Candela

Candela (from the French candelie - candle) is a luminous flux equal in energy to a photon corresponding to a radiation frequency of 5.4•10^14 Hz.
Candela is a unit of luminous intensity used in the SI system of units. It was introduced in 1979 and replaced the obsolete photon unit.

The candela unit was chosen as the unit of luminous intensity based on the following considerations:

1. A photon has an energy equal to hv, where h is Planck's constant, v is the radiation frequency.
2. Photons corresponding to frequency 5.410^14 Hz, have an energy of 6.62610^-34 J.
3. A photon corresponding to a frequency of 5.4 10^14 Hz and having a wavelength of 600 nm has an energy of (1.24 10^-19 J) 6.25 10^-20 J.
4. A photon with energy 1.2 10^-19 J (wavelength 600 nanometers) has momentum p = mv, where m is the mass of the photon.
5. A photon with mass m = hv / c^2 = 2.898 10^-33 kg has an impulse p = 2.518 10^-30 kg m/s.
6. The momentum of a photon corresponding to 5.4 * 10^14 hertz is equal to 2.65 10^-31 kg m/s, which is equal to the momentum of a photon with energy (1.24 * 10^-19) J.

Candela: SI unit of luminous intensity

In the International System of Units (SI), the candela is the basic unit of luminous intensity. Candela is used to estimate the intensity of light that is emitted by light sources in a certain direction. Indicated by the symbol "kd".

Candela is defined as the luminous intensity in a given direction from a monochromatic light source with a radiation frequency of 540 x 10^12 hertz and a radiation intensity in a given direction equal to 1/683 watts per steradian. Watt per steradian is a unit of light intensity that reflects the amount of light energy passing through a unit surface in a unit solid angle.

The value of 1/683 in the candela formula is the result of normalizing a standard light source, called a standard illuminant, which states that the luminous intensity is equal to 1 candela in the direction normal to the source.

Candela has important applications in various fields related to lighting and photometry. It allows you to evaluate the brightness of light sources such as lamps, lamps, car headlights and spotlights. Candela is also used to characterize light distribution and illuminance in architectural designs, event lighting, and even in the development of displays and LED technologies.

It is important to note that the candela only measures light intensity and does not take into account the spectral distribution of light or the perception of light by the human eye. Other photometric quantities such as lux and nit are used for these purposes.

In conclusion, the candela is a unit of luminous intensity that determines the intensity of light from a source in a specific direction. It plays an important role in photometry and lighting, providing precise measurements and standards for assessing the brightness of light sources.

Luminous intensity is a physical quantity that characterizes light radiation emitted by material bodies and is determined by the energy of photons incident on a unit area. The amount of luminous intensity is measured in candelas. To obtain a candela, only light of a strictly defined frequency can be used, the spectral composition of which is especially important: this frequency should be approximately 5 × 10 ^ 11 Hz. This is due to the fact that only an emitting source with such radiation parameters has approximately the average brightness of a human pupil when precisely illuminated by the light of an isotropic scene. When light with a significantly shorter wavelength is brought to the eye retina, the brightness of the fixed point decreases significantly, less than the light that does not reach the retina.