LED Retrofits in the home – to dim or not to dim?

This question may sound banal but draws attention to an important difference to places of work such as offices. There are many reasons for regulating the brightness of light. When using light in places of work it is almost always a matter of saving energy. This is not the case in the home. The incandescent lamps (high volt/low volt halogen, standard incandescent lamps), which were mostly used until a few years ago, are of their very nature not suitable for saving energy, because a reduction in voltage leads to an over-proportional reduction in luminous flux. The radiation efficiency shifts further and further into the non-visible red, better known as infrared. In this way the already modest luminous efficacy of a halogen low volt lamp drops from approx. 25 lm/W at 100% performance to approx. 1 lm/W at 1% performance. Nobody would ever dream of dimming an incandescent lamp to save energy.

However, dimming can substantially change the atmosphere in a room. Together with a change in brightness, dimming any thermal radiator leads to a change in light colour. Namely towards »warm«. And it is precisely this effect which we are interested in. As long as we have a high quality dimmer, it is possible to move along the whole Planckian curve, starting at 3 000 K and ending at 295 K (room temperature). An incredible flexibility which with a minimum of effort can immerse the dining area in a candlelight ambience when dimming to approx. 2 000 K.

Unfortunately, the term dimmability does not really tell us much. It merely describes the possibility per se. What is decisive  is the dimming quality. When comparing the above-mentioned thermal radiators and LEDs, differences become apparent. LEDs and, in particular, retrofits behave differently and require different technology for dimming. LEDs do not change colour when dimmed. The effect described above in the dining area can therefore only be achieved in a different way. When selecting products the designer/user must ask himself new questions: How far can the light source be dimmed? Does the dimming process take place smoothly and in a visually uniform manner? Is there flickering at certain stages of dimming or does dimming seem to proceed in recognizable sequences? Does noise develop during dimming?

Dimming performance of low volt halogen exceeds that of LED Retrofits

Despite all the justified criticism of thermal radiators with regard to efficacy, they can be dimmed quite easily from 100% to 1% with a simple phase dimmer whereby the percentages refer to the luminous flux. This performance is already quite amazing when you consider that almost all manufacturers of LED retrofit lamps restrict the dimmability of their products at approx. 10% of the nominal value. At this point we must ask what the sense of this is. The sensitivity of the human eye to changes in brightness does not proceed linearly but – to put it very simply – follows a common logarithm. That means that  when the luminous flux is reduced by  the factor 10 (dimming from 100% to 10%) we have the feeling that the brightness has been reduced by half.  

Professional dimming of LED systems 

To dim LED light sources professionally it is necessary to systematically adopt a suitable procedure such as CCR or PWM. When the technical requirements are planned carefully, it is possible to dim LED luminaires even into the lowest brightness zones, at least theoretically. Products which actually exist in the architectural lighting sector are frequently dimmable to »only« 1%. For many professional applications this is sufficient. However, in the home, especially in bedrooms and bathrooms , including the zones connecting these two areas, this is not sufficient. This applies not only to private homes but equally to hotels, medical facilities or homes for the elderly. 

An example: You illuminate a room with lively accents (project a spot onto a picture) and dimension the accent lighting to 1500 lx. Then, even when dimming to a possible maximum of 1%, you still have 15 lx on the picture. At an average degree of reflectance of 50% this generates a luminance of 2.4 cd/m². This value is in the upper mesopic zone and, for the eye completely adapted to darkness, this could already be described as dazzling. For clarification here is an example: The »brightest« street lighting according to EN 13201.2 (illumination class ME1) requires 2 cd/m². When half awake on the way to the bathroom at night nobody would wish for such a luminance. A »good, old« halogen low volt lamp, which, when used in the described way, would only produce 0.2 cd/ m², would be more than sufficient. And this would not only be functional but also have the appropriate quality. 

We are of course waiting expectantly for the first manufacturers of LED luminaires to discover this niche for themselves and offer systems which really can reproduce the complete dimming performance of incandescent lamps.