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Chapter 7:
Instrumentation and Measurement Issues

It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong. - Richard P. Feynman

This chapter reviews the methods and equipment used to take scatter measurements. As pointed out in Section 1.5, the BSDF is defined in differential form but is measured with the incremental limitations imposed by real instrumentation. The finite detector aperture, scatter created by the instrument, calibration inaccuracies, and other practical equipment limitations, such as noise, detector nonlinearity, and mechanical errors, all produce noticeable deviations between the true BSDF and the measured BSDF. In order to generate meaningful scatter specifications and fully utilize the data, it is important to understand the source and magnitude of these deviations. System calibration, an often-discussed issue, is described in detail. The chapter progresses from these basic concepts to discussions of other measurement techniques (curved samples, area raster scans, retroscatter, TIS, and out-of-plane scatter) and concludes with a section on error analysis. Chapter 1 provides necessary background information for understanding this material.

7.1 Scatterometer Components

The simple plane-of-incidence scatterometer outlined in Fig. 7.1 contains most of the components typically found in more-sophisticated systems. These are easily grouped into four categories: light source, sample mount, receiver (detection system), and computer/electronics package. A fifth important element is the controlling software package. This section outlines the need for and general operation of these modules.

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