www.osram-os.com Application Note No. AN097 IR OSLUX® — SFH 4780S in iris recognition applications Application Note Valid for: OSLUX® SFH 4780S Abstract This application note describes the use of the OSLUX® SFH 4780S in iris recognition (iris scanning) applications. In the first part of this application note the basics of iris recognition are briefly discussed. The second part deals with the use of the SFH 4780S, a component especially designed for iris scanning applications.
www.osram-os.com Table of contents A. Iris recognition system ............................................................................................2 The hardware / optical system ...........................................................................4 Iris recognition software .....................................................................................5 B. SFH 4780S ..............................................................................................................
www.osram-os.com In general, an iris recognition system consists of only a few components (see Figure 1): • Illumination module, containing the SFH 4780S • Camera (including the image sensor and the lens) • Optical bandpass filter in front of the camera to increase the signal-to-noise ratio in high brightness environments • Software algorithm including pattern extraction As everything can be mounted behind a glass cover it results in a very rugged system with a small footprint (compared to e.g.
www.osram-os.com Figure 2: Two options to arrange an 8° tilt of the SFH 4780S illumination module ~8° Cover glass ~8° SFH 4780S ~8° Light bending structure (i.e. prism type) SFH 4780S typ. 8° wedge to skip pcb Standard Option 1 Option 2 The hardware / optical system As empiric data suggests there are various suitable wavelengths for an iris scan system, depending on eye color. In general, two wavelengths look suitable, depending on the color of the human iris.
www.osram-os.com To simplify and optimize the overall system a single high-power 810 nm illumination module is best suited for all possible eye colors to unveil the rich structures of the human iris. It also allows the use of narrow optical bandpass filters in the camera module to maximize the signal-to-noise ratio of the iris scan pattern in high-brightness environments (e.g. sunlight).
www.osram-os.com Wavelength The SFH 4780S features a typical centroid wavelength of 810 nm (see also Figure 4), to match best the requirements for iris illumination for all eye colors. Figure 4: Definition of wavelength parameters for the SFH 4780S. λpeak ≠ λcentroid (due to the asymmetric spectrum) Opeak Rel. spectral emission 1.0 Ocentroid 0.8 0.6 FWHM 0.4 0.2 0.
www.osram-os.com temperature dependent wavelength drift (typ. 0.25 nm/K). Figure 5 and 6 present the typical wavelength behavior (λcentroid and FWHM) vs. ambient temperature and drive current. Using short pulses minimizes temperature dependent wavelength shift as well as spectral broadening due to internal heating of the LED (e.g. pulse width < 300 μs and repetition rate > 2 ms). Figure 5: Dependence of the centroid wavelength on ambient temperature and drive current.
www.osram-os.com Percentage of total power [%] Figure 7: Percentage of optical power within a given 100 nm wavelength range. This is for a standard SFH 4780S with centroid wavelength of 810 nm and FWHM of 30 nm. Thermal effects are not considered 100 80 60 40 20 0 650-750 700-800 750-850 800-900 850-950 Wavelength range [nm] As the data in Figure 7 suggests, the SFH 4780S is an excellent single emitter fit for the NIST mobileID “best practices report” (SP 500 — 280) recommendation.
www.osram-os.com 1.0 100 0.8 80 0.6 60 0.4 40 0.2 20 0 -40 -30 0 10 20 -20 -10 Wavelength range [nm] 0 30 Cumulated total optical power [%] Percentage of total power [%] Figure 8: Spectrum of the SFH 4780S (black line) with FWHM = 30 nm, centered around λcentroid. Total optical power accumulated from low wavelength up to the actual filter cut-off wavelength is indicated by the dark blue line (ideal shortpass filter).
www.osram-os.com Figure 10: Optical bandpass filter for SFH 4780S with operating conditions of 1 A/ 10 ms. The design ensures that 75 % of total optical power is transmitted at the ambient temperature extremes of -20 °C and 65 °C. Note that good connection to a heat sink (low Rth) is mandatory as Tj has direct influence on the passband (upper cutoff wavelength). No guardband/ angle of incidence effect considered Norm. filter transmission 1.0 0.8 0.6 0.4 0.
www.osram-os.com At 65 ° C ambient temperature the optical power is reduced by ~ 12 %, thus the overall required transmission at this temperature to still match 75 % overall needs to be around 87 %. By using of Figure 8 and Figure 9 (FWHM ~ 35 nm at Ta = 65 °C) the upper cut-off wavelength can be estimated to be: • λcut_off_upper = 810 nm + 13 nm (centroid distribution) + 0.
www.osram-os.com might feature a direct connection between the PCB where the SFH 4780S is mounted on and the metal frame of the mobile housing or other metal structure to support the heat transfer to the ambient. This is especially important as otherwise an increased junction temperature might require to limit the maximum drive current at higher ambient temperatures. In addition an increased junction temperature automatically leads to a reduced optical output power vs. time (at a given drive current).
www.osram-os.com Irradiance correction factor k Figure 12: Irridance correction coefficient k vs. viewing distance for the SFH 4780S 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0 2 4 6 8 10 12 14 12 18 20 Distance [cm] Eye safety For the SFH 4780S used for iris recognition the IEC-62471 standard is relevant.
www.osram-os.com With D as the duty cycle, tpulse as the pulse on time and tperiod the repetition time (tperiod,max = 250 ms). To consider the nearest distance the following extract from [4] might be helpful: “For analysis of the retinal exposure for small sources, such as a small diameter optical fiber, the closest distance at which the human eye can sharply focus is about 100 — 200 mm.
www.osram-os.com a • Example 1: SFH 4780S biased with 500 mA (DC) for 9 s, assuming Ie = 2.2 W/sr (estimated maximum Ie level at 500 mA). According to equation 5 / Figure 13 Ee,lim(cornea) = 3464 W/m². This level is reached at a distance from the SFH 4780S of around 2.5 cm (considering the correction factor k = 0.75, the actual Ee level at 2.5 cm is 25 % below this limit, resulting in an actual eye safe threshold of around 2.0 cm (not considering any optics / cover glass which cause additional losses).
www.osram-os.com Figure 14: Angular subtense index α versus viewing distance from SFH 4780S. Angular subtense D [rad] 0.10 0.08 0.06 0.04 0.02 SFH 4780S for all operating conditions / time ranges) 0.00 0 2 4 6 8 10 12 14 16 18 20 Distance from SFH 4780S [cm] In any way, the upper limit of α = 0.1 rad.
www.osram-os.com Figure 15: Exposure limit (LR) versus exposure time (retina) for different angular subtens (αmax = 0.1 rad, αmin according to IEC-62471) 106 Irradiance cornea Irradiance skin LR, LIR EL radiance [mW/mm²/sr] 105 104 103 102 10 1 10-5 10-4 10-3 10-2 10-1 1 10 102 102 102 Time [s] 2 --------------------------------------------6000 W m srL R lim 10 sec 13 • Example 1: SFH 4780S biased with 500 mA (DC) for t = 9 s, assuming Ie = 2.
www.osram-os.com might alter the overall radiation level in a way that it complies with the IEC-62471 standard. In case eye safety is a concern it is recommended to use a proximity sensor, e.g. SFH 7776 or directly the SFH 4780S in connection with e.g. SFH 7771 to ensure that the high-power operation of the IR-LED is disabled / reduced to reasonable levels in case “something” is close to the SFH 4780S.
www.osram-os.com Don't forget: LED Light for you is your place to be whenever you are looking for information or worldwide partners for your LED Lighting project. www.ledlightforyou.com ABOUT OSRAM OPTO SEMICONDUCTORS OSRAM, Munich, Germany is one of the two leading light manufacturers in the world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers its customers solutions based on semiconductor technology for lighting, sensor and visualization applications.
