When we look at lighting products, we will always see a lot of professional terms, but most designers are difficult to understand these concepts, in fact, the essence of these concepts is the data of light, of course, we can not only use data to design lighting, but if we can grasp the meaning of these data, we can imagine the impact of light on space to some extent, so we must master the language and unit of light. To help designers better understand these terms, a definition of each concept is given below.

Schematic diagram of the language and unit of light▲

1. Color temperature

Color temperature represents the data of light color, also known as "chromaticity", the unit is Kelvin (K), color temperature refers to the color change that the human eye can perceive under different energies of light waves, and is used to represent the scale of light color of the light source.

Color temperature diagram ▲
When the color of the light source is the same as the color of the black body at a certain temperature, the absolute temperature scale of the black body is the color temperature of the light source. The "black body" is an idealized object that absorbs all external electromagnetic radiation without any reflection or transmission.

The color temperature changes in the visible light field during the heating of the blackbody (for artificial light)▲
In other words, a blackbody has an absorption coefficient of 1 and a transmission coefficient of 0 for electromagnetic waves of any wavelength. Physicists use it as a standard object for the study of thermal radiation. In the process of heating the black body, with the increase of energy, it will enter the field of visible light, and the color temperature change in the field of visible light will rise in the order of red→orange→ yellow→ white→ blue and white. For example, at 2800K, the color light emitted is the same as that of a light bulb, so we say that the color temperature of a light bulb is 2800K.

The impression of the space changes depending on the color temperature▲
The same is true for natural light, which is common in daily life, for example, the color temperature of sunlight varies from early morning and noon to dusk, with reddish sunrise and sunset having a lower color temperature, and yellowish white sunlight having a higher color temperature at noon. The color temperature value determines whether visible light produces warm or cool light. In general, if the color temperature is low, it will have a touch of orange to give people a warm feeling, and if the color temperature is high, the light will have a touch of white or blue, giving people a refreshing and bright feeling. The light with different color temperatures in the space will determine the perception of visible light.

Indoor color temperature reference legend▲

2. Luminous flux

Luminous flux is the amount of visible light energy, that is, the amount of light passing through, and the more luminous flux of a light source, the more light it emits. It is also the sum of all the radiant energy emitted by a light source and perceived by the human eye in a unit of time, which can also be called a light beam, and its unit is lumens (lm).

Different light sources emit luminous fluxes of different intensities in different directions▲
Designers can skip the definition of backlight flux, but use the luminous flux of some common light sources listed in the table below to visualize what 1lm of light means. In the actual design work, the luminous flux parameter can be found in the product samples provided by the manufacturer.

Luminous flux of common light sources▲

3. Illumination

Based on the surface of the object irradiated by light, the luminous flux received per unit area can also be understood as the density of light on the receiving surface. The unit of illuminance is Lux (lux), the code is Lx, and 1Lux represents the brightness of 1m2 when the area is illuminated by 1lm of luminous flux [1Lux (illuminance) = 1lm (luminous flux) / 1 square meter].

Inverse square law of illumination▲
We often say that the reading desktop is not bright enough, which usually means whether the illumination is enough. In the case of the same area, the higher the luminous flux of the light source, that is, the higher the lumen value, the higher the illuminance will be. Generally speaking, if the working environment is required to be bright and clear, the higher the illuminance requirements.

For example, the general lighting brightness of the whole space of the study room is about 100 Lux, but the illumination of local accent lighting during reading needs to be at least 600 LuX, so the reading desk lamp is the best lamp for local lighting. The illuminance values of some common scenes are listed in the figure below, so that you can have an intuitive understanding of the comparison degree.