In the olden days, bulbs were easy.
We only had to decide three simple things.
First, the cap base…
Secondly, the shape of the glass, usually a "GLS"...
...but sometimes a candle or a golfball.
Thirdly, the wattage, partly because luminaires had maximum wattages to prevent their being burnt by the heat from the bulb.
But maybe the most important decision was how much light they would gave out. This was easy because the output was directly related to the wattage of the electricity being consumed, and everybody had an idea of how much light the common wattages (40W, 60W, 100W….) produced. And if you reduce the power, the bulb dims.
But now, with LEDs, it is all change. Anybody choosing a LED bulb has to take a lot more decisions. Yet what they are deciding is not well understood. I’ll take you through them here.
(Note that in the trade we call bulbs “lamps”, so that’s what I’m going to do from now on, except when referring to the classic GLS “light bulb”, like the one above.).
I’m going to look briefly at:
the differences between light bulbs and LED lamps
what came after the light bulb and before the LED lamp (i.e. at fluorescent strips and halogens)
lumens (brightness)
kelvin (colour temperature)
CRI
dimming
retrofit LED lamps vs dedicated LED light engines
size.
What are the differences between a light bulb and a LED lamp?
The Light
An incandescent lamp produces light the way the Sun does. This is fundamental: the Sun existed before the Earth did, so everything on the Earth has evolved in accordance with its characteristics, This includes our eyes. They developed the ability to see by exploiting a small proportion of the Sun’s energy.
An incandescent lamp produces light the same way as the Sun does. A wire is heated. So it produces the light we understand and were designed for.
Whereas a Light-Emitting Diode relies on fluorescence—the ability of phosphors to produce light. But they can only produce certain frequencies. They cannot produce the continuous range that something burning does. They can emit different selections from the spectrum so, when choosing a lamp, we have to decide which compromise to make.
The Object
An incandescent lamp is a hot wire in a glass enclosure. A LED light engine is a complex set of electronic components. Although light bulbs are simple, you need a big factory to make them. Whereas anybody can make up LED light engines in their garage: all they need to do is buy the bits.
The Quality
This means that there are a lot of cheap LED lamps out there. The golden rule is therefore to buy from a reputable reseller or a brand you know. If you don’t:
the LED may not last for very long
the colour of the light will vary from one to the next…
…and may not be the colour you expect. It may say on the box that it’s 2700K….
they may not give out as much light as they should, particularly after several hours of use
they may be dangerous.
Between the light bulb and the LED lamp
Before there were LEDs, there were:
Fluorescent tubes and Compact Fluorescent Lamps (CFLs)
A grim quality of light, but they did get better as manufacturers learnt to combine several fluorescing phosphors. Very many tubes are still in use (e.g. to disastrous effect in windowless rooms in hospitals). CFLs are less common now. Here are some:
Not only is their light dingy (and they don’t fail like other lamps do, they just get dingier and dingier….), they are also dangerous if they break, because they contain mercury vapour. Politicians urged people to use them but, as always in such situations, the politicians only consider one aspect at a time. Yes, they use less energy, but they are inefficient and dangerous.
However, now that there are reasonably good LED alternatives for most legacy lamp formats, the exemption that has allowed mercury to be used in lamps is about to be removed. This means that, in about a couple of years’ time, it won’t be possible to buy fluorescent lamps any longer. Click here to read Kevan Shaw’s useful blog post about this.
Halogen lamps
Whereas halogen lamps are brilliant in every way! They are also incandescent, so the give out proper light. They use less energy than a light bulb, they are a lot smaller, and they last longer—what’s not to like?!
Actually, what’s not to like is what British architects and electricians did with them. They lit spaces with grids of them in ceilings. This is not the place to rehearse all the reasons why this is so wring, and so unpleasant, but it did have two useful consequences. Whereas with a light bulb one only had to consider the cap, the shape and the wattage, now halogen lamps made possible tiny spot lights. So customers should have learnt to choose the right beam angle (though it seemed few did).
The other innovation was 12V lamps. Since LED lamps can also be 12V, and power the same little spots, we have to thank the specifiers of grids for introducing the public to beam angles and transformers….
The decisions you now have to take when buying a lamp
…and this is its specification:
More than just cap, shape and wattage! Some of these are self-explanatory, some are only required by specialists, but there are some that everybody has to know about. Let’s look at them.
Lumens
Lumens are a measure of brightness. We can’t use the wattage as a proxy any more, because in LEDs there is no direct correlation between the wattage and light output. The lumen output of a lamp will always be given on the box: the abbreviation is “lm”. Here’s a useful comparison table:
Lumens are not an absolute: they reflect the varying sensitivity of the human eye to different wavelengths of light.
Kelvin
The Kelvin scale is used to measure the ”colour temperature” of the light output by the lamp. Because a LED lamp can’t give out the full spectrum, as we have seen, you have to choose which bit of it you want to use for any given location. Here is a table:
The kelvin scale is inherently counter-intuitive when used to measure the colour temperature of light, because the whiter the light (i.e. the “cooler”) the higher the temperature. One might have expected the warmer light to be the “hottest”. This anomaly arises from the fact that the colour temperature is determined by finding the approximate theoretical temperature in Kelvins that a black-body radiator must be heated to in order to create a visible colour. To understand this, imagine a gas hob; the higher (hotter) you make it, the bluer the flame gets. The abbreviation for kelvins is “K”.
There is no “right” colour. For example, the closer to the Mediterranean you get, the cooler the default choice (3000K). In UK it is 2700K. But the most suitable colour will depend upon the space and what is in it—what is being looked at, by whom.
Ignore the references to daylight, blue sky &c. in the representation of such scales. Remember, the Sun’s light is the full spectrum of visible light. Yes, the light the Sun produces is affected by many things—e.g. cloud cover. But also remember that our eyes, working with the relevant parts of our brain, evolved to process the energy produced by the Sun. This is not done passively, like a camera. Thanks to “colour constancy”, we adjust for this. Thus, the Impressionists showed us that shadows could be blue and black, and grass all sorts of colours, but our eyes still see grass as green.
Monet: Essai de figure en plein-air: Femme à l'ombrelle tournée vers la gauche
We judge colours in daylight if we can (we take a garment out of the shop) not because daylight is always exactly the same, but because our brains have learnt to adjust for its changes.
So why do such tables make reference to daylight? Simple: it is all part of a conspiracy to get people to believe that it is normal for only bits of the spectrum to be available. This conveniently ignores the other effects that light has upon us, that are essential to a healthy life. It is like the makers of fake meat and fake milk extoling the fact that they look and taste like the real meat and milk, whilst ignoring the nutritional value of the real thing. Ultimately the public realize they’ve been conned: why do you think it is no longer possible to buy margarine?! Since we spend most of our days in artificial light, and because almost all new homes do not have open fires, it is no wonder that we are surrounded by competitive miserableness!!
Now, where was I? Oh yes.
CRI
Because the patchy spectrum of LEDs is not daylight, they alter the appearance of colours to us. This has led to another scale of measurement: the Colour Rendering Index (CRI). The CRI given for a lamp attempts to indicate how near to the performance of daylight it is, as far as human eyes are concerned.
This matters anyway, but it particularly matters to interior designers, because colours and textures will look different according to the CRI of the light source.
As with colour temperatures, the appearance of light from sources that have the same CRI, but are from different brands, may not be the same. The light from two lamps with the same CRI, but different colour temperatures (e.g. 2,700k and 4,000k), are also not going to be the same.
By definition, daylight is 100. So is the incandescent light bulb. CRIs of LED lamps used to be typically 80 to 85, but we now see more CRIs of 90 to 95 (much higher than this will never be possible). Note that the higher the CRI, the more expensive the LED is to make, and the more power it consumes.
Unfortunately, the current CRI scale was developed in the early days of fluorescent strips and is regarded by many as inadequate when judging LEDs. So a new, much more accurate, scale, “TM30” was developed. But in lamp specs, and on the box, it is CRI that is still used.
Dimming
You can dim any light bub and it is easy to do; you just reduce the power going into it. Remember, that’s because it is just a hot wire. But a LED is not a bit of wire; it is a very complicated set of electronic components, so to achieve a similar, simple effect is immensely complex. There are three important consequences.
The first is that they are not necessarily dimmable (only in UK do people insist on every lamp being dimmable) so, if you want to dim them, check first. It’ll be in the spec and on the box as a symbol like this one:
The second is that if they dim, that’s all they do—they get dimmer, thus creating a dingier, grey light. Whereas an incandescent lamp, when dimmed, casts a “warmer” light (remember the comparison with the gas hob). Fortunately, ever greater feats of engineering are resulting in LED lamps that “dim to warm”. Use these if possible, though they will be the most expensive.
The third is that dimmers the client already has are incompatible with LED lamps if they were bought before LED lamps took over. This is because non-LED lamps operate at 40W, 60W, 100W…, whereas LED lamps operate at 2W, 4W, 6W….
Cheap LED lamps may not dim smoothly down to nil. They may flicker below a certain level, or not go completely off. And some may need to be turned fully on before dimming them down. Imagine that they have to be kick-started, so if you just turn them gradually up from nil to 10%, say, they may not operate, whereas they are happy at 10% if they have already been up to 100%.
“Dedicated” or “retrofit”?
LED light sources can be LED versions of existing lamp types, with the same caps, shapes &c. These are known as “retrofit” because you can just screw them in where an incandescent lamp was before.
But many luminaires come with their own light engine fixed into them, known as “dedicated”.
Which is best?
Points in favour of the retrofit LED:
a luminaire that takes retrofit lamps is future-protected. There will always be E27 lamps even as the technologies change, whereas a dedicated light source may be irreplaceable once the manufacturer stops making that luminaire. Even if a replacement is still available, it may be different to the LED engine used originally, because the technical development of LEDs is still rapid. And often it’s not possible to replace a dedicated LED lamp without the whole luminaire having to be sent back to the factory….
there will always be a retrofit that is compatible with any dimming protocol, now and in the future
they don’t require a remote driver/transformer unless the halogen lamps they are replacing did
if you don’t like the colour temperature, or the brightness &c., you just change one retrofit lamp for another.
Points in favour of the dedicated LED:
they dramatically expand the possible shape, size and functionality of the luminaire
they can perform better.
A dedicated LED can perform better than a retrofit LED because the latter has to cram the LED, its cooling and its power supply, into a small container that was designed for a completely different technology (incandescent or halogen). It then has to be sold at a low price in a highly competitive market. Whereas with a luminaire that has a dedicated LED inside and a remote driver, each component can be designed to do its job properly, without any size restriction, or similar price constraint.
As an illustration, here is a 10W power supply. It is 115x34x19mm:
Here is a 10W GU10. The whole thing—LEDs, power supply, connectors and cooling—has to be Ø50mm L55mm.
That entire GU10 costs less than that power supply on its own!
Size
So the retrofit has to contain the components that make up the LED light engine as well as its power supply. It also has to be kept cool: if it gets above about 50°C, its life will be dramatically shortened. This meant that early retrofits were quite ugly. It’s not so bad now—we’ve gone from this…
---to this…
…one consequence of which is that the lamp now spreads its light more evenly, 360° from the whole glass area.
But some legacy shapes are too small to fit in everything that a LED requires. So either they are not available, or they are not efficient enough yet (the R7s) or they are too big. The most frequent size issue we have is with G9s being too big, so always check the dimensions when specifying a retrofit G9!
Takaways?
LED lamps do fail. No-one knows when: the extravagant claims of e.g. 50,000 hours are extrapolations from initial data. And sometimes they just fail quite soon for no apparent reason.
Buy the best you can. Where relevant, the illustrations in this email are of Zico lamps. Click here for their web site.
If the lamp has been specified by a lighting designer, replace like-with-like. They know all this stuff and will have specified what they specified for good reasons.
Otherwise, do work through the issues I’ve covered above. The data you need will be in the spec/on the box. Don’t avoid this step. If you do, the consequences can range from a disappointing scheme (and therefore a pissed-of client) to the hotel burning down.
One of the biggest causes of aftersales issues is someone in Maintenance replacing a particular lamp with another that has the same cap (so it is possible to screw it in) but which is not suitable for the luminaire. Or it’s just cheap.
Finally, remember that it is physically impossible for a LED lamp to cast the light the Sun does (whatever marketing people may say) and therefore to confer all the benefits the Sun’s light (the light we were designed to process) gives us. Only an incandescent light source can get anywhere near, and therefore a CRI of 100!
Artificial light matters because we now spend most of our time in it.
