AFTER a street-light opposite his home went out, your correspondent started noticing others on his hillside that had likewise gone dark. Twice, he reported the fact. Each time, he got a polite thank you from a city official, but no explanation nor promise of action. Then, several months later, an elevator truck arrived and replaced all the low-pressure sodium lamps in the street with solid-state light-emitting diode (LED) lamps. They have turned the front of his house from night into day. In the long run, they will also save the city money, though not as much as is commonly supposed.



Sodium-discharge lamps are efficient, because they produce virtually all their light at a wavelength near the peak sensitivity of the human eye. That translates into a rating of 200 lumens per watt of electrical power consumed. The white LEDs used for street lighting have a typical luminous efficacy of 100-125 lumens per watt. The compact fluorescent lights (CFLs), or “twisties”, used in the home produce around 60 lumens per watt. The traditional incandescent bulbs that CFLs were designed to replace emit a miserly 14 or so.



In terms of energy savings, then, LED street-lights are contenders, but definitely not champs. However, they have other advantages. One is longevity. They simply do not burn out, but gradually lose brightness over the years. The usual measure of an LED lamp's economic life is its L70—the time taken for the brightness to fall to 70% of its original value. Municipalities and commercial users reckon they ought to get ten years or more from LED lamps before this happens and they have to replace them. Low-pressure sodium lamps, by contrast, last typically only three or four before they start getting thirsty. Add to this the fact that the cost of changing an LED street-lamp is around $60, compared with over $200 for a sodium lamp, and the lack of maintenance costs alone can make the switch worthwhile.



There are also performance benefits, though. Unlike sodium lamps, LEDs turn on immediately, rather than taking minutes to warm up. They are also better at displaying colours. And recent research has shown that white light provides better visual perception than yellowish-orange sodium light. This is because of the way different photoreceptor cells in the retina perform their different tasks. In daylight, vision is handled largely by cone-shaped cells packed around the centre of the retina. Apart from processing colour, these cone cells help the eye perceive detail and rapid changes in surroundings. In the dark, by contrast, perception is handled almost exclusively by the more sensitive rod-like cells towards the retina's periphery.



For people driving at night, however, artificial lighting means conditions are usually neither pitch-dark nor light enough to see in a daylight-like way. Moreover, the driver's movement with respect to the lights means the illumination his eyes perceive is constantly fluctuating. In such circumstances, both rods and cones are required, but the demands on them are constantly changing. When it is briefly lighter, the eye is more sensitive to greenish-yellow light. When darker, it responds best to light that is greenish-blue. White LED lamps do a better job of meeting these conflicting requirements than sodium lamps can manage—and they do it at lower power levels, to boot. There could therefore be some energy savings from switching to LEDs after all.

Politics as well as optics have played a part in making LED lighting in general more popular. Among other things, the Energy Independence and Security Act, signed into law by President George W. Bush in 2007, with widespread support in both houses of Congress and backed heavily by the lighting industry, effectively outlawed the traditional incandescent light bulb, because of its profligate consumption of electricity.

Lighting accounts for roughly 17% of the electricity used in the United States. Reducing the amount of waste—through the introduction of solid-state technologies that convert a lot more of the energy into light—would reduce that figure. Incandescent bulbs convert no more than 6% of the energy they consume into light. The rest is turned directly into heat. LEDs convert over 75%.



The 2007 act required that, by the beginning of 2012, light bulbs which consume 100 watts would have to be 27% more efficient. By 2014, bulbs rated at 40 watts and up would have to do the same. While some of the latest incandescent bulbs more than meet the new efficiency standard, traditional tungsten-filament bulbs did not stand a chance.



The law did not explicitly ban the use of traditional incandescents; it merely prohibited their import and sale. As most of the cheap incandescent bulbs came from China, Congress had no fear that the ban might cause hardship to American lighting firms.

Unfortunately, the politicians did not bargain for a backlash from the public. Consumers objected vociferously to being forced to buy alternatives like compact fluorescents that cost three of four times more.



Besides, CFLs were by then beginning to get a bad reputation. Few lived up to their claim of longer life, especially when they were switched on and off repeatedly, or used in recessed ceiling lights where heat could build up and fry their inbuilt circuitry. Meanwhile, the procedures for dealing with the hazardous mercury vapour in spent or broken CFLs made people wary of having them around the house.



Fearing a consumer revolt, Congressmen who had backed the legislation rushed to repeal it. In the event, the prohibition on the sale of inefficient 100-watt incandescents was put off until October 2012. Californian retailers ceased to restock them in January 2011. But the prohibition is now in force, and people are grumpy. Though LED bulbs, which have a total-cost-of-ownership significantly less than that of sodium or other forms of industrial lighting, have proved their worth to municipalities and businesses, they have yet to do the same for residential users.



That is hardly surprising, given an up-front cost of anything from $25 to $60 (before possible rebates from utility companies) for an LED bulb that has a luminous flux equivalent to the 100-watt incandescent variety. To the average householder, it does not matter that such bulbs pay for themselves—by using considerably less energy than incandescents—in three to four years, and go on putting out useful light for 20 years or longer. What does matter is that swapping from incandescents or compact fluorescents to LEDs makes a big hole in the family budget in the here and now.



Your correspondent plans to upgrade his original 4.2 kilowatts worth of indoor and outdoor lighting with LED equivalents, one lamp at a time, as the existing bulbs (mostly compact fluorescents) burn out. Replacements for 100-watt flood lights tend to cost around $40 a piece. But the popular 60-watt-equivalent can now be had for as little as $14.



Not all LED lamps, however, are created equal. Some of the cheaper ones have a chilly blue cast. None has quite the full complement of colours of the profligate tungsten-filament bulb, with its wide pallet of wavelengths in the visible spectrum.



The usual measure of a light source's ability to reproduce colours faithfully is the Colour Rendering Index (CRI). By convention, incandescent bulbs are assigned a CRI of 100. Cheap LEDs can have a CRI of 50 or less. A really good one—such as the $50 Philips LED bulb that won the Department of Energy's $10m “L-Prize” for exceeding all the set requirements for a 60-watt-replacement—has a CRI of 93. Most LEDs have CRIs of 80 to 85. It is thus worth checking the quality of the light an LED bulb produces before splurging on a six-pack of them.

It is also wise not to take claims of an LED bulb being dimmable too literally. Though many are, they usually require approved dimmer switches. That can mean replacing existing switches along with the bulbs.



That said, these are early days. Like their industrial brethren, LED bulbs for domestic use will get better, cheaper and have more features. Your correspondent is particularly excited by the prospects of being able to communicate remotely with his lights at home via Wi-Fi and the internet. He also looks forward to being able to program the colour they produce on the fly. Solid-state lighting makes all this, and more, possible.



Bulbs like the Philips Hue or LIFX can have their brightness, colour and timing of when to come on and go off controlled wirelessly from a smartphone. The Philips programmable LED uses a separate router box, but the Australian LIFX bulb has the networking electronics built into the bulb itself. The LIFX can even be set up to change colour to music.



That may be a gimmick, but it gives some idea of how domestic lighting is poised to become yet another network appliance around the home, with each bulb having its own internet address. Once that happens, the possibilities become endless.