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A spectrometric transducer is a basic element of an optical measurement microsystem. This article explains the principle of operation of waveguide grating-based spectrometric transducers developed for telecommunication applications, their technological limits, and performance. These issues include optical loss (both total loss and the uniformity of loss as a function of channel number), crosstalk (both from adjacent or nearby channels and from those far away, called "background" in this article), polarization sensitivity (inherent in planar waveguide devices), and wavelength accuracy. The article also looks at commercialization issues such as long-term reliability, packaging, and cost. Even though the focus of this article is InP-based etched echelle grating demultiplexers, all of the lessons learned are independent both of the material system and the nature of the planar demultiplexer whether phasar or echelle grating.