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SFH3605 Datasheet(PDF) 6 Page - OSRAM GmbH |
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SFH3605 Datasheet(HTML) 6 Page - OSRAM GmbH |
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6 / 14 page  August 13, 2010 page 6 of 14 devices, whereas the latter uses narrow- angle IREDs, necessary to achieve total internal reflection. It is worth to mention that the emitting light guides radiation characteristics depend on the design of the outcoupling structures and the emitter’s radiation characteristics in combination with the coupling arrangement. To support design activities OSRAM provides raytrace models, available at the OSRAM website. For narrow-angle applications, the e.g. MIDLED® SFH46XX series provides a viable solution, whereas the TOPLED® SFH42XX without lens family is an excellent choice for standard wide-angle requirements. Both feature slim packages and a flat-top to minimize the air-gap between the fiber and the IRED. 3.4 Projector-based with Diffuse Illumination In projector-based touchscreens with IR illumination from the backside it is desirable to achieve a diffuse and homogenous illumination. Suitable high power products are the DRAGON IRED series (SFH423X). In large projector applications it is recommended to split the screen into subscreens and use several IRED-arrays for illumination purpose. Alternatively high power IRED-based modules with homogenous and diffuse fields are recommended, like the OSRAM OSTAR® Observation product family (SFH47XX). 3.5 Projector-based with FTIR The FTIR principle features different aspects. The mathematics behind is based on Snell’s law: n 1·sin( ε 1) = n2·sin( ε 2), with ε as the angle between the surface normal and the light path and n as the refractive index of the material (see Fig. 6 for an illustration). Using Snell’s law, the boundary condition for total internal reflection at the glass – air interface is around ε c = 42° (assuming a refractive index of around n 2 = 1.49 for acrylic glass). To achieve efficient FTIR a high number of total internal reflections per unit length inside the acrylic glass are desirable. Direct coupling (also called butt coupling, like depicted on the left side in Fig. 5), employs IREDs with a wide half-angle to achieve this target. However, to increase the level of internal reflections (increasing the light density to achieve a more efficient FTIR) it might be preferable to couple light into the acrylic glass under angled conditions. This can be achieved efficiently by either tilting the standard wide-angle emitter by e.g. 45° or by an inclined glass edge (see also Fig. 5, coupling from the right side). A suitable arrangement is e.g. cutting the glass edge up to around γ g ≈ 35°. To ensure a maximum of total internal reflections simultaneously with a high power density, components with a high radiant intensity and narrow half-angle are recommended. Under above conditions, emitters with a half-angle of up to around 15° are preferable. Compared to wide-angle emitters, narrow- angle IREDs might require a tighter component spacing to avoid ‘dark’ spots close to the coupling location. To ensure a high coupling efficiency and minimal Fresnel-losses (which typ. add up to at least 2 x 4 % at the emitter – air-gap – glass interfaces) a plain cut glass surface is n1=1.0 n2=1.5 1 = c 1 2 IRED 2 = 90° total internal reflection refraction n2=1.5 n1=1.0 total internal reflection ε ε ε ε ε Fig. 6: Definition of Snell’s law and critical angle ε c for total internal reflection. The left schematic also illustrates that under certain conditions all light coupled into the light guide is subject to total internal reflection. |
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