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LED vs Halogen

Halogen lamps used to be the conventional light emitters for many image processing illuminators. But now the LED, with its longer life, lower operating costs and the associated increased operations security is ready to take over.

In the past, energy costs, maintenance intervals and operating costs were only rarely brought into the equation when image processing systems were being evaluated – the focus was on whether the system could do the job. Today, however, operating costs are being given increasing weight.

The illumination has a significant influence on the efficiency of the system as a whole. For an industrial image processing system it is as important as the location for a building. This means that when you design a new light source the choice of emitter is equally vital to its efficiency. Here you need to focus not only on the equipment’s optical performance but also on its expected service life and efficiency.

In recent years, light sources have generally used halogen lamps as emitters. These typically have a colour temperature of about 3200 K and a service life of a few thousand hours, but this relatively short life and the correspondingly poor operations security are no longer adequate for modern applications.

There are a number of efficient alternatives to halogen lamps, such as long- arc gas discharge lamps (such as fluorescent tubes), short-arc discharge lamps (such as the metal halide lamps used for vehicle headlights) and LEDs, but discharge lamps can be used only to a limited extent because many image- processing applications call for a light flux that is stable over time. That leaves the LED as the optimal alternative.

The LED as optimal alternative

Among the advantages of LEDs are their markedly increased service life of up to 50,000 hours, their straightforward activation, their mechanical robustness, their compact dimensions, the extremely flexible design of illuminator models, their lower operating costs and an excellent price- performance ratio. These are convincing arguments for choosing LED emitters and explain why LEDs have already become established as the most frequently used emitters for ring lights and similar illuminator models.

In view of the advantages of LEDs over conventional halogen light sources, the Swiss illumination specialist Volpi set out to extend this pioneering technology to the field of fiber-optic illumination, which had previously been largely the preserve of so-called “cold light” sources on the basis of halogen or metal halide technology. In this type of light source the generated light is transmitted to its target location by means of a flexible optical waveguide made of glassfiber. This offers two considerable advantages over direct illumination: Because fiber-optic cables do not transmit any heat they do not stress the object being examined, and in addition they are often the only way to illuminate inaccessible places.

Volpi’s development team wanted to combine the advantages of LEDs with those of fiber-optic illumination, implement this as a product suitable for industrial use, and in this way develop a new type of LED light source. There was no problem about using fiber-optics as an optical waveguide – this functioned as perfectly as ever – but they needed to find a new LED emitter for their new-generation light source that was able to meet the market’s sophisticated requirements.

After intensive preliminary investigations their choice fell on multichip LEDs of type OSTAR manufactured by OSRAM, which offered the best combination of luminance, brilliance and service life. The emitters used are semiconductor chips of gallium nitrite (GaN), which emit blue light that passes through a conversion layer to yield white light. Unlike previously used methods, in which conversion dyes were simply blended into the encapsulating material, OSTAR LEDs make use of a new technology called ThinGaN, a highly efficient thin-film technology in which the conversion layer is printed directly onto the semiconductor chip. This results in improved colour accuracy and better homogeneity, a marked increase in efficiency and a significantly longer service life for the emitter. All the semiconductor chips are switched in series to ensure that the identical current passes through them all, producing an even brilliance over the whole surface.

The colour temperature of these white LED can be chosen by using an appropriately formulated conversion layer. One generally aims at a colour temperature of between 5000 and 6000 K, the range designated “cold white,” because humans perceive this as white light. In contrast, a number of manufacturers offer “warm white” LEDs whose colour temperature is in the range 3000 to 4000 K, which corresponds to the colour temperature of a halogen lamp.

Increased efficiency

It has recently been possible to effect a gratifying increase in the efficiency of white LEDs, i.e. the number of photons emitted relative to the energy consumption. A few years ago their efficiency was around 10 lm/W (lumens per watt); at the moment various manufacturers are supplying LEDs with an efficiency of 40 to 70 lm/W, and white LEDs with efficiencies as high as 70 to 90 lm/W have been announced for later this year (summer/autumn 2007). By comparison, the efficiency of a halogen lamp is around 30 lm/W and that of a discharge lamp about 90 lm/W.

Volpi’s developers implemented the advantages of OSTAR LEDs into their new LED light source, designated IntraLED 2020.

The very high efficiency of these LEDs makes it possible to generate white light with an intensity of over 3.5 Mlx for a power consumption of only 15 W. By comparison, a DC light source with a 150 W halogen lamp consumes ca. 180 to 200 W electrical power and outputs yellow light with an intensity of about 9 Mlx, but since the LED offers a better balance of red, green and blue components than a halogen light source the resulting difference in illumination amounts to a factor of about two. This means that you only need to open your camera’s aperture by one stop to be able to replace the halogen light source by an LED source.

The IntraLED 2020 can achieve a service life of over 30,000 hours, depending on its mode of operation. The intensity of illumination can be specified in 5% steps, either using buttons on the front of the device or digitally via an RS232 interface. Its compact heavy-duty housing means that the new LED light source can easily be integrated into an automation system.

The IntraLED 2020 is supplied for 12 VDC with a universal plug-type power unit (100 240 V 50/60 Hz). It produces no scattered light and is suitable for clean-room applications. Since this new type of illumination system gets by with considerably less power than conventional light sources the IntraLED 2020 is eminently suitable for use in air-conditioned environments. And since the price of this new system is well below that of the previously used halogen light sources it can pay for itself within a few months.

The first successfully realized applications

This newly developed LED technology is already being successfully used. In one such application Volpi has fitted an automated filter checking plant with an IntraLED 2020 that uses a light probe to illuminate the test subjects from within. This LED-based illuminator replaced a 150W halogen light source.

A further example is Volpi’s high-resolution surface checking system for spherical caps, which also makes use of an IntraLED 2020.

To sum up:

The energy and maintenance costs saved will very quickly recoup the investment of replacing halogen lamp illuminators by LED illuminators. The potential for replacing outdated halogen illuminators is probably in the tens of thousands of halogen lamps.