Basic concepts of lighting engineering

In lighting engineering, as in any branch of science and technology, there are a number of concepts that characterize the properties of lamps and luminaires in standardized units of measurement. The most important of them are given below in brief.

Light and radiation.
Light is understood as electromagnetic radiation that causes a visual sensation in the human eye. In this case, we are talking about radiation in the range from 360 to 830 nm, which occupies a tiny part of the entire spectrum of electromagnetic radiation known to us.

Luminous flux F.
Unit of measurement: lumen [lm].
Luminous flux F is the total radiation power of a light source, assessed by the light sensation of the human eye.

Luminous intensity I characterizes the power of the luminous flux of a lamp F in a solid angle W

Luminous intensity I.
Unit of measurement: candela [cd].
A light source emits a luminous flux Ф in different directions with different intensities. The intensity of light emitted in a certain direction is called luminous intensity I.

Representation of luminous intensity in polar coordinates

Illuminance E.
Unit of measurement: lux [lx].
Illuminance E reflects the ratio of the incident luminous flux to the illuminated area. Illuminance is equal to 1 lx if the luminous flux of 1 lm is uniformly distributed over an area of ​​1 m ² .

Illumination E

Brightness L.
Unit of measurement: candela per square meter [cd/m2 ^] .
The brightness of light L of a light source or illuminated area is the main factor for the level of light sensation of the human eye.

Brightness L

Basic lighting formulas:

Force of the World I [kd]

Illumination E [lx]

Illumination E [lx]

Brightness L [cd/ ^m2 ]

Luminous efficiency h [lm/W]

Luminous efficiency h .
Unit of measurement: lumen per watt [lm/W].
Luminous efficiency h shows how efficiently the consumed electrical power is converted into light.

Colour temperature.
Unit: Kelvin [K].
The colour temperature of a light source is determined by comparison with a so-called “black body” and is displayed by the “black body line”. If the temperature of the “black body” increases, the blue component in the spectrum increases and the red component decreases. An incandescent lamp with warm white light has, for example, a colour temperature of 2700 K, and a fluorescent lamp with daylight colour has a colour temperature of 6000 K.

Color of Light
Different people perceive the same color differently. Figuratively speaking, the concept of a particular color is just the result of an unwritten agreement between people to call a certain sensation of the optic nerve a specific color, for example, "red". Moreover, in the book by C. Padgham and J. Saunders "Perception of Light and Color" it is mentioned that "there is information about differences in the pigmentation of the lens in different races, which can lead to differences in color vision". It is also known that with age the lens turns yellow, which leads to disturbances in color identification. That is, we can say that adequate color perception is the result of a psychological process rather than a physical one. As you can see, science had to work hard to systematize and strictly scientifically determine the characteristics of different colors of the spectrum!

If the color of the surface of an unheated non-radiating object, that is, one of its reflective (and therefore filtering) characteristics, can be described by the wavelength or its inverse value - frequency, then we will act differently with heated and radiating bodies. Let us imagine an absolutely black body, that is, a body that does not reflect any light rays. For a primitive experiment, let it be a tungsten spiral in an electric bulb. Let us connect this unfortunate bulb to an electric circuit through a rheostat (variable resistance), throw everyone out of the bathroom, turn off the lights, apply current and observe the color of the spiral, gradually lowering the resistance of the rheostat. At one fine moment, our absolutely black body will begin to glow with a barely noticeable red color. If we measure its temperature at this moment, it will turn out to be approximately equal to 900 degrees Celsius. Since all radiation comes from the speed of atoms, which is zero at zero degrees Kelvin (-273C) (which is the basis of the principle of superconductivity), we will forget about the Celsius scale and use the Kelvin scale.
Thus, the beginning of visible radiation of an absolutely black body is observed already at 1200K, and corresponds to the red border of the spectrum. That is, simply put, the red color corresponds to a color temperature of 1200K. Continuing to heat our spiral, measuring the temperature, we will see that at 2000K its color will become orange, and then, at 3000K - yellow. At 3500K our spiral will burn out, since the melting point of tungsten will be reached. However, if this had not happened, we would have seen that upon reaching a temperature of 5500K the color of the radiation would have been white, becoming bluish at 6000K, and with further heating up to 18000K increasingly blue, which corresponds to the violet border of the spectrum.

These numbers are called the "color temperature" of the radiation. Each color has its own color temperature. It is psychologically difficult to get used to the fact that the color temperature of a candle flame (1200K) is ten times lower (colder) than the color temperature of a frosty winter sky (12000K). However, this is true, the color temperature differs from the usual temperature.

The color of light is very well described by the color temperature. There are the following three main colors of light:
warm white < 3300 K
neutral white 3300 - 5000 K
daylight white > 5000 K.
Lamps with the same color of light can have very different color rendering characteristics, which is explained by the spectral composition of the light they emit.

Color of white light of some sources

Colour Rendering
Depending on the installation location of the lamp and the task it performs, artificial light must ensure the best possible colour perception (as in natural daylight). This capability is determined by the colour rendering characteristics of the light source, which are expressed using different degrees of the “overall colour rendering index” Ra.
The colour rendering index reflects the degree of correspondence between the natural colour of an object and the visible colour of this object when illuminated by a reference light source. To determine the Ra value, the colour shift is recorded using the 8 standard reference colours specified in DIN 6169, which is observed when the light of the test or reference light source is directed at these reference colours. The smaller the deviation of the colour of the light emitted by the test lamp from the reference colours, the better the colour rendering characteristics of this lamp. A light
source with a colour rendering index Ra = 100 emits light that optimally reflects all colours, like the light of a reference light source. The lower the Ra value, the worse the colours of the illuminated object are rendered.