, a brief account of the complex is the definition of principles, methods and experimental apparatus. China's electricity consumption in 1982 is complex calibration radiometer luminous intensity candela units, the total uncertainty as ± 0.28%. Finally, on the development direction of Candela's a brief description.

Candela is the intensity of light-emitting unit of the International System of Units (SI) of the seven basic unit of one of the symbols used "cd", said.

As we all know, human beings alone, as well as the nature of biological existence and development of an important material. Scientists found that people's eyes, and other sensory organ, from the outside world to receive all the information, there are more than 70 percent from the light. The light of human understanding, has also experienced the phenomenon by the nature of the development process, optical metrology is also accompanied by the understanding of the process of arising and development. This article will be only the definition of luminous intensity, and the evolution of the complex is a brief introduction.

One. Luminous intensity of the original definition - Candlelight

The measurement of luminous intensity of the original through the eyes of people feel. About 200 years ago, has been the use of "candlelight" as a unit of luminous intensity. It is a candle in the horizontal direction of luminous intensity. Such as in 1860, the United Kingdom city lights and regulations adopted by the "whale wax." Later, one after another the use of standard light sources are vegetable oil lamp, the lamp-pentane and pure light, such as amyl acetate.

At the same time, scientists have explored the theory luminous intensity. 1727, P. Baoji Er published his book "light on graduated optical experiments," which is no doubt that the visual light measurement is the first major attempt. In 1760, long a white, in his monograph, and detailed definition of a luminous flux, luminous intensity of illumination, the brightness of the light, and other important parameters, but also explained the link between the mathematics. These studies on the amount of photometer is important to the development.

In 1881, the International Electrotechnical Commission in accordance with the development of science and technology and requirements of the "candlelight" for the provisions of international units and the definition is as follows:

Will be a pound of whale oil candles made from 6 to 120 cells per hour so that the speed of combustion, in the direction of the level of luminous intensity for a candlelight vigil.

The above definition of grain quality for the unit. In this definition we can see that the intensity of light-emitting fuels is not only relevant but also with the wick, a high flame, and other factors, it is complex and stability are far from satisfactory.

In 1879, Vionnet as to avoid the inconvenience of flame standard, the proposed solidification process of pure platinum trough of 1 square cm surface light intensity as a luminous intensity standard. For a variety of reasons (for example, the purity of platinum is not higher), making it less repetitive, not used. However, the idea of later development of a certain light.

In 1879, Edison invented incandescent, artificial lighting into realities, but also promoted the development of the volume of photometer. In 1909 Britain, France and the United States relating to research institutions, in order to enhance the unity and the international scope of the luminous intensity of the agreement to adopt the definition of carbon filament incandescent light-emitting intensity, that is,

Pentane derived from the lights and a group of 45 which consists of carbon filament incandescent average luminous intensity. Also known as the "International candlelight."

The stability of the carbon filament incandescent better, but is less complex, almost two out of the legal system as luminous intensity of the incandescent lamp, so that the base is not entirely satisfactory, it is only temporary.

Second, the new definition of luminous intensity - Candela

In 1908, Waiolner and Burgess put forward by immersion in the platinum filled with a solidification of the tank as a light-emitting black body strength. In 1933, the nature of the blackbody theoretically provide a luminous intensity of the solution method, using the principles of the new light units will be built on the platinum freezing point temperature of the blackbody radiation on. As a result, in 1937, the International Commission on Illumination (CIE) and the International Committee decided measures starting from 1940 the use of "candlelight" for the intensity of light-emitting unit, and is defined as follows:

Full-body radiation in the platinum solidification of the brightness temperature of 60 square centimeters each of the new candlelight.

In other words, platinum freezing point (2042.15K), the absolute size of the blackbody 1cm2 of 1 / 60 part of the luminous intensity for a candlelight vigil. Due to the delay of the Second World War, this standard had not been implemented.

In 1946, the International Measurement Committee in 1933 under the 8th international measurement of the powers conferred by the General Assembly, the promulgation of the decision:

1 = 1.005 New Old candlelight candlelight.

In 1948, the 9th International measures adopted by the General Assembly with Latin - candela (Candela) to replace the new candlelight, Candela means "animal oil produced by the candle."

In 1967, the 13th International General Assembly to consider measures to the wording of the definition is also due to close, it was decided that Candela is defined as:


Candela was 101,325, under pressure newton per square meter, a platinum solidification temperature of the blackbody 1/600000m2 surface in the vertical direction of the light intensity.

In 1971, the 14th General Assembly of the International Measurement by the pressure of the Newton unit per square meter of the special name "Pascal", as the symbol Pa. In this way, changing the definition of candela:

Candela 101325Pa is under pressure from platinum in the solidification temperature of the blackbody 1/600000m2 surface in the vertical direction of the light intensity.

There are two definitions of this term need to explain:

1. The platinum freezing point: We can understand the sense of solid to liquid to melt, and then the formation of steam in the process, although they co-exist when the temperature is fixed, but they can not be fixed in any state, in the Will be solid in the process of heating, the temperature gradually increased, at the beginning of a melting when the temperature is fixed, then melt completely solid start to heat up, in which the junction is the freezing point.

2. Bold: blackbody is a hypothetical energy sources, in the evaluation of other sources, using it as a relatively light source and reference light source, a radiation only depends on its temperature of the radiator. Such an object, it can at any temperature, it will be to radiation on the surface of any wavelength of all the energy absorbed. In other words, in any radiation-temperature blackbody spectrum absolute absorption rate equal to 1.

For example, if a complete closure to the room marked with small holes into the light, light on the constant reflection on the wall, but can not return to the original hole, and all the radiation to the inside. If the piece of metal into the Chi-empty, the radiation is internal, not reflective light to the outside. If this piece of metal in exchange for the freezing point of platinum into an empty, it is not radioactive radiation, this can be defined as an empty black body. With a very small peephole generally close to the hole in bold.

In fact, based on the performance of the physical form of full-light-emitting black body is full.

According to this definition of luminous intensity, many countries around the world have established a benchmark candela of blackbody radiation, and the international than the right.

Third, the latest definition of candela

70 years, several national laboratories bold use of the base complex convergence is the Candela, the different data from several international comparative results, a difference of about ± 1%. This shows that countries are in the resumption of Candela, you may still exist have not yet found a system error, which also revealed the existence of the above definition of Candela. As a result, people started to re-consider the definition of Candela. At the same time, radiation measurement of the rapid development of technology, already used in some countries it is a complex Candela, and to achieve the same accuracy. In 1975, W. R. Blevin and others propose to redefine Candela, the international committee and radiometric measurement of the Advisory Committee (CCPR) support and encourage countries to use experimental methods of measuring the value of Km (next largest visual light spectrum, as the performance value of 6831m / W). 1977 to date, 10 countries (including China) will study the measurement of their own measurements to inform the committee, most of the countries close to measuring the value of 6831m / W, from a theoretical calculation of the value of Km is also just a few that end. As a result, CCPR decided to adopt 6831m / W as the Km value, so as to maintain the continuity of the light units. As a result, on October 8, 1979 held in Paris in the 16th International General Assembly on measures to repeal the measures of the International Committee in 1933 under the 8th international measures authorized by the General Assembly in 1946 and decided in 1948 by the 9th International measures approved by the General Assembly And the 13th General Assembly of the International Measurement revised definition of Candela. At the same time, adopted a resolution on re-definition of Candela's an important decision. The new definition is:

Candela is a light source in a given direction of luminous intensity, the frequency of light to issue 540 × 1012 Hz of monochrome radiation, and once in the direction of this radiation is 1 / 683 watt per steradian.

The definition of 540 × 1012 Hz radiation wavelength is about 555nm, it is the sense of the human eye's most sensitive wavelengths.

This definition has the advantage of easily replicable, and the luminous intensity as a result of radiation according to the definition of the relationship between the fixed, and the light intensity units and other power units (such as Watt), a simple relationship can be developed in a variety of experimental methods And to be able to better control the accuracy of the experiment.

Fourth, the complex is candela

At present, various countries are complex Candela There are two main ways: First, the power radiometer calibration method, the radiation spectrum is the law. The former complex is a high accuracy, while the latter later Spectrophotometric due to weak signal and difficult to measure accurately, and therefore a greater error. The following brief account of electrical calibration radiometer law is complex and its main principles of Candela's experimental device.

1. Candela's theory is complex

Photometry from the basic principles, we can see light at a fixed distance under the light of the degree of Ev with the corresponding spectral irradiance Ee, λ for the relationship between the

Km-in for maximum visual spectrum out performance, V (λ) for the observation of the CIE standard light spectrum efficiency.

When the Add V (λ) filter, the restrictions in the diaphragm of the radiation received by the degree of under-calculated:

Τ555-in filter for 555nm in the Department of transmission, τ (λ) for the relative transmission spectrum, l as a light source to limit the distance between the diaphragm, △ light to filter through later amendments to the optical path.

From the two above formula can be seen clinging filter aperture restrictions radiator when the Department of illumination for

In-Ek, λ light source for the relative spectral power distribution. Because the two points, there is no spectral irradiance Ek, λ common factor, so do not need to calculate it, and use the relative spectral power distribution of Ek, λ replaced.

EV obtained, according to the distance inverse square law, you can calculate the light intensity of the light-emitting IV:

IV = L2EV.

This is the complex is the main principle of luminous intensity candela.

2. Candela is the complex experimental installation

Device as shown in Figure 1, on the 6m-long light rail, radiator in a special block of the stent, its precise location can be adjusted. Following the experimental device in the main section: cavities cone-3 radiometer to measure the radiation power of the base, it will put heat shielding box, so that the tapered radiometer to avoid the ambient temperature changes have the potential thermal drift. During the experiment, with its standard measurement of radiation in the light of restrictions arising from the diaphragm on the surface of the irradiance. V (λ) filter 4 by 4 colored glass, to amend radiometer spectral sensitivity to the visual spectrum out with the efficiency of light as V (λ) for its similarity. 7 screen is used to block to block stray radiation into the radiator inside.

Filament will be flat, light-water shutter gate 5, 6, HU-4, radiator central diaphragm restrictions on the precise adjustment on the same axis measurement, and so limit the filament plane and the axis perpendicular to the diaphragm, the radiometer Thermal shielding box placed around 10h, until the temperature distribution, you can start measuring.