Photogrammetric color scanning (2002)

Scanning of analog image material remains a key function in the photogrammetric production chain as long as digital aerial cameras have not completely taken over. Therefore it remains an important goal to achieve optimum scanning results. This contribution reviews how the scanning procedures have evolved since 1990, compares photogrammetric and non-photogrammetric scanning applications and focuses on the scanning of color material. The implementation of such concepts is presented by several examples, including the need for color calibration. 1. FROM CENTRALIZED SERVICE TO DISTRIBUTED FUNCTION After an initiation in about 1990, photogrammetric scanning by now has become a routine and standard element of each digital photogrammetric workflow (Baltsavias & Kaiser, 1999; Leberl et al., 1992). Initially, the concept was to set up scanning service bureaus, which would centralize the scanning function for many photogrammetric mapping workshops. But since the photogrammetric equipment manufacturers were able to develop increasingly more affordable products the scanning function has become very distributed and has been brought into each photogrammetric operation. This improves the workflow and offers the photogrammetric production managers complete control over the end-to-end workflow for extraction of information from blocks of initially analog photography. The photogrammetric industry has come back to a situation that existed when analog and analytical technologies “ruled”, with a clear interface between the raw data creation by means of aerial photography specialists, and the information extraction by photogrammetric processing specialists. 2. PHOTOGRAMMETRIC VERSUS NONPHOTOGRAMMETRIC SCANNING However, bringing the scanning function into a photogrammetric mapping operation has led to a need to better understand the workings of scanners on a routine basis. “Document scanning” is a rather traditional function in industry with many different manifestations. There are the color graphics and printing industries with their need to change a color photograph into separate cyan-magenta-yellow-black channels for offset printing, producing the justification for the largest investments in scanners compared to other industries. Since about 1990, this has been changing over from a simple task of color separation into an input to digital image processing systems (see figure 1). There is medical scanning of X-ray films and of histological specimens. The manufacturing industry has for several decennia studied the need to convert paper drawings into CAD-documents. And the most recent developments have been inspired by the transition to the “paperless office”, leading to the need for low cost, low quality scanners in each office. Today, millions of scanners are being produced each year as peripherals to personal computers. Figure 1: Non-photogrammetric scanning examples. Left is a medical application (x-ray, monochrome, up to 4.0 OD) and to the right a graphic arts application with a wide dynamic range. In this context, photogrammetric scanning is a niche with very distinct requirements which are set apart from all other scanning applications, first and foremost by the need to offer a 1. high and well defined and verifiable geometric accuracy. In analogy to photogrammetric coordinate measuring equipment, this accuracy is typically seen in the range of ± 2 μm. Other specifications typical for photogrammetry exist as well, to include the need to work with 2. a comparatively large format at 25 cm x 25 cm (Fig. 2); 3. transparent film; 4. uncut and rather heavy rolls of film; 5. comparatively large data quantities, given 23 cm x 23 cm color imagery and small pixels; 6. high geometric resolution of up to 5000 dpi. Figure 2: An aerial image, 23 cm by 23 cm, results in a digital file of several hundred megabyte of data (e.g. 705 MByte @ 15 μm pixel size and 3 by 8 bit color depth) While some attempts have been made to convert graphic arts scanners into photogrammetric systems, these have generally not been successful, except where the application was for low quality ortho-photo production (Thorpe, 1992). Today it is generally understood that the needs of the photogrammetric application can only be met by specialized scanning systems. 3. NEED FOR HIGH RADIOMETRIC PERFORMANCE Converting analog film into digital information requires the ability to resolve the radiometric range of the film (Kolbl, 1999; Neumann & Baltsavias, 2000). We know from densitometric measurements that b/w film material may cover a dynamic range from approximately 0.1 D up to 2.5 D, perhaps up to 3.0 D in special cases (Fig. 3). Investigating color dia-positive material one may need to coop with an even larger dynamic range, e.g. up to 3.5 D. Figure 3: B/W negative aerial frame with a large dynamic range of 3.1 to 0.3 D. Details like cracks in the asphalt are clearly visible in the digital image after radiometric enhancement of the image detail. 4. THE NEED FOR COLOR When photogrammetry was based on analog measurement devices, color was a factor of extra costs when copies had to be made for work and finally color was expensive and critical in the analog production line. This has no longer any meaning in the digital photogrammetric workflow. Therefore the color information of photogrammetric products is highly appreciated. Scanning color images has become an every day job and photogrammetric scanners need to support the operator through a smart user interface and to show the technical ability to transfer color information from the analog film to the digital file. 5. DIGITAL WORKFLOW FAVORS USE OF COLOR