Indoor LED lighting technology based on vision and information inversion

As an emerging light source, LED has received widespread attention. The current research focus of the industry is on improving its optical and thermal performance. This article attempts to explore the advantages of LED for indoor lighting from the perspective of visual ergonomics and intelligent lighting.

As a light source that has developed rapidly in recent years, LED has significant advantages over traditional light sources, such as ultra-long life, high light efficiency, fast response, artistry, safety, flexibility, and reliability. However, the main application occasions of LED are still the backlight field and outdoor lighting fields such as street lights and landscape lights. In the field of indoor lighting, it still has some shortcomings, such as high cost, poor color rendering, high brightness, and consistency needs to be improved. Combining the above advantages and disadvantages of LED, from the application perspective, in indoor lighting, we should make full use of the advantages of LED and avoid its disadvantages. In the future, LED will be widely used in indoor lighting, and its technology combined with lighting control can achieve better results.

The key to combining LED and lighting control

Lighting control is a distributed lighting control system composed of computers, wireless communication data transmission or power carrier communication technology, and electrical control technology to achieve the intensity adjustment and timing control of light brightness, and realize regional and personalized control of lighting scenes [1]. Generally, bus control is adopted, requiring the controlled lamps to be digitally dimmed and have good fast response characteristics.

At present, lighting control generally adopts bus form to digitally dim halogen lamps, fluorescent lamps with digital dimming rectifiers, etc., to achieve regional lamp combination dimming, ambient illumination adjustment, or color temperature adjustment through fluorescent lamps with different color temperatures. The lighting expression method is still relatively simple. The emergence of RGB mixed light LED technology makes it possible to adjust the ambient light color, and it is very convenient. LED adjustment generally adopts PWM duty cycle adjustment method, which can be easily implemented in digital form. As long as a digital interface is reserved on the driver, it is easy to achieve communication with other digital systems, making the system construction very simple. As shown in Figure 1.

The key to combining LED with lighting control is to use its flexibility and artistry to select the appropriate system topology. LED lighting systems generally have a large number of nodes and communication channels, a large system scale, and high requirements for system response speed.

At present, many lighting control systems use RS485 bus control, which is a control method based on a master-slave structure. The controller issues control instructions or queries sensor detection values ​​through master-slave response methods and timed inspections. RS485 only specifies the physical layer, which is convenient for manufacturers to customize system communication. The common practice in the field of lighting control is to build an MCU control into each LED lamp and connect it to the bus through an RS485 interface chip. This method is simple and reliable, but the application layer standards are not unified, and engineering application configuration is inconvenient.

Another widely used control method is based on the DALI bus. This method addresses the nodes in the system. The data signal is composed of instructions and addresses. Different lighting units can be flexibly grouped to achieve multiple combinations. 16 lighting scenes can be set. The system has good reliability, stability, and compatibility. However, each DALI network node does not exceed 64, which is not convenient for large-scale lighting applications. In addition, the refresh speed in the DALI system is slow, which limits the artistic expression of LED.

The DMX512 protocol is currently widely used in the field of LED control. This protocol uses frames as units. Each frame of data consists of a synchronization header and 512 bytes. Data is sent and received in a serial manner that complies with the EIA 485 standard. The data transmission rate is 250KBit/s. This method has a fast transmission speed and a large system scale, and is widely used in stage lighting and outdoor LED landscape lighting. However, this protocol is a one-way transmission mode. The system cannot receive lamp information or external sensor signals, and cannot meet the closed and intelligent requirements of indoor lighting.

After research, we believe that a distributed topology based on Zigbee wireless communication can be used in indoor lighting. The operating frequency is the 2.4GHz wireless frequency band, which can accommodate more than 60,000 nodes at the same time. The control range can reach up to 10,000 meters. It can be used in a variety of indoor lighting places such as homes, hotels, and commercial lighting [2]. The Zigbee network has the advantages of low power consumption, self-organization, and easy positioning. It has considerable advantages in building wireless sensing and lighting control networks.

The system adopts a star network structure controlled by a central controller. The main controller sets the lighting scene and processes the feedback information. It receives the data from each sensor through the wireless network, performs relevant algorithm calculations on the collected data, obtains the adjustment instructions for each node, and sends the instruction parameters to the corresponding node through the network. In addition to the lighting nodes and sensor nodes, there is also a handheld remote control for simple control operations.

Each node is equipped with a wireless communication module. The Zigbee module has the characteristics of simple protocol, low rate and low power consumption. Since the protocol is relatively simple and the requirements for the control chip are low, the cost of the wireless module is low. Its transmission rate is about 10k~250Kb/s, which is suitable for applications with relatively small data volume such as lighting control. The module has low power consumption and can work for six months to two years when powered by button batteries or ordinary dry batteries. It is very suitable for the working mode of each indoor wireless node. The system block diagram is shown in Figure 2.

Application of visual comfort

In indoor work and life, people not only require visual functional satisfaction, but also have high requirements for visual comfort. It is required to reasonably control glare, choose appropriate color temperature and light color, etc.

In terms of glare in the field of view, white light LED is both an advantage and a challenge. Compared with traditional light sources, under the condition of equal light flux, the volume of LED light point is very small and the beam angle is also small, so its light emission direction is easy to control, but on the other hand, the surface brightness of LED is usually very high, the light intensity is relatively concentrated, and if the maximum light intensity distribution is not properly handled, it is easy to cause glare to the observer. Studies have shown that uncomfortable glare will not only cause a decrease in work efficiency, but also cause adverse reactions such as visual fatigue and headaches. Therefore, in indoor applications, there are high requirements for the primary and secondary optical design of its lamps.

In addition, since LED is a narrow spectrum light source, the white light generated by white light LED lacks red and yellow components, which is quite different from the white light of fluorescent lamps that people are accustomed to, and the color temperature is usually high. If RGB mixed light LED is used, although white light of any color temperature can be formed, the large color difference will affect the overall comfort and beauty of the field of view. In addition to improving the powder coating process and selecting high-quality chips, indirect lighting can be used as much as possible in the application, or the color temperature of the light source can be selected to match the required illumination. Figure 3 shows the comfort zone corresponding to illumination and color temperature [3].

In terms of control, the combination of light color sensors, infrared sensors, illuminance sensors, etc. with LEDs and flexible system configuration can effectively improve the user's comfort. Constant illuminance lighting can be achieved by automatically dimming the light by detecting external illuminance, or by adjusting the LED light intensity, light color or color temperature in combination with time changes, so as to simulate the changes in illuminance and color temperature to natural light and make the lighting conform to the human physiological rhythm curve. It is also possible to make the lighting conform to the user's functional and psychological needs in specific application scenarios through personnel sensors and the matching of different light colors, which is the so-called situational lighting.