A brief history of light - from the Big Bang to today

Introduction and Background

"And God said, Let there be light; and there was light. And God saw that the light was good, and God separated the light from the darkness." I'm sure most of us are familiar with these first two sentences in the Book of Genesis, and whether you believe it or not, the fact remains that there is indeed darkness and light in our world. But what is light? Where does light come from? How is it produced? What will it look like in the future?

These are all good questions, but I don't want to answer them all in this article. I just want to review a brief history of light, from about 14 billion years ago to today. Why 14 billion years ago? Because we know that 14 billion years is about the age of the universe, according to NASA's Wilkinson Microwave Anisotropic Probe (WMAP). This probe is used to probe the universe with exquisite accuracy, detecting microwaves from the most distant depths of the universe, where the cooling fireball formed from the Big Bang itself. Of course, our own solar system (including the sun and the earth) is much younger than the universe itself, only about 4.5 billion years old. Therefore, the main source of light on Earth is the sun. Later, about 4 billion years ago, the moon formed. No one knows exactly how the moon was formed, although there are 4 main hypotheses, namely fusion, capture, co-formation and giant impact (for more information on these hypotheses, see Wikipedia). However, no matter how the moon was formed, it provides us with an additional light source at night by reflecting photons from the sun back to the earth's surface.

It is generally believed that humans (Homo erectus) appeared on Earth about 1 million years ago, which is just a blink of an eye in the galactic universe! Obviously, the sun was the main source of illumination for these early humans, as artificial sources of illumination did not appear until much later. In fact, many scientists believe that there is irrefutable evidence that early humans created controllable fire about 125,000 years ago. That is, humans created torches, which became the first artificial light source. However, it was not until 17,000 years ago that prehistoric humans used lamps for lighting. These lamps were usually made of shells, rocks, or animal horns, filled with animal or vegetable oils as fuel, and used a fiber wick. It took another 10,000 years for these lamps to include olive, nut, sesame, or fish oils as fuel. Over the next 5,000 years, the materials used in these lamps changed many times. Then, around 500 BC, Pythagoras proposed the "particle" theory of light. This theory posits that all visible objects emit a steady stream of particles that bombard our eyes. He further proposed that "light consists of rays, which are like tentacles, and which travel in straight lines from the eye to the object, and when these rays touch the object, vision is obtained."

The next advance in lighting did not come until 400 AD, when the candle was invented. For the next 1,400 years, candles were the primary source of lighting. However, the most important discovery of modern lighting came in 1666, when Isaac Newton, then just 23 years old, performed his famous prism experiments. He noticed and recorded that sunlight was white light, which contained all the colors of the spectrum. Similarly, in 1752, Benjamin Franklin performed his famous experiments with kites. He invented the lightning rod and explained the phenomena of positive and negative electricity. Benjamin Franklin's work was so significant that his inventions and principles were used when the incandescent light bulb was invented about 100 years later. Next, in 1792, William Murdock used gas produced by heating coal to illuminate his home and office in Cornwall, England. This was the first time that gas was used as a fuel to produce artificial light. Then, after the discovery of natural gas in the early 1800s, gas lighting became mainstream in homes, offices, factories and street lamps.

In 1877, Thomas Edison became interested in and experimented with electric lighting. A year later, with the help of a friend, he founded the Edison Electric Light Company, whose purpose was to "own, make, license, and operate all different apparatus for producing light, heat, or power by electricity." Although Edison did not invent the filament-based light bulb, he did transform the theory into a practical form and was one of the first to successfully market the incandescent light bulb. The first patent covering the incandescent light bulb was actually filed by Henry Woodward and Matthew Evans in 1874, about five years before Edison developed the light bulb. However, German chemist Herman Sprengel was perhaps the first to invent the vacuum light bulb, which he invented in 1865.

Although incandescent bulbs have been the dominant technology for more than 100 years, they are not immune to the threat of new technologies. Another new technology is about to overturn the dominant position of incandescent bulbs in the lighting field. This is the white light emitting diode (LED).

White LEDs and the soon-to-be-extinct incandescent

lampsLEDs are semiconductor devices that emit incoherent narrow-spectrum light when a forward bias voltage is applied, creating a phenomenon called electroluminescence. In other words, a solid-state phosphor converts electrical energy directly into light under the action of an electric field. The color of the light emitted depends on the chemical composition of the semiconductor material used and can be near-ultraviolet, visible, or infrared.

LED technology has significantly improved over the past few years, with advances in heat dissipation, packaging, and processing, resulting in brighter, more efficient, longer-lasting, and lower-cost LEDs. Unlike incandescent bulbs, LEDs have no filaments to burn out, and they tend to run cooler. In addition, incandescent bulbs waste 95% of their electrical energy, converting it into heat.

The light output of high-power or high-brightness (HB) LEDs has passed the critical milestone of 100 lumens per watt, or 100 lm/W. In fact, some manufacturers have announced that they have achieved 200 lm/W in the lab. Clearly, then, LEDs have surpassed incandescent bulbs in terms of luminous efficacy (a typical 60W bulb has a light output of 15 lm/W). Or, another way to put it, luminous efficacy is the ratio of the light output of a source measured in lumens to the power consumed to produce that light output measured in watts. Not only that, but LEDs with a light output of 150 lm/W are expected to be available on the market steadily in the next year. Another benefit of LEDs is their long life. Depending on how you calculate it, white LED lamps have a lifespan of at least 50,000 hours, with some claiming more than 100,000 hours, compared to about 1,200 to 1,500 hours for incandescent bulbs.

The price of high-brightness LED lamps is also falling very quickly. A few years ago, the price of a single white light diode (several such diodes make up an LED lamp and account for the majority of the cost of an LED lamp) was about $4, but now it has dropped to less than $1. Many LED industry analysts predict that the cost of replacing incandescent lamps with LED lamps will reach a level acceptable to consumers in the next year. Some LED manufacturers have announced that they have designed light-emitting chips that can power LED lamps, making the light output of LED lamps comparable to the 75W incandescent lamps commonly used in most households. Such LED chips typically only need about 9W of power to produce the same amount of light.

These advances are significant because the U.S. Department of Energy has stated that lighting consumes 22% of the electricity produced in the United States. Widespread use of LED lighting has the potential to cut lighting electricity consumption in half. To put this in perspective, by 2027, LED lighting has the potential to reduce annual energy use by the equivalent of 500 million barrels of oil, with the accompanying reduction in carbon dioxide emissions.

Cars Need LEDs, Too

This year, the market for high-brightness LEDs is expected to reach $12 billion, and by 2015, it is expected to grow to $20.2 billion, a compound annual growth rate of 30.6% (according to research by Strategies Unlimited). One of the key application areas driving this significant growth is LEDs used in automobiles. Applications range from headlights, daytime running lights and brake lights to instrument panel display backlighting and all kinds of interior vanity lighting. However, to maintain this impressive growth rate, LEDs must not only provide higher reliability, lower power consumption and more compact form factors, but also improvements in contrast and color accuracy. Furthermore, in the automotive environment, all of these improvements must be optimized while also withstanding the relatively harsh automotive electrical and physical environment. It goes without saying that solutions used in automobiles must provide a very flat and compact footprint while also improving overall cost-effectiveness.

But what are the factors behind this impressive growth potential in automotive lighting? First, LEDs are 10 times more efficient than incandescent bulbs and nearly twice as efficient as fluorescent lamps (including cold cathode fluorescent lamps), thereby reducing the electrical power required to provide a given amount of light output (measured in lm/W). As LEDs are further developed, their utility, or ability to produce light output from a power source, will only continue to increase. Second, we live in an environmentally conscious world, and LED lighting does not require the handling, exposure, and disposal of toxic mercury vapors common in cold cathode fluorescent lamps (CCFLs). In short, LEDs are "green." Finally, incandescent bulbs often need to be replaced after about 1,000 hours of use, while fluorescent lamps can last up to 10,000 hours. However, these figures pale in comparison to the more than 100,000 hours of life provided by LED lighting.

In most applications, this longer operating life allows the LED to be permanently embedded in the end application. This is obviously important for backlighting of automotive dashboards, instruments, and infotainment system displays, but long operating life has also become a "must have" for headlights and brake lights, as these lamps do not need to be replaced during the operating life of the vehicle. In addition, LEDs can be several orders of magnitude smaller and more compact than other lamps, and can be configured with red, green, and blue LEDs to provide an infinite variety of colors.

However, one of the biggest obstacles facing automotive lighting system designers is how to optimize all the features and benefits of the latest generation of LEDs. Because LEDs generally require an accurate and efficient current source and a dimming method, LED driver ICs must be designed to meet these requirements under a variety of operating conditions. In addition, their power supply solutions must be very efficient, rugged and reliable, while also being very compact and cost-effective. Arguably, one of the most demanding applications for driving LEDs will be headlight assemblies (consisting of high and low beams, daytime running lights, fog lights and turn signal indicators) because these lights are in a harsh automotive electrical environment and must adapt to a wide range of temperature changes. At the same time, these lights must be able to fit into very constrained spaces and have an attractive cost structure.

The new LED driver IC

LT3791 for automotive headlight applications is a synchronous 4-switch buck-boost LED driver and regulator controller that is ideal for driving high-brightness LEDs in automotive headlight applications. The controller operates with an input voltage that is higher, lower or equal to the output voltage. The device has an input range of 4.5V to 60V and an output range of 0V to 60V, and can also seamlessly transition between operating modes.

Figure 1: The LT3791 drives a 3A LED array at up to 100W

A ground-referenced voltage feedback pin (FB) serves as the input for several LED protection features and enables the converter to operate as a constant voltage source, as shown in the schematic of Figure 1. Fault protection is provided to withstand and report open or short LED conditions, while a timer allows the LT3791 to run continuously, latch off, or restart in the event of a fault.

The LT3791 has a proprietary current mode topology and control architecture that uses a current sense resistor in either buck or boost mode. The sensed inductor current is controlled by the voltage on the VC pin (see Figure 2), which is the output of feedback amplifiers A11 and A12.

Figure 2: LT3791 Block Diagram

The VC pin is controlled by 3 inputs, one from the output current loop, another from the input current loop and the last from the feedback loop. Whichever feedback loop voltage is higher takes precedence, forcing the converter into either constant current mode or constant voltage mode.

The LT3791 is designed to switch completely between the two operating modes. Referring again to the block diagram shown in Figure 2, current sense amplifier A1 senses the voltage between the IVINP and IVINN pins and provides pre-gain to amplifier A11. When the voltage between IVINP and IVINN reaches 50mV, the output of A1 provides IVINMON_INT, which inverts the output of A11 and the converter is in constant current mode. If the current sense voltage exceeds 50mV, the output of A1 increases, causing the output of A11 to decrease, thereby reducing the current provided to the output. In this way, the current sense voltage is regulated to 50mV.

The output current amplifier operates similarly to the input current amplifier, but with 100mV instead of 50mV. The output current sense value is adjustable through the CTRL pin. Forcing CTRL to less than 1.2V forces ISMON_INT to the same value as CTRL, providing current control. The output current amplifier provides rail-to-rail operation. Similarly, if the FB pin is above 1.2V, the output of A11 is driven low to reduce the current value and regulate the output. This is constant voltage mode.

The LT3791 provides monitoring pins, IVINMON and ISMON, whose voltages are proportional to the voltages on the input and output current amplifiers, respectively.

Conclusion
The benefits of using LED lighting in any type of environment, including automotive, have several positive implications. First, these LED lamps never need to be replaced, as they have a reliable lifespan of more than 100,000 hours, which is equivalent to 11.5 years of use. In the case of automotive headlights, this allows automakers to permanently embed LED lamps into the body of the vehicle, rather than having to remove the LED lamps for replacement. Styling can also change significantly, as LED lighting systems do not require the depth or area of ​​an incandescent bulb. Finally, LEDs are also generally more efficient than fluorescent lamps in providing light output (measured in lm/W). This has two positive effects. First, LED lamps consume less of the vehicle's bus electrical power, and, just as importantly, this reduces the amount of heat that needs to be dissipated in the headlights, eliminating the need for bulky, expensive heat sinks.

Finally, I hope you enjoyed this brief history of light, from the Big Bang nearly 14 billion years ago to the rise of LEDs today. Looking back, we can clearly see that the pace of innovation in artificial light sources has accelerated exponentially. Clearly, the future of LEDs is bright, while the era of incandescent bulbs is bleak. However, we still have a lingering question: "What will be the next big thing in lighting?" Stay tuned.