Energy efficiency is good solid worthy way to encourage home owners to switch to away from incandescent bulbs.
But they might as well switch to compact fluorescent alternatives because they cost only a quid each.
And then some of them will take them out again and go back to their elicit stock of 100W bulbs because compact fluorescents really are dimmer and really do take ages to warm up.
So how do you encourage a fashion-conscious home owner to pay a lot for an LED alternative.
Let dimming,dimming,dimming be your marketing mantra,I would propose,to be replaced by colour in a couple of year's time.
Industry is working hard,but has only just begun to scratch the dimming and colour-tuning possibilities of power LEDs.
To be anything other than a novelty,at least initially LED lights have to fit straight into the sockets vacated by incandescent bulbs and work with no changes to the house wiring.
And it is happening.
Triac-dimmable is the current watchword amongst LED power chip makers,which means the chip can work with work with dimmers intended for incandescent bulbs,and the LEDs will dim according to the knob on the wall.
This is no trivial task in the chip,as the intensity curve of a LED is nothing like that of a light bulb so some sort of translation has to be included.
NXP was early on the scene with its SSL210x range and there are drivers from National Semiconductor,Power Integrations,iWatt and others.
A word of warning.Not all wall dimmers are triac-based.Some use transistors and not all chips are compatible with both types.
And there are a lot of dimmers,some of them awful,so anyone wanting to make a dimmer-compatible LED light bulb better be prepared to do some wide-ranging testing.
Even with a great dimmer,a great chip and a great circuit,you will be lucky to get 100:1 dimming range which is a lot better than an incandescent bulb,but not the 500:1 or more than can be used in the home.
500:1 might be a complete waste of time in a kitchen,but in a bedroom this range gives you a night light as well brilliance to Hoover by.
A downside of keeping a dimmer intended for incandescents in the circuit is that none of them will work with LEDs at the lower end of the current range,so all the triac-dimmable chips have to bleed,and therefore waste,current through the dimmer to keep it operating.
German chip maker ZDM has recently introduced a chip that will dim using the standard wall switch.
Dubbed ZLED7030 and aimed at 12V lighting,the application circuit uses a 220μF capacitor to run the chip through power interruptions up to 2s and interpret the hiatus as a control signal.
So by blipping the wall switch,the home owner can cycle around 100%,50%,20%,100%....current through the LED string.
Slightly clever than that,there are two pins on the chip that can change this to 100-60-30-100 or 100-30-100,or no dimming.
It seems to me that have missed a trick here,as the range is still a maximum of 5:1 which once the eye has logarithm-ed it will look like halving the brightness.
The same firm does a more conventional chip that can be dimmed by applying a PWM or voltage signal,that it claims offers 1,200:1 range.
On this subject,many chips are available that offer PWM and voltage dimming.
A smart designer could use this kind of interface with a microcontroller and probably get a 1,200:1 range entirely by switch flicking-the challenge would be to fit that into a bulb,and persuade the home owner to flick a switch a lot or add a time dependency into the control algorithm.
There is a lot of scope to develop this technique,and if lighting makers could get their acts together there could be whole new class of'LED dimmer'that could signal to compatible bulbs in the form of complete mains cycles that would produce very little interference to other circuits.
Wireless control is another possibility,over ZigBee,EnOcean,Bluetooth Low Energy,or something similar.
Just a note on PWM dimming here.
If the PWM signal is passed onto to the LEDs,you are creating the equivalent of a multi-firing flash gun.
Done at a high enough rate this is invisible to the static eye,but if the leds are in the field of vision,the flashing can be immediately noticeable to anyone looking from one object to another in the room.
I suspect most people get used to this quickly,but it will drive others nuts.
Power supply translated into lighting-speak is'ballast',and ballast-makers are sorting out how to use and dim LEDs in commercial premises-developments that are also applicable to multi-downlighter installations in the home.
Some offer triac dimming,but the de-facto standard dimming option for premises without control wiring is'pushbutton dimming'-offered by almost every LED ballast maker on at least part of their range.
No additional wiring is needed for pushbutton dimming,only reconnection.
Instead of the wall light switch switching the live on and off,the switch is replaced with a push button and the ballast wired live the whole time.
A 240V control signal is sent by the ballast down to normally-open pushbutton which replaces the wall switch.
This can be used to dim the LEDs up and down by the usual long-push short-push to dim and change dimming direction favoured by touch-controlled incandescent dimmers.
This is certainly a low-hassle arrangement for the installer,and some of them give smooth flicker-free dimming over a wide range,but it is less elegant as the ZDM approach as it leaves the ballast permanently energised,dissipating several Watts in many cases,even when the LEDs are very dim or off.
For buildings with control wiring,there are LED ballasts that implement traditional 0-10V and 1-10V dimming.
This scheme translates well to LED dimming,although falls apart somewhat if very wide range dimming is needed as dc offsets get confused with intensity instructions-power LEDs produce visible light at a few dozen microamps-so if the die is visible,offsets will be noticed.
Digital control schemes are also available for use with control wiring with DALI(digitally addressable lighting interface)looking the most popular.
Being digital,the ballast can look after its own offsets so there is no excuse for poor dimming control at the low end.
And colour?
My crystal ball is on the blink so I can make no predictions as to what form it will take when it does catch on.
The technology is already there:red green and blue power LEDS are all in production-some high colour rendering index white LED fittings already use different coloured LEDs to fill out their spectrums.
As far as I can see,control will have to be over some form of digital interface,over the mains wiring or by wireless remote control.
The technology is simply three-channel dimming-or two colour if the need is just to add a variable amount of red to adjust the'warmth'of a room.
The trick will be to presenting the choice of a billion colours to the punter in a meaningful way,or to limit the choice in an acceptable way.
