Analysis and discussion on the application of white light LED in space station cabin lighting

1 Introduction

In the future, the country will build a space station. The lighting system is an important subsystem in the space station. Comfortable lighting can provide a good lighting environment for the astronauts' cabin life and work, and ensure the personal safety of the astronauts; at the same time, the power consumption control of lighting also plays an important role in the smooth implementation of the entire space mission. In the process of cabin lighting design, in order to establish a highly reliable, efficient, and human-machine functional lighting environment for the space station, it is necessary to select a suitable lighting system to achieve the relevant cabin lighting indicators and meet the requirements of volume, weight, power consumption and reliability of the lighting system.

At present, the general lighting source in each cabin of the International Space Station is fluorescent lamps. Fluorescent lamps have mature technology, stable performance, high reliability, high light efficiency, uniform light emission, and low surface brightness. However, their disadvantages are: containing mercury, flickering, difficult to dim, and short maintenance cycle.

In contrast, white light-emitting diodes (LEDs), as a new type of lighting source, have the characteristics of small size, shock resistance, long life, fast startup, low operating voltage, no flicker, and no mercury. They are very suitable for the high reliability and safety requirements of lighting systems for space flight missions. At the same time, in cabin lighting applications, the luminous efficacy, color rendering index, color temperature, and glare prevention of LED light sources should also be considered.

Therefore, in response to these problems, it is very necessary to analyze and study the application of LED light sources in spacecraft and space station cabins.

2. Space station cabin lighting indicators

The lighting indicators are implemented to meet the requirements of the space station cabin environment for lighting quality and quantity, that is, to ensure:

(1) Visual comfort: astronauts feel comfortable.

(2) Visual function: Astronauts can complete visual tasks quickly and accurately even under difficult conditions and after long hours of work.

(3) Visual safety: The ability to see objects near the operating targets inside and outside the cabin to prevent misoperation or detect abnormal situations.

Since there are currently no lighting standards for aerospace applications, the lighting indicators for the space station cabin are mainly based on the International Commission on Illumination's indoor workplace lighting standards. The indicators for the lighting in the space station cabin should include:

Among the above indicators, (1), (2), (3), and (4) are mainly determined by the number of light sources, the design of the optical system, the installation location and distribution of the lamps, the lighting method, the size and shape of the space inside the cabin, and the material properties of the illuminated surface. (5) and (6) are also affected by the spectral power distribution of the light source, the secondary optical system, and the driving power supply. (7) is mainly guaranteed from the two aspects of safety and reliability design and testing.

The following combines relevant indicators to analyze the performance characteristics and key application technologies of white light LEDs in cabin lighting applications.

3 Color Temperature and Color Rendering Index

The correlated color temperature of a light source describes the apparent color of the light emitted by the lamp (the chromaticity of the lamp). The choice of color temperature is based on the illuminance, the color of objects in the room and the surrounding environment, the surrounding climate and the application site. Usually, the color temperature of cold light is more popular in warm climates, while the color temperature of warm light is more popular in cold climates. The general color rendering index Ra of a light source describes the color rendering characteristics of the light source. The smaller the Ra, the lower the quality of the color appearance. Lamps with Ra less than 80 should not be used in cabin spaces where people work or stay for a long time.

Under the premise of meeting the cabin illumination standards, the cabin lighting should use LED light sources with stable performance, and its color rendering index Ra should not be lower than 80. The color temperature of LED needs to be determined based on relevant human-machine efficacy experiments. Generally speaking, LED light sources with a color temperature between 3000K and 5000K can be used for cabin lighting.

The light spectrum of LED will be affected by the operating junction temperature of LED chip, the current and the lens material. These factors should be controlled in the application to ensure the lighting quality.

4 Key application technologies of LED cabin lighting

4.1 Device Screening

Unlike ordinary lighting, aerospace missions have high requirements for product quality. LED devices used in lighting systems must be screened and qualified before they can be used; on the other hand, the optoelectronic performance of LED devices screened according to relevant screening specifications is similar, thus ensuring the stability of the working performance of the entire lighting system. The screening content is determined according to the specific screening object and screening specifications.

4.2 Design of secondary optical system

For a low rectangular space inside a space station, astronauts are in a microgravity environment, their eyes may be facing in various directions and at different distances from the light source, so the lamps must meet the requirements of glare limitation.

Fluorescent lamps have a large luminous area, low surface brightness, and soft light. Only some diffuse transmission materials are needed to reduce glare and produce a comfortable lighting effect. In contrast, LEDs are point light sources with a small luminous area and high brightness. If the light angle of the lamp is not properly controlled, glare is likely to occur. Therefore, the light angle and intensity of the LED light source should be redistributed according to the installation position and method of the lamp in the cabin, so that the LED is changed from a point light source to a uniform surface light source, and the brightness of the light-emitting surface is reduced to a low enough level to achieve the purpose of limiting glare. In the selection and design of the optical system of the cabin lighting, a diffuse transmission system with high transmittance can be used, or a light guide, plane mirror reflection system, etc. can be used to produce a uniform surface light source. The design should be based on the restrictions on the weight, volume, and power consumption of the lighting system by the space mission, and a good balance should be achieved between them and the uncomfortable glare value.

4.3 Design of driving power supply

The LED driver power supply provides stable power supply for the LED lighting in the cabin and must be equipped with circuit protection. The power supply output current should meet the requirements of stability, output noise and startup time. Circuit protection should include input protection, voltage limiting protection and open circuit overvoltage protection. The efficiency of the driver power supply should be high enough to achieve reasonable utilization of the power supply in the cabin.

In addition, the LED driver power supply should have continuous dimming function and be designed for radiation resistance and electromagnetic compatibility.

4.4 Heat Dissipation Design

An efficient heat dissipation system can ensure that LED lighting lamps work stably and reliably. Since LEDs generate excess heat while emitting light, if this heat accumulates too much inside the device, it will seriously affect the performance of the LED. Specifically, heat accumulation will cause the junction temperature of the chip to rise, which will in turn reduce the chip's luminous efficiency, cause the color temperature to drift, and deteriorate the color rendering index, ultimately shortening the life of the LED lamp.

Figure 1 shows how different color LED samples experience a drop in light output due to an increase in junction temperature, which causes a mismatch between the output light and the phosphor.

Figure 1. Relative light output versus heat sink substrate temperature.

In addition, the junction temperature will also affect the life of the chip. Figure 2 shows the change of the life of a company's product with the junction temperature. The effective life of the sample (B10, L70) is defined as the time when 10% of the light sources drop to 70% of their initial luminous flux. It can be seen that at a current of 350mA, the increase of the junction temperature from 127℃ to 135℃ reduces the life of the LED by about 30,000 hours, a decrease of 50%. That is, the failure probability of the LED increases with the increase of the junction temperature, which must be avoided in the application of cabin lighting.

Figure 2 Relationship between lifetime and junction temperature.

In view of the above situation, in addition to selecting LED devices with good performance, the use conditions must ensure that the LED can fully dissipate heat and control the size of the driving current to prevent the LED junction temperature from being too high. That is, on the one hand, a good heat dissipation system should be designed for LED lamps to reduce the thermal resistance from the chip to the cabin; on the other hand, the stability and accuracy of the constant current power supply should be ensured. In order to dissipate heat, the LED lamp can be installed on an aluminum bracket and dissipated through the cabin. The space station cabin is generally a constant temperature environment, which is beneficial for LED work.

4.5 Reliability Design and Estimation

Due to the particularity of space flight missions and the different application conditions from those on the ground, the reliability of LED lighting systems in the harsh environment of space should be considered to ensure the smooth implementation of space missions and the safety of astronauts. These influencing factors include acceleration and impact vibration caused by rocket launch, high and low temperature environment in space, thermal vacuum environment, etc.

Therefore, device upgrade screening and reliability analysis and design of LED cabin lighting optical system, heat dissipation system and driving power supply should be carried out, and the lamps should be subjected to a series of reliability tests including aging test, thermal cycle test, thermal vacuum test, radiation resistance test, acceleration test and impact test.

The Shenzhou VII spacecraft launched by my country in September 2008 used LED lights to provide extravehicular lighting for astronauts. This verifies that the LED lighting system can work reliably and stably in the complex and harsh extravehicular space environment and has a high degree of reliability. Figure 3 shows the prototype of the extravehicular activity light for astronauts on the Shenzhou VII spacecraft. Since the environment inside the cabin is not as harsh as outside the cabin, this flight experience also shows that the LED lighting system also meets the high reliability requirements of space missions when used inside the cabin.

Figure 3 Prototype of the astronaut extravehicular activity light on the Shenzhou VII spacecraft.

Generally speaking, LED is a solid-state light-emitting device with a long life, no mercury, small size, and resistance to impact and vibration. It has better reliability than gas discharge light sources in cabin lighting.

5. Security, Power Consumption and Efficiency

The safety of the lighting system is the highest requirement that must be met in space flight missions. The safety of LED lighting systems is mainly considered from the following two aspects.

(1) LED light sources are solid-state devices encapsulated by silicone, semiconductor materials, metal materials, etc. They have a solid structure and will not loosen or break during normal use. Since LEDs are cold light sources and the drive circuit does not have high voltage, they will not cause electric shock accidents due to high temperature or breakage. They are both very safe and safe from the outside environment.

(2) LED light radiation may cause photochemical damage to the skin and eyes, near-ultraviolet damage to the eyes, blue light photochemical damage to the retina, photochemical damage to the retina, thermal damage to the retina and thermal damage to the skin. In aerospace lighting applications, such damage should be strictly avoided according to relevant standards (CIE S009/E: 2002 and IEC 60825).

In addition, the shell temperature of the cabin lighting should be controlled within 45°C to avoid burns to astronauts. Compared with fluorescent lamps, LED light sources do not contain mercury, a substance that is harmful to the human body.

In terms of power consumption, an efficient cabin lighting system achieves reasonable use of cabin power supply. On the premise of meeting cabin lighting indicators, the efficiency of the driving power supply and the efficiency of the lamp should be improved as much as possible. At the same time, the luminous efficacy of LEDs will also be affected by the junction temperature. During use, the junction temperature should be strictly controlled at a certain level to ensure that the lamp has sufficient light output.

6 Others

Solving the above problems is the basis and premise for the application of LED in space station lighting. In addition, LED light sources have high luminous efficacy, fast startup time, wide startup temperature range, low operating voltage, and no flicker after lighting; LEDs are easy to design special light distribution, and the secondary optical system is highly efficient; they are impact-resistant; and they are easy to achieve color and brightness control. These characteristics provide advantages for LEDs to replace fluorescent lamps for cabin lighting.

7 Space application experience of LED lighting system

In addition to the application of white light LED lighting systems in my country outside the orbital module of the spacecraft, the National Aeronautics and Space Administration of the United States conducted experimental comparative tests of solid-state lighting modules and general lighting assemblies in the cabin of the International Space Station in 2008 to verify the feasibility of LED applications in space lighting. Figure 4 shows the fluorescent lamp (GLA) in the cabin of the International Space Station, and the alternative LED lamp (SSLM) is shown below. Table 1 shows the test results of the two lighting lamps published by NASA.

Table 1 Comparison of fluorescent lamps and LED lamps on the International Space Station

Figure 4 Fluorescent lamps and LED lamps for general lighting in the International Space Station cabin.

From the above examples, it can be seen that using LED light sources instead of fluorescent lamps for cabin lighting is advantageous and feasible.

8 Conclusion

Compared with fluorescent lamps, LED lighting systems have greater advantages in space station lighting, including (1) long life, long maintenance cycle, reduced transportation weight and reduced number of space missions; (2) small size and weight, easy installation; (3) 0-100% dimming, changing the cabin illumination and color temperature, and thus achieving switching of lighting scenes. Therefore, it is feasible to use LED light sources to replace fluorescent lamps to provide general lighting in the space station cabin, and we should start to develop LED lighting systems suitable for future manned space stations.

From the above analysis, we can know that the key technologies for the application of LED in the interior lighting of the space station are: the selection of LED devices; the heat dissipation design and junction temperature control of LED lamps; the design of LED secondary optical systems suitable for interior space lighting; the efficiency and life of the driving power supply. Improving the above application technologies will play a great role in giving full play to the advantages of LED in the application of space station lighting and ensuring the high-quality implementation of space missions. With the improvement of technical level, there is still a lot of room for improvement in the light efficiency, color rendering index, life, etc. of LED. LED will definitely have a good application prospect in the field of space station lighting in the future.