tor Sensitivity The standard detection device for high speed Ir communications is the photodiode, which has a detection sensitivity, or minimum threshold irradiance E, expressed in uW/cm2. In standard power mode (see the Section "IrDA PHY Layer, p. 282"), the IrDA standard specifies a minimum emitter power of 40 mW/sr. From Eq. 5.3, the minimum power density at a receiving photodiode at a range of 1 metre will be 40 mW/m², or 4 uW/cm². The sensitivity of a photo diode detector depends on the incident angle of the infrared source relative to the detector axis in a similar manner to that shown in Figure 5-2 for the beam pattern of an LED. Photodiode sensitivity also depends on the incident infrared wavelength as shown for example in Figure 5-4, and in any application a detector will be chosen Power density P = I & R 1 4 2 1g/2R2 Solid angle S Area = SR2 Source Distance from source Relative distance from source Figure 5-3: Inverse Square Distance Relationship of Radiant Power Density Infrared Communication Basics Wavelength (um) Figure 5-4: Typical Photodiode Sensitivity vs. Wavelength with a peak spectral sensitivity close to the wavelength of the emitting device. Ir Link Distance The maximum link distance for an Ir link can be calculated as the distance R at which the equivalent power density (P) drops to the level of the detector's minimum threshold irradiance (Ee). Eq. 5.3 gives: E = I / R2 mW/m² (5.4) The effective range of an Ir link can be increased substantially, up to several tens of metres, using lenses to collimate the transmitted beam and focus the beam onto the receiving photodiode. Alignment of the lenses and of the transmitting and receiving diodes will be critical to the effectiveness of such a system. As shown in Figure 5-5, a misalignment of approximately 1/3° would be sufficient to break a focussed link over a range of 10 m. Ir areal coverage may be increased in a home or small office environment by reflecting the Ir beam from a wall or ceiling in order to access a number of devices. In order to p