LED lights are a source of light that is replacing the usual incandescence lights1. They are even preferred to halogen lamps and compact fluorescent ones of low consumption. This fact is due to the the following advantages:

1.- Very low consumption down to 80% less:
    From 80 to 130-150 lumen/wat, versus values of 20-25 lumen/wat for halogens ones and of 10-12 lumen/wat for incandescence bulbs.

So that, a 10 W LED lamp gives the same “amount of light”, around 600-700 lumens, than an incandescence or halogen lamp of 60 W, or a fluorescent one of 20 W.

2.- A low amount of heat is produced, so a large amount of energy is saved.

3.- A very long life:
    30.000-60.000 hours (10-20 years) versus 1.500 hours of incandescence bulbs, 3.000 hours of halogens ones or 6.000-9.000 hours of compact fluorescent lamps.

4.- They are produced with different light temperatures depending on the use: warm white for homes, sun white for offices, cold white for outdoor lighting. They are also produced in different colors, a feature that is very useful in decoration and other applications.

5.- Lighting is instantaneous after switching on.

6.- They support an extremely large number of switching on/off cycles.

7.- They admit regulation of light intensity.

8.- They are manufactured in any shape: bulbs, panels, strips, etc., and with any usual connections: Edison, bayonet, pins sockects, etc. It means that no changes in the electric installation have to be done.

9.- They are highly ecological: mercury is not present; there is not ultraviolet light emission; CO2 emission is decreased due to the lower energy consumption; their very long life implies lower expending of row materials and lower production of waste materials that, in any case, are recyclable at the end of their useful life.

10.- Their price is lowering continously; furthermore, due to their much lower energy consumption the higher price is compensated before one year; from that moment on, it is a net savings source lasting along their long life.

From the above it is concluded that LED lamps are the future of illumination in every aspect: lighting supply, decoration, light indicators, etc.


Nowdays most used outdoor discharge lamps are:
    a) Mercury vapor lamps (MV); they can be of low (fluorescent) or high pressure. Only high pressure ones (HPMV) will be commented here:

    b) Sodium vapor lamps of low and high pressure. Only high pressure sodium vapor lamps (HPSV) will be commented here:


There are several basic factors that condition the life of discharge lamps:

1.- First one is flux depreciation. It is produced by darkening of tube surface where electron emitting material from the electrode coating is depositted. In lamps that use fluorescent substances there is an additional cause from lost of eficiency of those substances.

2.- Second one is aging of lamp components due to electrode degradation because of depletion of the emitting material of its coating.

3.- Other causes are a gradual change of composition of the refill gas and its leaking in the case of high pressure lamps.

Most important external factors that condition the lamp life are:

- Ambient temperature: They will be more or less afected depending on the construction characteristics (nude tube, external bulb, ...). For instance, high pressure lamps have start problems under low temperatures.

- The influence of number of switch-on is very important to determine the life of a discharge lamp because deteroration of the emission susbtance of the electrodes depends largely on this factor.

Light efficiency: LUMINOUS FLUX

Nor all light emitted from a source reach the human eye and produces a sensation, neither all energy consumed, for instance by a bulb, is converted into light. All this have to be evaluated in some way, consequently new quantities have to be defined: luminous flux

- Luminous flux is defined as the power (W) emitted as a luminous radiation that is perceived by the human eye. That is, the power that is converted to visible light. Its symbol is Φ and the unit is the lumen (lm).

- The equivalence between wats (W) and lumens is call luminous equivalent of energy and it is defined as:

      1 W-light at 555 nm = 683 lm

- Another important quantity is theluminous efficacy; it is defined as luminous flux per consumed wat; its unit is lumen/wat (lm/W). The efficacy of the mercury vapour lamps of high pressure is in the range 40-63 lm/W, whereas that of sodium vapour lamps of high pressure is in the range 70-130 lm/W.

Therefore, as it is shown in the figure, only a fraction of power yields visible radiation because it is lost in a large extent as heat and non-visible radiation.

Color of Emissions

The human eye is not equally sensible to all wavelengths of sun light. In fact, maximum sensibility is at 555 nm what correspons to yellow greenish colour. Then sensibility decreases as we move away from the maximum towards the spectrum ends (red and violet). This a reason to use yellowish colour for danger or advise signals, emergency lighting or foglamps.

Tradicionally, we diferentiate between cold and warm colours. First ones are violets, blues and dark green. They give a sensation of coolness, sadness, meditation and confinement. On the contrary, warm colours, yellows, orange and clear green produce sensation of happiness, stimulating and friendly atmosphere, and room spaciousness.

For discharge lamps several things can be done to get good colour reproduction and high yied:
- Increasing of gas pressure inside the lamp
- Addition of solid substances to the gas is another solution since theY emit complementary monocromatic radiations at vapourizing.
- Finally, the internal wall of the bulb can Be covered by a fluorescent substance to convert ultraviolet radiation into visible light.

a) High pressure Mercury vapour lamps

As mercury vapour pressure is increased inside the discharge tube, the importance of the ultraviolet radiation characteristic of low pressure lamps goes down with respect to radiation in the visible zone.
However, light of blue greenish colour emitted in these conditions lacks of red radiations. To solve this problem fluorescent substances that emit in this zone of the spectrum are usually added. In that way, the cromatic features of the lamp are improved. Colour temperature is around 3500-4500 K. Useful life, taking into account depreciation, is established in around 8000 hours. Efficacy is between 40 and 60 lm/W and it increases with power; for a given power, it is possible to increase efficacy incorporating an internal coating that converts ultraviolet light into visible light.

b) High pressure Sodium vapour lamps

High pressure Sodium vapour lamps have a spectral distribution that covers most of the visible spectrum yielding an orangish ligh much more confortable that the ligh emitted by low pressure lamps. As a consequence of this, HPSV lamps have a much better performance and capacity to reproduce colours than low pressure lamps. Nevertheless, efficacy has to be given up to get these features; although its value, around 130 lm/W, continues being a high value if compared with other types of lamps.
Mean life of this type of lamps is around 20.000 hours and their useful life between 8.000 and 12.000 hours. Several causes limit the lamp duration; besides flux depreciation we have to include leaks in the discharge tube and a progressive increasing of the necessary lighting voltage to levels that impede a correct operation.

Adventages of LED lamps

In comparison with that writen above, LED lamps present a number of advantages with respect to high pressure lamps of mercury (HPMV) and sodium (HPSV):

    1- Very low power comsumption, down to 80% lesser.
    2- They produce a very low amount of heat.
    3- They last a very long life (very long mean efficient life).
    4- They bear numerous switching on/off cycles.
    5- They light instantaneously after switching on.
    6- They can produce any colour shade to fit to their use.
    7- They are highly ecological (without any pollutant and 100% recyclables).

LED lamps have a mean useful life much longer that high pressure lamps of mercury (HPMV) or sodium (HPSV):

LED lamps have a luminous flux much higher that high pressure lamps of mercury (HPMV) or sodium (HPSV):

Adventages of LED lamps: LIGH COLOUR SHADES

LED lamps are produced to give ligh of colour shades adequate to each use:

a) White
   - Warm white (2700-3000 K) for homes similar to HPSV lamps.
   - Sunligh white (4000 K) for offices.
   - Cold white (6000 K)  for outdoors similar to HPMV lamps.


b) Different colours: Very useful feature in decoration and other applications.

Advantages of LED lamps: ECOLOGICAL ASPECTS

a) No mercury present
b) No emission of ultraviolet ligh
c) Their low power consumption makes lower CO2 emissions
d) Their long duration implies lower expenditure of raw materials and lower waste production
e) Fully recyclables at the end of their useful life

   1 Incandescence bulbs have to completely disappear after a few years following EU regulations.