Knowledge: Several Important Foundation of LED Optical Design

Different from the energy-saving lamps or incandescent lamps on the market, LED is point light source. As the name implies, LED light source comes from one point. Not only emitting light, the point has the other major characteristic that the light is available for only one direction (all traditional back reflectors are meaningless to LED). If this special light would be used as illumination, it is necessary to deal with the light to meet our requirements. The optical design is exactly used here to change a relatively concentrated point light source to be astigmatism with a certain angle that is known as the straw hat LED – a form of capsulation professionalized for lighting design. On one hand, it can make the concentrated point light source become the astigmatic source; on the other hand, lower luminous decay, more light would be transmitted out.

Although the straw hat LED has qualitative leaps in the optical fields, relative to the plane light source which is familiar to us, it is still a point light source with an extremely concentrated brightness, causing some damages to our eyes when looking directly at the light source. As a result, when being applied in home lighting, LED lighting needs to be processed further and added some optical designs to be soft light for people’s direct looking. Furthermore, the straw hat LED has 120 degree beam angle itself, but it is still necessary to improve its beam angle to prevent the dark ceiling when it is used in home lighting. We should take use of the transparent bubble outside to disperse the light to adapt to the normal household lighting.

 

Difference of High-power LED and Low-power LED

With the development of LED technology, people deliver single LED chip of higher and higher power to adapt to large area lighting, and now the highest power of single LED chip can reach to 200W in the world with a high cost performance.

At present, single power 0.06W is common in the industry and its luminance can up to 7. Now we calculate it by 6LM, if we want the power to be 1W, then we need 17PCS same LED, and the whole luminance would be 17*6=102LM, which means it can up to 100LM/W. If we adopt single chip of 1W, its highest luminance can be 80LM while the common is 60LM. That is the main difference in luminance. Thus, it is better to choose low-power LED for home lighting.

From the product costs, high-power LED is higher than low-power LED, which comes from two aspects: one is the cost of LED itself, the other is high-power LED needs to be added with aluminum heat sink while low-power LED only needs the heating panel with natural dissipation.

From the maintenance costs, the failure light can be brought to electrical repair to change the broken LED. The cost of one piece 0.06W is 1 RMB and would be not more than 5 RMB with repair fee. However, the cost of changing 1W LED is 8 RMB and finally reaches to 15 RMB with repair fee. Comparatively speaking, low-power LED is available in any electrical market while high-power LED is not the case.

High-power LED is a market trend and will be a mainstream in the future. Nevertheless, the LED technology did not reach the desired outcome and is not suitable for home lighting applications now.

 

The Importance of Thermal Design

LED itself is a semiconductor device which has a certain requirement for temperature, including environment and working temperature.

Generally, the environment temperature of semiconductor device should be lower than 80℃ for normal working. The LED would be broke when the inside temperature of PN junction reaches to 140℃. Actually, the temperature would be dissipated by pin or special seat and then be radiated to the air through the circuit board or aluminum base to ensure the normal working of LED. Certainly, it is concerned with the quantity and density of LED. Low-power LED also needs to consider about the thermal design for its concentrated density. That is just like the electrical products around us, such as television, monitor and mainframe computer. Wrong thermal design would short the life span and speed up the luminance depreciation.

  1. Now the upper limit of junction temperature mostly can reach to about 120. The highest one is CREE, can reaches to 150.
  2. The thermal resistance of LED chip changed with the packaging material structure.
  3. At present, the life span of most LED lights is 20000-50000H.
  4. Structural layout is actually the question of touched thermal resistance and the bottlenecks of heat conduction from the aspect of dissipation.
  5. Higher drive efficiency, better quality.

Conclusion covers overall consideration: the temperature rise should be controlled under 35℃ for a good thermal design. Junction temperature is lower than 80℃ with a life span of 50000H.

 

The current problems of the thermal design:

  1. Natural dissipation is large limited by the spatial size.
  2. The huge scope of natural dissipation is limited.
  3. The heat sink now is all made by metal, the heat conduction is enough for a relatively small space.
  4. Most of the thermal design only can focus on the choice of LED chip, radiating areas, interface touched thermal resistance and thermal radiation. It almost goes into a dead end.
  5. Now the breakthrough point of thermal design relies on the whole system of light, which is the achievement of combining the experience of actual production, material, structure and thermal design.

 

(From China LED.net)

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