Luminous intensity

The unit of luminous intensity is candela. This, figuratively speaking, indicates the density of light which a light source emits. The denser the light, the brighter and more intense it appears, and conversely, the less dense it is, the dimmer it appears. The unit of luminous flux is lumen. It indicates the total amount of light emitted by a light source. The unit of illumination is lux, which gives the "density" of light falling on the surface on which the light source shines.

The more we direct the total amount of light (lm) into a narrower cone, the "denser" the light becomes, i.e., it increases its luminous intensity (cd), and thus the illumination (lx) also increases. At the same time, however, the illuminated area will decrease. An example can be car headlights, where the total amount of light (lm) is directed into a narrow cone to achieve high luminous intensity (cd) and thereby increase the distance for the desired illumination (lx).

Honestly, when you want to choose a bulb for your home, it's not that important. It's more about understanding the relationships between these quantities to be able to compare, for example, two products according to their intended use and to have a basic orientation about what is stated on the packaging.

Several examples

• The most important quantity is usually luminous flux (lm). This is always stated on LED bulbs and tells us how much the bulb will shine. The higher the value, the more light the bulb will emit.
• Next in line is the emission angle. For instance, if we want to illuminate a dining table, bulbs with a narrower emission angle will provide higher illumination (lx) of the table. A bulb with a wider angle will shine more to the sides and the table will not be as illuminated, or the light will need to be reflected from a shade, which will result in a loss of illumination. Such a bulb is more suitable for illuminating the whole room, e.g., with ceiling lights.
• In some narrowly directional light sources (e.g., flashlights or cycle lights), the main parameter is the unit of luminous intensity (cd). This is stated because we are not so interested in the total luminous flux (lm), but rather in how "sharp" the light will be. This better expresses the property of how far the light will shine, or how intense the illumination (lx) of a distant object will be.
• Sometimes you can also find information about illumination (lx). However, this tells us almost nothing about the bulb unless it is supplemented with the distance, as the illumination decreases with increasing distance. However, without information about the emission angle, it is not possible to assess the total luminous flux (lm), only to derive luminous intensity (cd). This unit is rather used to determine if, for example, a workplace is sufficiently lit, depending on the type of work being performed. This determination can be made by calculation, but in practice is usually conducted directly on site with a lux meter.

One candela is the luminous intensity of a light source which in a given direction emits monochromatic radiation of a frequency of 540×10¹² Hertz and whose radiant intensity in that direction is 1/683 watt per steradian.

One lumen is the luminous flux emitted into a solid angle of one steradian by a point source with a luminous intensity of one candela in all directions.

Illumination of one lux is caused by a luminous flux of one lumen falling on an area of 1 m².

We can also mention color temperature. It is used to express the color tone emitted by a bulb. It is given in kelvins (K), the same as actual temperature, because the same color tone would be found in an absolutely black body heated to that temperature (an analogy could be heating iron. Cold, it is dark, upon heating it turns red, and with increasing temperature it turns white. The Celsius scale used by us has the same degree size as the Kelvin scale but is offset by 273.16 degrees higher. Therefore, 0 degrees Celsius is 273.16 degrees Kelvin and 100 degrees Celsius is 373.16 degrees Kelvin.)

Visible color temperatures for the human eye are approximately from 800K to 18,000K. The higher the color temperature, the color shade changes from red to blue. For example, a classic bulb emits light by heating a tungsten filament to a high temperature. At a low temperature, it first glows red and at higher temperatures, this color changes through yellow to white. Its possibilities end there because at a higher temperature the bulb's filament would melt.

Modern light sources such as fluorescent lamps and LED bulbs work on different physical principles than filament heating, allowing them to achieve higher color temperatures than traditional bulbs without any part of them reaching actual high temperatures. In the case of LED bulbs, it can even be said without exaggeration that they produce so-called cold light, where the light-producing chip is usually only a few dozen degrees warmer than the surrounding temperature.

The human eye is a perfectly reliable tool for comparing color shades. It can distinguish between two different color shades. Therefore, if you are choosing bulbs for one room and want the overall impression to be compact when all the light sources are turned on, you will reliably recognize if bulbs with the same color temperature are used. There may be an impression that, for example, the difference between 2700K and 3000K is not very large and that it "doesn't really matter" and won't be noticeable. The opposite is true, you will immediately recognize that it "does matter". Therefore, it is good to consider which bulb shade is suitable for the given purpose and whether "jumps" in color temperature might be disruptive.