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Chapter 6:
Radiometry
Abstract
Some years ago, a monochrome laser projection system was shown on the cover of one of the first “lasers and optics” magazines, illustrating the state-of-the-art in laser system R&D for future consumer products. More than 30 years later, this vision has started to come to fruition in the form of large-screen IMAX theaters using high-brightness semiconductor lasers for a three-color projection system that does not use large, inefficient arc lamps. These projectors are an ongoing area of product development, with Necsel IP, Inc. winning a 2014 SPIE Prism Award for its high-power (3 W) green lasers for use in cinematic laser-projection engines. With further miniaturization and cost reduction, these large-screen systems will evolve into smaller, home-based consumer products with a much larger market potential (Fig. 6.1). While high-brightness lasers emitting into a small divergence angle can easily be coupled to projection-system optics with small aperture and long focal length - i.e., large f/# - it is laser intensity (units of W/sr) that is a key requirement for the use of lasers for directed energy. In this case, the laser is a source not of light but of heat, but the effectiveness of the laser depends on its power and divergence angle. Radiometric concepts are thus critical to understanding the use of lasers for laser displays, directed energy, manufacturing, laser radar, fluorescence microscopy, and so on. As we will see this chapter, it is not the laser power by itself that determines performance, but the laser’s brightness (Section 6.1) and intensity (Section 6.3). As the power available is a key component of brightness and intensity, Section 6.2 reviews how to estimate the power collected by a telescope, camera, or microscope directly from a laser, including material, truncation, and obscuration losses. Laser brightness, intensity, and collected power are then combined in Section 6.4 to estimate radiometric performance using a systems engineering tool known as a power budget. An example power budget is illustrated with the range equation for laser radar and active imaging.
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CHAPTER 6
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