1. What are the basic terms used to describe the optical parameters of a light source?
Generally, parameters describing the light emitted by a light source include luminance, luminous flux, luminous intensity, and illuminance, as shown in the diagram below.
Luminous flux is the amount of light emitted by a light source per unit time, measured in lumens (lm). This quantity describes the total amount of light emitted by the light source; the greater the luminous flux, the more light it emits. It is directly proportional to the light power.
The luminous flux emitted by a light source per unit solid angle in a given direction is defined as the luminous intensity of the light source in that direction, measured in candela (cd). Luminous intensity refers to a point light source and describes how "bright" the light source is. The greater the luminous intensity, the brighter the light source appears, and under the same conditions, objects illuminated by that light source will also appear brighter.
Luminance refers to the physical quantity describing the intensity of light emitted from the surface of a light source. When the human eye observes a light source from one direction, the ratio of the luminous intensity in that direction to the area of the light source "seen" by the eye is defined as the luminance per unit projected area, i.e., the luminous intensity per unit projected area. The unit of luminance is candela per square meter (cd/m²). Luminance is the human perception of light intensity.
Illuminance refers to the luminous flux received per unit area of an illuminated surface, measured in lux. Illuminance is an important indicator of environmental conditions. On a moonlit night, illuminance is typically 0.02–0.3 lux; on a cloudy day, outdoor illuminance is typically 50–500 lux; and on a sunny day, indoor illuminance is typically 100–1000 lux. The illuminance required for reading is generally 50–60 lux.
2. What is a light distribution curve? Why is light distribution necessary?
A light distribution curve is a curve that describes the spatial luminous intensity distribution of a light source or luminaire. It records information such as the luminous flux, number of light sources, and power of the luminaire. We can imagine placing a lamp or light source at the center of a sphere in space, and then measuring the luminous intensity at a cross-section passing through the center to obtain the light distribution curve. Of course, to better understand the spatial distribution of light emitted by the light source or lamp, we can measure the luminous intensity data at multiple spatial angles.
(The light distribution curve reflects the spatial luminous intensity distribution of the light source or lamp.)
When a light source emits light, the light rays travel in all directions. To utilize a light source for desired illumination, specific mechanisms are needed to control the light, readjusting its spatial distribution to achieve the desired effect. This control is called light distribution.
3. How does the spectrum of an LED differ from the spectrum of other lighting fixtures?
A spectrum is the pattern of monochromatic light dispersed by a dispersive system (such as a prism or grating) and arranged sequentially according to wavelength (or frequency). It is also called the optical spectrum. The entire electromagnetic spectrum includes radio waves, infrared radiation, ultraviolet radiation, and X-rays. They differ in wavelength. The largest part of the spectrum, the visible spectrum, is the portion of the electromagnetic spectrum visible to the human eye, with wavelengths between 400 and 760 nanometers constituting typical visible light.
Currently, most LEDs use a blue LED chip to excite one or more phosphors, ultimately mixing the blue light and the phosphor's emitted light to produce white light. Therefore, the spectrum of a typical LED typically has more than two peaks, while other wavelength ranges have relatively low radiant intensity.
(Differences between LED spectra and those of other lighting fixtures)
4. What is color temperature? What sensations do different color temperatures evoke?
Color temperature is a measure of the color (color surface) of a light source. When the color emitted by a light source is the same as the color emitted by a black body at a certain temperature, the black body temperature at that temperature is defined as the color temperature of the light source, expressed in Kelvin (K). Because most lighting sources emit light commonly referred to as white light, the color surface temperature or correlated color temperature of a light source is used to indicate the degree to which its light color is relatively white, quantifying the light color performance of the light source. The black body locus on the CIE color coordinate system shows the process of a black body changing from red to orange-red to yellow to yellowish-white to white to bluish-white, as shown in the diagram below.
(Different color temperatures evoke different feelings.)
Different color temperatures of light sources result in different colors of light: higher color temperatures produce a more bluish light, commonly known as cool white light; while lower color temperatures produce a more reddish light, commonly known as warm white light. Color temperatures below 3300K create a stable and warm atmosphere; color temperatures between 3000 and 5000K are considered intermediate and provide a refreshing feeling; color temperatures above 5000K create a cool feeling. Different light colors from different light sources create the optimal environment.