Explosion protected indicator lights have used LEDs as a light source since the mid-1970s in hazardous process areas.
Because diodes available at the time had a luminous intensity of just 30 to 50 millicandelas (mcd), it was necessary to use clusters of LEDs in order to achieve the equivalent brightness of conventional flameproof indicator lights.
For example, a D30 (30mm) lamp required seven LEDs arranged in the form of a wreath in order to replace a four or five Watt incandescent or glow lamp. At the time LEDs were almost exclusively monochromatic and could be produced only in the colours red, yellow and green.
Despite this restriction, the LED solution had one crucial advantage. The service life of these diodes is up to 50000 hours, compared with the 1000–2000 hours of an incandescent lamp. In hazardous environments, the circuit generally needs to be disconnected in order to change the lamp. This problem can be alleviated either by using an expensive special lamp holder, or by shutting down part of the system for maintenance, so extending the life of the indicator is a very tangible benefit (Fig.1).
Today, light is available from three main sources: radiation emission of a solid in thermal equilibrium (thermal radiation of the incandescent bulb); collision excitation of atoms, ions and molecules in the gas phase (radiation spectrum of discharge lamps); and recombination of positive and negative charge carriers in solids (light-emitting diodes).
LEDs number among electroluminescence radiant light sources. As on a solar cell, the basis of an LED is a semiconductor crystal. In simple terms, the LED is the reverse of the solar cell. On the latter, light is converted to electrical current and, on the LED, the current flowing through generates light. The semiconductor consists of two layers, one of which is doped with impurity atoms in such a manner as to produce an excess of electrons. Additional atoms in the other layer produce an excess of positively charged holes. If we connect a voltage source to this ‘pn’ junction in forward direction, the electrons migrate towards the positive pole and collide with the positive holes at the pn junction. The charge carriers recombine and release electromagnetic radiation during this process. The wavelength of this radiation depends on the energy gap between conduction band and valence band (Fig.2).
As early as 1907, British engineer HJ Round observed light phenomena while investigating the detection of radio waves with crystal detectors at Marconi. Since the crystal did not heat up, the light in question was termed ‘cold light’. However, this effect was not further investigated for many decades. In 1962, General Electric offered first commercial red Ga-As-P (Gallium Arsenide Phosphorous) LED with modest light output.
The efficiency of diodes was constantly improved into the 1990s, reaching that of modern fluorescent lamps. However, the radiation spectrum was still restricted using the available semiconductor materials Al Ga As or Al In Ga P to the colours red, yellow and green. The small band gap (energy gap) of these meant these materials were unable to produce the high-energy light emission required for blue or ultraviolet radiation. This is a drawback for signalling in an electrical installation. The new colour coding of indicating lamps defined in 1992 (EN60204,Part1,1992) gave the signal colours blue and white new significance, so that was no longer possible to dispense with these indicating colours. While white light can be simulated using yellow-green diodes with a white-tinted diffuser lens, this technique still does not achieve the blue colour.
It was only in 1994 that Shuji Nakamura, in Japan, managed to develop an effective LED which emitted luminous blue light using a Ga N combination on a sapphire substrate.
The development of a blue LED with adequate intensity made it possible to produce white light in the same may. It is still the case that the individual LEDs emit light in a narrow spectral range, but combining blue, green and red leads to white light, if the colour components are weighted correctly. Unfortunately, even if correctly balanced during production, additive colour mixing has its problems. Each colour LED ages differently, so the original spectrum shifts during this process. This effect can be counteracted with a sophisticated electronic control system but the costs of such a solution far outweigh the benefits. The solution is light conversion, a monochromatic LED with an ultraviolet or deep blue spectrum is enriched with a luminescent material that absorbs this light and then radiates a broad spectrum in the visible band.
The new generation of 8010 and 8013 indicator lights from R STAHL are designed with flameproof enclosures, type ‘d’ and Increased safety ‘e’ for use in zone 1 and zone 2 hazardous areas, (an Exe enclosure is shown in Fig.3). These indicator lights are based on white light-emitting diodes.
A special power circuit was designed to ensure simple integration into a variety of applications, coping with a broad range of power requirements. Voltage ranges of 2–270V for dc and ac voltage can be used. The circuit was designed for low power loss, ensuring minimal little heat dissipation and thereby increasing the service life of the indicator lights up to a maximum of 100000 hours.
The light direction is controlled using a lens, this means the signalling of indicator lights can be clearly seen by the operator, even under difficult lighting conditions. The signal colours red, yellow, green, blue and white are achieved with coloured filters. R STAHL incorporates suppression to prevent miss-signalling due to electromagnetic interference (EMI).
The white LEDs now available have a light output of around 20lm/W (Lumens per Watt) and a power consumption of about 70mW. This corresponds to a luminous flux of 1.4lm, meaning that application in the industrial sector is restricted to indication and signalling.
However, if the light output could be boosted to between 30 and 50 lumen, similar designs can be used for lighting, for example in a flashlight. Recently developed white LEDs achieve a light output of 30lm/W owing to improvement of deposition technology increasing the quantum efficiency. The power handling can be increased by increasing the chip surface area and dissipating heat more effectively, thus achieving an order of magnitude of a few Watts.
Dr Thorsten Arnhold is Director, Marketing and Product Manager with R STAHL Schaltgeräte GmbH.