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Optimizing Radiometric Fidelity to Enhance Aerial Image Change Detection Utilizing Digital Single Lens Reflex (DSLR) Cameras Andrew D. Kerr and Douglas A. Stow Abstract Our objectives are to analyze the radiometric characteris- benefits of replicating solar ephemeris (Coulter et al., 2012, tics and best practices for maximizing radiometric fidel- Ahrends et al., 2008), specific shutter and aperture settings ity of digital single lens reflex (DSLR) cameras for aerial (Ahrends et al., 2008, Lebourgeois et al., 2008), using RAW image-based change detection. Control settings, exposure files (Deanet al., 2000, Coulter et al., 2012, Ahrends et al., values, white balance, light metering, ISO, and lens aper- 2008, Lebourgeois et al., 2008), vignetting abatement (Dean ture are evaluated for several bi-temporal imagery datasets. et al., 2000), and maintaining intra-frame white balance (WB) These variables are compared for their effects on dynamic consistency (Richardson et al., 2009, Levin et al., 2005). range, intra-frame brightness variation, acuity, temporal Through this study we seek to identify and determine how consistency, and detectability of simulated cracks. Test- to compensate and account for, the photometric aspects of im- ing was conducted from a terrestrial, rather than airborne age capture and postprocessing with DSLR cameras, to achieve platform, due to the large number of images collected, and high radiometric fidelity within and between digital multi- to minimize inter-image misregistration. Results point to temporal images. The overall goal is to minimize the effects of exposure biases in the range of −0.7 or −0.3EV (i.e., slightly these factors on the radiometric consistency of multi-temporal less than the auto-exposure selected levels) being preferable images, inter-frame brightness, and capture of images with for change detection and noise minimization, by achiev- high acuity and dynamic range. ing a balance between full dynamic range and high acu- The applications contexts for conducting this study are ity. DSLR cameras exhibit high radiometric fidelity and can detecting post-hazard damage and monitoring changes in effectively support low-cost aerial image-based change urban infrastructure. The technical context is Repeat Station detection, such as for post-hazard damage assessment.Delivered by IngentaImaging (RSI), where image capture over time occurs at nearly IP: 192.168.39.210 On: Fri, 24the Sep identical 2021 station10:11:27 points in the sky, and image registration Copyright: American Society for Photogrammetryand change and detection Remote are Sensingperformed on a frame-by-frame basis Introduction (Coulter et al., 2003; Stow et al., 2003). Due to the dynamic Consumer-oriented digital single lens reflex (DSLR) cameras of nature of urban scenes, a challenge is to minimize noise ever increasing spatial resolution and quality are an economi- sources due to variations in illumination characteristics and cal and readily available sensor option for airborne remote scene conditions and features, sensor variability, and appar- sensing. Increasing coverage per image frame and higher im- ent image motion (AIM) motion blur, through the selection of age fidelity, along with low barriers to entry (i.e., affordability an appropriate shutter speed. The goal is to automate image and ease of use) inherent to DSLR cameras, make them viable processing and analysis as much as possible, but with the ex- for many remote sensing applications. Consumer DSLR cam- pectation that a human analyst will make the final analysis of eras have features and incorporate processes that differ from whether damage or other land surface changes have occurred. traditional aerial cameras, as they are designed with photom- This study was conducted in such a way that the charac- etry in mind, rather than high radiometric fidelity associated teristics of the lens used, such as the specific or various focal with many aerial imaging sensors. length(s) and relative aperture(s), had minimal bearing on the Photometry seeks to measure light, as closely as possible replicability and adaptability to other users, can be performed to how it is perceived by the human eye, whereas radiometry without specialized equipment, and can be tailored to specific seeks to normalize or even measure the absolute spectral radi- collection parameters. The study design also should allow for ance. Having photometric accuracy as the primary concern of future camera models to be tested, provided that they have a camera manufactures, has led to the development of several variable aperture lens, bayer array, and image sensor similar photometric materials and processes, including specific lens to a CMOS sensor. We captured high quality images with vary- or sensor spectral coatings, and onboard image processing ing exposure parameters using consumer grade DSLR cameras, steps, to achieve greater photometric accuracy, sometimes at and the resultant image sets were utilized to empirically the expense of radiometric linearity or greater radiometric ac- address the following research questions, in the context of curacy (Lebourgeois et al., 2008). RSI-based change detection. A dearth of research articles exist that explore remote 1. With what combination of exposure settings can the sensing based on DSLR digital cameras (Clemens, 2015). These dynamic range of image brightness values be maximized, studies have tended to focus on vegetation remote sensing while achieving high image acuity? (Dean et al., 2000, Ahrends et al., 2008, Lebourgeois et al., 2008, Richardson et al., 2009), change detection for wide area Photogrammetric Engineering & Remote Sensing aerial surveillance (Coulter and Stow, 2008), and generation Vol. 84, No. 3, March 2018, pp. 149–158. of color indices for soil identification (Levinet al., 2005). 0099-1112/17/149–158 In investigating these topics, past studies have outlined the © 2018 American Society for Photogrammetry Storm Hall 307B, Department of Geography, San Diego State and Remote Sensing University, San Diego, CA 92182-4493 ([email protected]). doi: 10.14358/PERS.84.3.149 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING March 2018 149 03-18 March PR Print.indd 149 2/20/2018 4:35:47 PM 2. What is the characteristic spatial trend in brightness atmospheric path radiance, and transmittance conditions. response within image frames, how do these trends vary The sites chosen for these image collections (Figures 1 and with different exposure parameters, and how well can 2) have urban scene composition, and an image capture within image trends be balanced or normalized? azimuth nearly parallel to the solar azimuth, so as to create 3. How can between image differences in radiometric bright- illumination and shadow conditions similar to those for aerial ness response due to noise be minimized and due to signal imaging. Figure 3 is the camera and tripod set-up for station- be maximized for multi-temporal RSI pairs, by proper ary oblique imaging. Additionally, images were collected on selection of exposure parameters? a clear and a hazy day, so the dataset contains two different atmospheric conditions. We find no previous studies in the literature that evaluate the The experimental variables that pertain to the research radiometric characteristics of consumer grade DSLR cameras questions in this study are image exposure value and light me- for capturing aerial imagery in support of change detection ter measurement zones for automatically estimating the appro- applications, nor for determining best practices for optimal priate exposure level for particular ambient light magnitudes, radiometric fidelity through optimization of camera con- aperture size, and shutter speed for controlling the amount trol parameters. In addition, this study is novel in both its of light reaching the detector, ISO (a carry over acronym from ground-based image capture procedures that emulate airborne film speed metrics of the International Standards Operation) imaging, including simulation of vertical path radiance in the for controlling the sensitivity of the detector and white bal- horizontal plane, and near replication of solar geometry, and ance for inter-band radiometric consistency. Specific variables its damage change detection metrics, which were developed pertain to each of the research questions, as not all of the vari- for measuring change detection using a type of signal to noise ables are relevant for addressing each research question. metric, and two quantitative change detection evaluation The first research question pertains to maximizing dynam- methods based on pixel transects/profiles. ic range of a given image frame, while achieving high image acuity, for which the image exposure value (EV), light meter measurement zones, relative aperture, shutter speed, and ISO Methods are the five relevant variables. The frame-specific EV is a major determinant of dynamic range, as it sets the camera exposure Experimental Variables and Image Capture Strategy controls to yield an image that is not over- or under-exposed, The methods chosen for this study were designed for col- thus ensuring a maximized dynamic range in the resulting lection of imagery from a terrestrial, rather than an airborne image. The light meter measurement zone configuration may platform. The rationale is that for each image collection of 86 have an impact on a maximized dynamic range as well, as unique camera exposure setting combinations, the capture the “overall” and “center weighted” settings for determin- location
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