Light Bulbs

In the old days, buying a light bulb was simple.
• How many Watts?
• Pearl or clear?

There were some special bulbs, such as for projectors and for photography, but these weren’t mainstream and you’d have to go to a specialist supplier to get one.

Bewildering choice

Nowadays, the choice is bewildering, and something I’m having to resolve as I’m refurbishing my house, so I thought it would be useful to summarise some of the considerations.

Incandescent bulbs were progressively phased out in the UK from 2009 to 2014, being initially replaced by ‘compact fluorescent’ or CFL bulbs and subsequently by LED bulbs.

‘Halogen’ incandescent lamps are still permitted but are generally only used where a small light source is needed, such as in projectors, car headlamps and for some decorative uses.  These are most efficient when operated at a low voltage (generally 12 volts) so these need a transformer in domestic use.

LEDs are the norm

It is fair to say that LEDs are now the norm.  They are bright, come on immediately, generate little heat and have a long life.  They are not perfect.  The main disadvantage is that they can give poor colour rendering.  So lets look at this.

Black body radiation
The CIE colour space and black-body colour temperatures

An incandescent bulb radiates light due to the high temperature of the filament.  The radiation [nothing to do with radioactivity]  is close to a ‘black body’ radiation, which is dependent on the temperature of the heated object – in this case the filament.  We are used to seeing things under such illumination, as this is approximately what we see under sunlight (ignoring the effect of atmospheric absorption).  A black body emits light in a continuous spectrum which peaks at a certain frequency (colour) according to its temperature.  The higher the temperature, the bluer the peak of its spectrum. The ‘Colour temperature’  of a lamp means the temperature of a ‘black body’ when heated sufficiently to glow at the same colour that the lamp gives out.  It is usually expressed in ‘degrees Kelvin’ which are 273 degrees more than degrees Celsius – in other words, water freezes at 273 degrees Kelvin.  0 degrees Kelvin is ‘absolute zero’, where an object has no thermal energy.

Colour Temperature

On the colour temperature scale, a bright red glow is 1000 degrees Kelvin.  At 2000 degrees Kelvin, there is a bight orange glow, rising to a bright yellow glow at 3000 degrees, a yellow-white glow at 4000, an almost white glow at 5000 and a pure white glow at 6000.  At 7000, glow is a blue-white and at 8000 it is distinctly blue.  By 10000, we are looking at a bright sky-blue colour.  The colour temperature of a typical incandescent or halogen bulb is about 3200 Kelvin.

How LEDs work

However, LEDs don’t work by heating an object.  They work by ‘exciting’ electrons to vibrate within atoms, which when they fall back to their rest state emit a photon, depending on the material they are made from.  Nowadays, most LEDs emit photons in the blue or near ultraviolet range, but the blue/UV light is absorbed by a phosphor coating that emits visible light in the yellow range of frequencies (which is why the surface of the LED looks yellow when not illuminated).  By adjusting the balance between the blue of the LED and the yellow of the phosphor, the light can look white (often ‘warm white’, or ‘cool white’  but in reality it omits large parts of the spectrum, especially in the red.  This can mean that the colour rendering of LEDs can be very poor, particularly for skin and other surfaces containing a lot of red.  For this reason, lamps are now given a ‘colour rendering index’ (CRI) which indicates how closely the lamp reveals the colours of an object compared with a natural light source.  A CRI of 100 means that the lamp shows colours exactly as they appear under ‘standard’ daylight.  The test is done by looking at special test colour samples under the lamp and under light of the reference ‘colour temperature’ and rating the differences observed.

Colour rendering
Spectrum of different lights

Typical ‘white’ LEDs have a CRI around 83,  which is better than old fluorescent tubes, but far from ideal,  so this information is often omitted in marketing details.  However a CRI above 90 is needed for good colour reproduction.  It is possible to get better colour rendering by using phosphors that emit red, green and blue light.

A warm white LED has a colour temperature of 2700 K, which is considerably more yellow than an incandescent bulb.

A particular difficulty can exist for film and video lighting, because the spectrum of the LED, even with a high CRI, may not match that expected by the colour sensors in the camera.  For this reason, a special colour rendering index has been developed for video use.

Luminous efficiency

In the old days, you knew how bright a 100 w or a 60 w bulb would be.  LEDs need far less power but the amount of light they give out (which is measured in lumens) depends on their design.

The two-colour white LEDs have the best efficiency, around 120 lm/W whilst 3-colour LEDS produce around 70 lm/W, although the amount of power they need also depends on the efficiency of their control circuitry.

By comparison, an incandescent lamp produces about 15 lm/W and a CFL produces 63 lm/W.

This means that to a rough approximation

Incandescent  LED     Lumens
100 W                24 W        1800
75W                    15 W        1000
60W                    11 W           900
40 W                      6 W           400
20 W                      3 W           300

Dimmable?
The circuitry inside an LED lamp

There is yet another consideration – can they be dimmed?  This is not primarily anything to do with the light-emitting diode, but the electronic circuitry within the lamp.  All domestic LEDs have control electronics in the base of the lamp, because the LED must be fed with direct current at 2 to 3 volts.  There are a variety of ways of reducing the mains voltage of 240 volts to this low value, but usually by ‘chopping’ the incoming AC mains so that it is only on for part of the cycle.  The chopped mains ‘fills’ a capacitor until it reaches a certain a low voltage, and then stops the current, which then discharges into the LED.  Depending on how this is done, the lamp may not work with a dimmer, which also chops the mains voltage to drive less power into the lamp.  Some lamps can be dimmed, but only with a ‘trailing edge’ dimmer.

The lamp cap/base

There is yet another consideration – the lamp base.  Traditionally, only the ‘bayonet’ cap was used in the UK, but with the influence of Europe, ‘Edison Screw’ fittings have become very common. Both these ‘caps’ are available in different sizes.  It is not part of this article to consider the pros and cons of the two types of fitting. Most of the lamps described above are available in these two ‘caps’.

Bi-pin caps

There are also many types of ‘bi-pin’ fittings which originated for different purposes. G4 is a small bi-pin fitting originally designed for low-voltage halogen lamps.  G9 is a slightly larger fitting used with higher-power bi-pin mains voltage lamps, and G10 is a large bi-pin fitting intended for high-power mains halogen lamps.  However, these have all be re-purposed so care must be taken that your lamp is for the correct voltage as well as the correct cap.

Considerations when choosing

So, when choosing and LED bulb, the main considerations are:

  • the cap/base to fit the lampholder
  • the voltage of the bulb 230/240 volts in the UK, but may be 12 v in fittings with a transformer to replace some halogen bulbs
  • The colour temperature
    • 2700 = warm white, (i.e. yellow)
    • 4000 = natural white (i.e. sunlight)
    • 6000 = cool white (i.e. skylight)
  • Dimmable or not
  • Colour rendering (if you are doing art/design work)
  • Lumens output
The Circuitry

For those interested in the technology, this titchy bulb contains some quite sophisticated electronics.  I have decided to cover this in a separate blog called LEDs – The circuitry

On one side we have what is essentially the input circuitry – the big yellow rectangular blob is a smoothing capacitor and I assume that the small rectangular things form a bridge rectifier.  Together they create a nice smooth 12 v DC input from the 12 v AC that they are supplied with.  This is fed to the circuitry on the other side.  Although I can’t read the designations on the small chip in the middle, it is something like the one shown in the circuit diagram below.  In essence, it is a current/voltage regulator that reduces the 12 volt input to that needed to drive the LEDs.  I am rather inclined to think that this will not work with a dimmer – in fact it does its best to maintain the LED current whatever the input voltage.

 

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