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Brightness alone is not everything

Infrared LEDs supply the required light for a high image quality in many camera-based sensor and surveillance systems. The latest generation of these emitters sets new standards with regard to optical output and efficiency. 3D sensors also benefit from this, for gesture recognition, for example.

Illumination using infrared light is a developing application sector for infrared LEDs (IREDs). One reason for this is in the growing use of camera-based systems ranging from video surveillance to night vision assistants in automobiles, to all types of sensors based on intelligent image processing. In addition to this, there are systems, which measure distance to register movement in all directions, for example, for gesture recognition. Almost all of these solutions illuminate the area that is being watched with infrared light; IRED with a wavelength of 850 nm has established itself as the most common light source. This spectrum is hardly even perceivable by humans but can still be detected very well within the sensitivity range of camera sensors. This approach has only become an economical possibility since suitable high power IREDs are available. A series of innovations has now resulted in dramatic brightness and efficiency improvements in these emitters. They form the basis for compact and efficient designs as well as for the development of new application variants.

Illumination using IRED

Closed Circuit television or CCTV is frequently used for safety purposes in public areas, railway stations an airports, in industrial complexes, banks or museums. Many solutions illuminate the area with infrared light, to obtain good images irrespective of the ambient light conditions. In most cases, the lighting units comprise of several IREDs and are integrated in the devices. Intelligent, camera-based sensors perform complex analysis of the images that are taken, for example, for reading license plates or for vision systems in industry. They frequently employ infrared light, to achieve the required quality for the images. Such solutions are also establishing themselves within automobiles, for instance as attention assistant. In this case, a camera with infrared illumination mounted behind the steering wheel takes an image of the driver’s face and detects whether he is becoming drowsy based on the eyelid blinking frequency. Another example is pre-crash sensors, which monitor the direct surroundings and analyse the images to predict possible accidents. All of these applications require the lighting units to be as compact as possible and therefore require small emitters with high optical output. Furthermore, the IRED must allow for high operating currents in continuous operation (DC). If large ranges are required, for example for the surveillance of large outdoor areas, the emitter must also provide a high radiant intensity – in other words, a lot of light per solid angle.

3D systems employ short light pulses to measure the distance between the camera and an object in order to be able to detect movements in all directions. We are familiar with gesture recognition with games or to operate computers. In future, this feature could also be integrated in cars to operate the cockpit functions. Industry is now even using 3D cameras for the intelligent detection of objects in production, as an example. Automobile safety systems that protect pedestrians by registering their movements, for example, are also based on this technology. These frequently employ so-called TOF (Time of Flight) or PMD (Photonic Mixing Device) cameras. One links these to a source of infrared light and measures the travelling time of the light pulse from the light source to the object and back to the detector for every pixel. Alternately, the infrared light is modulated at frequencies higher than 20 MHz and one measures the phase shift of the signal. Such solutions have only been possible since high power IREDs are available as fast switching light sources that can be modulated.

Infrared LED

High power LEDs and IREDs for illumination applications are based on efficient, large area chips and on packages with excellent heat dissipation. Osram developed thin film chip technology for this purpose. The company already introduced an 850 nm emitter with a 1 mm² sized chip in the Dragon package in 2008. Since then, the subject matter of the development work was mainly to improve brightness. The brightest, conventional 850 nm IRED, the Oslon Black SFH 4715A, delivers an optical output of around 800 mW at 1 A. With a typical efficiency of 48pc, it is currently the most efficient IRED at an operating current of 1 A.

With the SFH 4715A, the developers finally succeeded in increasing brightness by 30pc over its predecessor SFH 4715. Amongst others, various measures to continue to reduce the light losses within the chip were involved with this. On the package side, the successful Oslon Black design in use for some time by Osram for visible LEDs with an optical output of more than 500 mW was optimised even further. Oslon Black can outcouple about 15pc more light than the flat Dragon with its silicone lens specially adapted for the package. Another benefit is its low thermal resistance, typically of 6.5 KW, which simplifies the operation at DC current. With the SFH 4715A, the brightness has now been improved by a further 10pc by improvements to the lens material, amongst others.

Stack IRED exceeds the 1 watt mark

For applications requiring even more light from a tiny space, Osram developed nanostack technology. It implements two emission centres in one chip and produces about 70pc more optical output (Figure 4). In 2011, the Stack Oslon SFH 4715S exceeded the 1 watt mark. Now the technology has been adopted in the latest generation of thin film chips. The result is the recently introduced Oslon Black SFH 4715AS with 1370 mW of optical output at 1A operating current.

Conclusions

The record brightness levels in infrared LEDs for illumination solutions available for some time in the market are the result of various innovation jumps in the chip and package technology. Without the steep development of IRED with 850 nm wavelength, the high dynamics on the application side would not be possible, opening up many new applications for camera-based or distance measuring systems. OSRAM has continued to serve this sector with new, tailor-made, developments. This also includes expanding the technology to more wavelengths for special applications in addition to the continuous expansion of the 850 nm emitter portfolio.

Thomas Kippes, developer at OSRAM Opto Semiconductors, speaks on adapting the high performance IREDs to specific market requirements.

Mr Kippes, with the development of the SFH 4715A and SFH 4715AS, OSRAM sets milestones in terms of efficiency and brightness. You have also introduced other modifications for these new IRED, such as new lenses. Why?

If one considers the various applications for the high power IRED, one can see that even though optical output is very relevant, it is not the sole criterion. An important point is the range, for example, and that depends on how the light is spatially distributed. The more narrow the solid angle in which the optical output is emitted – known as the radiant intensity – the greater the range. So, as a result, we had to bundle the newly acquired optical output to enable our customers to achieve a long range, but also to illuminate the target area widthways. Therefore, we developed two 90° and 150° beam angle lenses for the Oslon Black. The 90 degree lens interacts particularly well with external lenses that shape the beam for the respective application. The 150° version is used to illuminate a large area in the vicinity, for example, for gesture recognition systems. Also, the 150° lens is ideal with narrow cones of light with reflector-based optics, to achieve high ranges.

Also the electrical chip contact has changed – that is also due to users?

Yes. We moved the electrical contact into the corner in the newest generation of 850 nm thin film chips. The design improves the current input over the surface, thereby optimising the chip for high DC currents. In addition, the light beam is no longer disturbed by the centrally located bond pad. This is very important for users imaging objects onto the camera chip. In the past, they had to elaborately eliminate the interference effects caused by the bond wire from the detector signal.

Were there any other changes?

An important point is the thermal resistance of the chip and package. The better the IRED dissipates heat from the chip, the longer it can run at a certain current – or increase the current at a fixed pulse. The heat dissipation also determines the lifetime of the component. Many customers use the IRED with continuous light, so this aspect is particularly important for them. After we initially focused our developments on increasing the brightness, the youngest generation of chips was optimised for high current handling and allows up to 3 A in pulsed mode. Our most recent measure was the introduction of silicon substrates for infrared emitters. OSRAM already uses this technology for LED chips and we are using it for the first time in our IRED with the new stack chip in the SFH 4715AS. In the Oslon Black, the thermal resistance therefore decreases by 1 K/W to a typical value of 5.5 K/W.


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