The LED dichotomy

Solid state lighting has evolved over time. One of the luminiare design goals is to emulate natural lighting by transforming tiny high intensity light sources into surface lighting soothing for our eyes.

The purpose of this blog is to share some innovative technology which is needed to perform this light transformation. As the intensity increases, there will be hot spots and glare. Light transformation will be very important for human centric lighting.

A simple challenge: If a packaged LED could achieve say 300 lm/w and assuming 50% light extraction efficiency at system level, we only need 13 pieces LEDs to make a 2000 lumens lamp. This is good for a light bulb form factor. However, for the 120 cm or 4 ft T8, this would mean putting 13 pieces LEDs on a 4 ft tube instead of replacing the massive 2 ft by 4 ft ceiling troffers in most commercial buildings or we shall be seeing glaring STARS when we look up.

The LED challenge

Moore’s law is the observation that the number of transistors in a dense integrated circuit doubles approximately every two years.

As for LED, Haitz’s law states that every decade, the cost per lumen (unit of useful light emitted) falls by a factor of 10, and the amount of light generated per LED package increases by a factor of 20.

 

Fig. 1 Haitz’s Law states that the amount of light per package increase by a factor of 20 every decade

 

In March 2014, Cree announced another prototype with a record breaking 303 lm/W efficacy at 350 mA.

Fig. 2 Cree’s new milestone achieved in March 2014

The LED System Challenge

Luminaire designer will need to integrate the components into different form factor lighting. The biggest challenge is balancing light extraction efficiency with dollar per lumen per watt ($/lm/W). Fig. 3 depicts the various LED light form factors as compared to the older generation technology. People are purchasing lamps by watt. What really matters is the ‘lumens’ to deliver the right lux level.

Fig. 3 What really matters is the amount of ‘lumens’ of light

Innovative approach to transform tiny light sources into a surface light

Fig. 4: MCPET transform 120 deg beam into a 330 degree beam pattern

 

Fig. 4  shows an innovative approach to LED beam transformation. The light bulb is made by integrating many tiny LED sources (refer to the LED components on the green color printed circuit board). LED component has about 120 degree beam pattern. This means a light bulb without any beam forming when installed in a lamp shade would exhibit dark area below the light bulb.

By adding a lens or manipulating the physical led placement positions or using a reflective material as shown in Fig. 4, a beautiful lamp could be created with light reflected backwards. This adds cost to the lamp.

How it works

Observations:

  • Most reflective material acts like a mirror delivers specular reflection. There is high glare and light loss. A good example is the reflector used in the fluorescent troffer. There is about 30% light loss. This explains why a 32W fluorescent with initial 3000 lumens could be replaced by a 2000 lm LED tube.
  • The above sequences of pictures show that the reflective material glowing when placed flat and the glow becomes stronger by using a cupped reflector approach. This means there is internal reflection effects.

Fig. 5: Light from individual LED is diffused and reflected resulting in homogeneous surface and low glare lighting soothing for the eyes

Conclusion

As LED efficacy increases, the light intensity will increase many folds over time; thus, there is a need for innovative light transformation or indirect lighting approaches to create human-centric lighting for the future. It is about the quality of light and lighting ergonomics.

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