Unified Optical Scanning Technology

Chapter 4.3.6 - Scanner Devices and Techniques: Passive Scanning for Remote Sensing

4.3.6 Passive Scanning for Remote Sensing

Figures 1.1 and 1.2 at the beginning of this work identify, respectively, active and passive scanning. The distinctions between the two appear in the nomenclature and in the reciprocal optical path directions of a conjugate imaging system. Ray directions propagate from left to right in Figure 1.1 and are reversed in Figure 1.2. On the left in Figure 1.1 appears the single reference point (source), which is analogous to the single photodetector on the left in Figure 1.2. Similarly, the scanned "moving focal point" (or "flying spot") on the right in Figure 1.1 corresponds to the "moving sampling point" on the right in Figure 1.2.

Concentrating now on remote sensing represented by Figure 1.2, the region on the right provides for scanning in object space. Similarly, on the left appears the opportunity for scanning in image space. Because the scanning of infrared radiation (a principal application) is conducted often by the articulation of mirrors or prisms, and multispectral scanning also adapts well to the use of reflective devices, almost all of the mechanisms discussed above are candidates for remote sensing. Related comments at the ends of Sections 1.5.1.1 and 1.5.1.2 are noteworthy.

Fundamental forms of one-dimensional (line) scanning are represented by Figures 1.8.1.9, and 1.10. The second dimension is often provided by quadrature motion (Fig. 3.9) of the sensing vehicle. In these illustrations, ray directions are to be reversed, so that former (active) scan lines become the sampled paths in object space. In Figures 1.8 and 1.9  (with reversed rays) the oscillating or rotating mirrors appear "beyond" the objective lens, in image space, now directing the changing beam angle to a fixed axis A that leads to a fixed detector (not shown). When remote sensing over great distances, the object-side flux arriving at the objective lens is essentially collimated, forming converging beams on the image side to focus on the detectors. In Figure 1.10  (reversing ray directions), the scan mirror now appears in object space, directing the sampled changing-angle beams to a fixed axis through the lens to radiate on a fixed detector (not shown). Here again, the range from a distant object to the scan mirror could be great, prop-

Fig. 4.9 Remote sensing image space scanner and recorder. Radiation from the scene is imaged by the focusing objective lens FOthrough the vertical and horizontal scanners SM1 and SM2 onto the detector. Simultaneously a raster image of the scene is formed by modulating a light source with the detected signal and projecting the beam through lenses L1 and L2 and adjacent portions of the same scanners. From [Bar].

agating essentially collimated light to the objective lens to forward the beam to a fixed detector.

An interesting remote sensing system is represented in Figure 4.9 [Bar]. It not only views and detects a sensed scene in two dimensions but with an added light source that is modulated with the detected signal, it doubles up on the scan mechanism to record a two-dimensional raster image, essentially in real time. The passive scanning portion (in the foreground) accepts the substantially collimated radiation from the scene over a changing field angle and conveys it through the focusing objective lens FO to the vertical deflector SM1 and then to the horizontal deflector SM2, to be directed to a fixed detector. This illustration is schematic and not to scale. Lens FO must provide a sufficiently large aperture to include the full field of descanned flux. If lens FO is placed closer to deflector SM1, which in turn is placed closer to deflector SM2, consistent with avoiding interference of the scanned beams, the field overage may be more practical. A good way to evaluate these moves is to consider the detector as a light source, reverse ray directions, and review the consequences of variation of the pupil relief distance, per Section 4.3.5 and subsequent discussion. The active scanning portion (in the background) illustrates some of this consideration by placing lenses L1 and L2 closer to the deflectors. Furthermore, lens L1 and L2 apertures that are large enough to accept the scanned angles are required.

 

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