Page 29

EETE DEC 2015

Fig. 2: Iris recognition in a smartphone: an infrared LED illuminates the eye and the characteristic iris pattern is read out from the camera image. A bandpass filter above the detector provides a good signal-to-noise ratio. 2.4 mm with a radiation angle of 20°. The SFH 4780S delivers a typical 2900 mW/sr radiant intensity at 1A and thereby offers the currently best ratio of component size to performance. Figure 4 shows the brightness ranges achievable with the SFH 4780S at a distance of 40 cm. A region with about 8 cm width can be illuminated with irradiances between 1.31 and 1.83 mW/cm2. The choice of the bandpass filter above the camera sensor should also take place with a view to an efficient use of the available light. The filter has to let pass all light of the IRED if possible, but simultaneously has to ensure a good signalto noise ratio. The optimum filter width orients itself towards the centre wavelength and to the spectral emission width (half width) of the IRED. In addition, the temperature dependence of the wavelength, the half width and the optical performance has to be considered in order to ensure that the sensor also operates reliably under extreme conditions. In addition to the ambient temperature, the operating current and the pulse duration also enter these considerations. Figure 5 shows, as an example for the SFH 4780S a bandpass filter, which guarantees a sufficient illumination also for extreme conditions. Its permeable region is chosen with 780 nm to 850 nm in such a manner, that, during operation at 1 A and a pulse of 10 ms with ambient temperatures of -20°C and +65 °C, 75% of the optical performance is still transferred. Fig. 4: An IRED with suitable radiation characteristics directs if possible all available light to the eyes. The distribution of the irradiance is shown here, which is achieved with the SFH 4780S (20° beam angle) at a distance of 40 cm. The guidelines for eye safety have to be considered in any case when designing an iris scanner. The light performance which the eye receives over a certain period is significant for this. The system can be designed safe for eyes with a suitable combination of the pulse duration and the operating current. Indications for this can be found in the Osram application documents regarding eye safety and iris scanners. Fig. 5: The optical bandpass filter of the iris scanner has to be chosen in such a manner that sufficient light reaches the sensor even under extreme operational conditions. For the SFH 4780S, 75% of the light performance still reach the sensor through a filter which is permeable from 780 to 850 nm with an ambient temperature of -20 °C or +65 °C. Fig. 3: The Oslux SFH 4780S with a wavelength of 810 nm was developed especially for iris scanners. With its high radiant intensity of typically 2900 mW/sr at 1 A, it provides for reliable iris recognition in mobile devices for the first time. www.electronics-eetimes.com Electronic Engineering Times Europe December 2015 29


EETE DEC 2015
To see the actual publication please follow the link above