To judge whether an LED light source is what we need, we usually use an integrating sphere to test, and then analyze the test data. The general integrating sphere can give the following six important parameters: luminous flux, luminous efficiency, voltage, color coordinate, color temperature, and color rendering index (Ra). (Actually, there are many other parameters, such as peak wavelength, dominant wavelength, dark current, CRI, etc.) Today, let’s discuss the significance of these six parameters for light sources and their mutual effects.

Luminous flux: Luminous flux refers to the radiation power that can be felt by the human eye, that is, the total radiation power emitted by the LED, in lumens (lm). Luminous flux is a direct measurement and the most intuitive physical quantity to judge the brightness of LED.

Voltage: The voltage is the potential difference between the positive and negative poles of the LED lamp bead, which is a direct measurement, in volts (V). It is related to the voltage of the chip used by the LED.

Luminous efficiency: luminous efficiency, that is, the ratio of all luminous flux emitted by the light source to the total input power, is the calculated amount, in lm/W. For LED, the input electric energy is mainly used for lighting and heating. The high luminous efficiency indicates that there are few parts used for heating, which is also a reflection of good heat dissipation.

It is easy to see the relationship between the above three. When the current is determined, the luminous efficiency of LED is actually determined by the luminous flux and voltage. High luminous flux and low voltage lead to high luminous efficiency. As far as the current large-scale blue chip is coated with yellow green fluorescence, because the single core voltage of the blue chip is generally about 3V, which is a relatively stable value, improving the light efficiency mainly depends on increasing the luminous flux.

Color coordinate: the color coordinate, that is, the position of the color in the chromaticity diagram, is the measurement quantity. In the commonly used CIE1931 standard colorimetric system, the coordinates are represented by x and y values. The x value can be considered as the degree of red light in the spectrum, and the y value is considered as the degree of green light.

Color temperature: a physical quantity measuring the color of light. When the radiation of the absolute black body is exactly the same as the radiation of the light source in the visible area, the temperature of the black body is called the color temperature of the light source. Color temperature is a measurement quantity, but it can be calculated by color coordinates at the same time.

Color rendering index (Ra): used to describe the restoring ability of light source to object color. It is determined by comparing the appearance color of objects under standard light source. Our color rendering index is actually the average of the eight light color measurements calculated by the integrating sphere for light gray red, dark gray yellow, saturated yellow green, medium yellow green, light blue, light blue, light purple blue, and light red purple. It can be found that it does not include saturated red, which is commonly known as R9. Because some lighting needs more red light (such as meat lighting), R9 is often used as an important parameter to evaluate LED.

The color temperature can be calculated by color coordinates. However, if you carefully observe the chromaticity diagram, you will find that the same color temperature can correspond to many color coordinates, while a pair of color coordinates only corresponds to one color temperature. Therefore, it is more accurate to use color coordinates to describe the color of the light source. The display index itself has nothing to do with the color coordinate and color temperature, but the higher the color temperature, the colder the light color, the less red components in the light source, and it is difficult to achieve very high display index. For warm light sources with low color temperature, there are more red components, wide spectrum coverage, and closer to the spectrum of natural light, so the color rendering index can be naturally high. This is why LEDs above 95Ra on the market have low color temperature.