High Dynamic Range Photography Research Articles

MetaHDR: single shot high-dynamic range imaging and sensing using a multifunctional metasurface.

We present MetaHDR, which is a single-shot high-dynamic range (HDR) imaging and sensing system using a multifunctional metasurface. The metasurface is capable of splitting an incident beam into multiple focusing beams with different amounts of power, simultaneously forming multiple low dynamic range (LDR) images with distinct irradiance on a photosensor. Then, the LDR images are jointly processed using a gradient-based HDR fusion algorithm, which is shown to be effective in attenuating the residual light artifacts incurred by the metasurface and the lens flare. MetaHDR achieves single-shot HDR photography and videography that increases the dynamic range by at least 50 dB compared to the original dynamic range of the photosensor. It can also perform single-shot HDR sensing, including reflectance calibration and surface curvature estimation of reflective materials. MetaHDR's demonstrated functionalities could be broadly applied in surveillance and security, microscopic imaging, advanced manufacturing, etc.

Evaluation of sky spectra and sky models in daylighting simulations

Sky models in daylight simulations represent the luminance variation across the sky-dome for different locations, dates, times and weather conditions, but skies are typically modelled as colourless. Recent studies explore techniques for incorporating the spectral content of daylighting in simulations. This paper provides an evaluation of the existing spectral sky models in lighting simulation software. The comparisons are made between the available mathematical sky models and naturally occurring skies that were recorded using high dynamic range photography and spectrophotometric measurements. The results show that recently developed sky models present progress compared to colourless sky models, but further research is needed to accurately simulate daylight spectra.

CALIBRATION OF A LIGHT HEMISPHERICAL RADIANCE FIELD IMAGING SYSTEM

Abstract. A light hemispherical radiance field imaging system based on fish-eye camera was developed for the measurement of the surface incident radiance in an urban environment, which is often affected by radiometric heterogeneity problems. A linear radiometric model and a polynomial fish-eye lens model are used. A temperature-dependent dark level model is proposed to improve the dark correction for high dynamic range photography. This paper describes the calibration procedure for spectral and geometrical radiance field measurements and presents the results of the calibration. The spectral radiometric calibration error is 2.07%, 1.34%, and 0.98% for blue, green, and red bands, respectively. The mean geometrical calibration error is 2.037 pixels.

The Modulo Radon Transform: Theory, Algorithms, and Applications

Recently, experiments have been reported where researchers were able to perform high dynamic range (HDR) tomography in a heuristic fashion, by fusing multiple tomographic projections. This approach to HDR tomography has been inspired by HDR photography and inherits the same disadvantages. Taking a computational imaging approach to the HDR tomography problem, we here suggest a new model based on the Modulo Radon Transform (MRT), which we rigorously introduce and analyze. By harnessing a joint design between hardware and algorithms, we present a single-shot HDR tomography approach, which to our knowledge, is the only approach that is backed by mathematical guarantees. On the hardware front, instead of recording the Radon Transform projections that my potentially saturate, we propose to measure modulo values of the same. This ensures that the HDR measurements are folded into a lower dynamic range. On the algorithmic front, our recovery algorithms reconstruct the HDR images from folded measurements. Beyond mathematical aspects such as injectivity and inversion of the MRT for different scenarios including band-limited and approximately compactly supported images, we also provide a first proof-of-concept demonstration. To do so, we implement MRT by experimentally folding tomographic measurements available as an open source data set using our custom designed modulo hardware. Our reconstruction clearly shows the advantages of our approach for experimental data. In this way, our MRT based solution paves a path for HDR acquisition in a number of related imaging problems.

Luminance parameter thresholds for user visual comfort under daylight conditions from subjective responses and physiological measurements in a gymnasium

In a gymnasium in Harbin, China, a survey was performed under different lighting conditions to determine the factors influencing the user assessment of daylight environments and define visual comfort thresholds for mass sports activities. Nine subjects experienced the simulated visual environment of basketball shooting by observing the backboard from a designated position, and their visual comfort preferences were evaluated through subjective questionnaires and objective physiological measurements. A total of 108 diverse visual environments were recorded using high dynamic range photography to acquire luminance. The questionnaire factor analysis suggested that the user assessments were mainly determined by their emotional state, glare perception, and visual clarity. The physiological baseline state threshold was proposed based on the physiological measurement data; it described the daylight luminance of the subjects when their physiological value under only natural lighting reached their physiological value under electric lighting. The analysis of the luminance and questionnaire data defined user visual comfort thresholds under only natural lighting: an acceptable average luminance threshold of 42 cd/m2 and a comfortable average luminance threshold of 80 cd/m2 in the entire field of view (FOV); an acceptable maximum luminance threshold of 4800 cd/m2 and a comfortable maximum luminance threshold of 6700 cd/m2 in the entire FOV; a recommended average luminance threshold of 100 cd/m2 in the target area; an acceptable luminance ratio threshold of 42 and a comfort luminance ratio threshold of 21 between the average luminance in window area and the entire FOV.

Tutorial: Luminance Maps for Daylighting Studies from High Dynamic Range Photography

ABSTRACT In the field of lighting, luminance maps are often used to evaluate point-in-time lighting scenes from the occupant’s vantage point. High Dynamic Range (HDR) photography can be used to generate such luminance maps. The aim of this tutorial is to present a comprehensive overview of a step-by-step procedure to generate a 180° luminance map of a daylit scene from a sequence of multiple exposures with semiprofessional equipment and the Radiance suite of programs. The procedure consists in capturing a sequence of multiple exposures of the visual scene; selecting the useful exposures; merging the exposures to generate the HDR image by using the predefined camera response function; nullifying the exposure value; resizing and cropping the HDR image by using the predefined fisheye view coordinates; adjusting the projection of the HDR image by using the predefined distortion function; correcting the vignetting of the HDR image by using the predefined vignetting curves; correcting the alterations of the HDR image due to the Neutral Density (ND) filter if one was used, by using the predefined ND correction function; adjusting the photometry of the HDR image by using the measured spot luminance value; editing the HDR image header by using the predefined projection type and real viewing angle; and checking the validity of the HDR image by using the measured vertical illuminance, and, if needed, the predefined luminous range. To conclude, an analysis of errors is made and attention points to adapt the procedure for electric or circadian lighting studies are discussed.

Impact on visual comfort and well-being: Integration of wood coating in the space

Daylight and lighting comfort may improve indoor environments. While white coatings have the capability to better distribute natural light, the use of wood could provide additional features. According to the biophilia theory, the integration of wood has positive psychological impacts on human beings and changes the appreciation of indoor spaces. The main objective of this research is to evaluate the relationship between the luminous ambiance and the degree of visual satisfaction, considering three spatial configurations, varying wood types and proportions. The relationship between a quantitative and a qualitative analysis with scale models (1:10 – 1:20), high dynamic range photographs, CIELab system and surveys, reveal that the use of wood triggers favourable responses compared with rooms that use white coatings only.

Neuroanatomical aspects of the temporo–parieto–occipital junction and new surgical strategy to preserve the associated tracts in junctional lesion surgery: fiber separation technique

To describe the location of each white matter pathways around the temporo-parieto-occipital junction (TPOJ) and the dissections performed using focal fiber dissection and fiber separation techniques (FST). These areas play an important role in human language processing and high-level brain functions. Thus, it is important to identify the association of the fascicles as well as their courses to plan safe and effective surgical target vectors. Thirty formalin-fixed human hemispheres and two formalin-fixed human heads were dissected with focal fiber dissection and FST under 6-40x magnification using a surgical microscope and real-time magnetic resonance imaging navigation system. Two- and three-dimensional anatomical and surgical pictures were obtained and processed using high dynamic range photography (Photomatix) and were correlated to radiological images of the case with glioblastoma of the TPOJ. The trajectory and connectivity of these fibers as well as their surgical importance in performing FST in cadaver dissections were demonstrated with correlation of the surgical, radiological, and anatomical images. The microanatomy of the TPOJ and related structures, emphasizing the position of each white matter pathway, has been described. Understanding the composition of each region is critical in preventing intraoperative brain injury that could lead to functional deficits.

Luminance and vertical eye illuminance thresholds for occupants’ visual comfort in daylit office environments

This paper investigates field measured visual parameters inside a daylit office space to define occupants' visual comfort thresholds, particularly for a view direction parallel to windows. Twelve human subjects performed a computer-based reading and typing task and provided subjective responses on various luminous environments, which were created by adjusting interior venetian blinds and electric lighting fixtures. A total of 120 different lighting conditions were tested and documented for both quantitative and qualitative analysis. High dynamic range photography was conducted to capture the luminance distributions of interior surfaces in addition to horizontal and vertical illuminance measurements by sensors. Statistical analysis of the collected data defined occupants’ visual comfort thresholds: a luminance of 2420 cd/m2 and luminance contrast ratio of 11.7. These new thresholds lie perfectly within the existing visual discomfort threshold ranges and confirm that a higher glare source luminance is tolerant in a view direction parallel to windows compared to view facing windows. A vertical eye illuminance threshold was also defined, but there are large discrepancies among the existing and newly defined thresholds. The collected subjective survey data also revealed 53%–60% evaluation accuracy rates of the existing glare indices. Daylight Glare Index and Unified Glare Rating metrics show the highest evaluation accuracy when daylight is the only light source. The CIE Glare Index metric shows the highest accuracy when both natural and electric lighting are introduced. Full understanding of visual comfort parameters and validation of the existing glare indices would help to achieve successful daylight harvesting in buildings.

Measuring circadian lighting through high dynamic range photography

The human ocular system functions in a dual manner. While the most well-known function is to facilitate vision, a growing body of research demonstrates its role in resetting the internal body clock to synchronize with the 24-hour daily cycle. Most research on circadian rhythms is performed in controlled laboratory environments. Little is known about the variability of circadian light within the built and natural environments. Currently, very few specialized devices measure the circadian light, and they are not accessible to many researchers and practitioners. In this paper, tristimulus colour calibration procedures for high dynamic range photography are developed to measure circadian lighting. Camera colour accuracy is evaluated through CIE trichromatic (XYZ) measurements; and the results demonstrate a strong linear relationship between the camera recordings and a scientific-grade colorimeter. Therefore, it is possible to correct for the colour aberrations and use high dynamic range photographs to measure both photopic and circadian lighting values. Spectrophotometric measurements are collected to validate the methodology. Results demonstrate that measurements from high dynamic range photographs can correspond to the physical quantity of circadian luminance with reasonable precision and repeatability. Circadian data collected in built environments can be utilized to study the impact of design decisions on human circadian entrainment and to create guidelines and metrics for designing circadian friendly environments.

High-Dynamic-Range Ultrasound: Application for Imaging Tendon Pathology

Raw ultrasound (US) signal has a very high dynamic range (HDR) and, as such, is compressed in B-mode US using a logarithmic function to fit within the dynamic range of digital displays. However, in some cases, hyper-echogenic tissue can be overexposed at high gain levels with the loss of hypo-echogenic detail at low gain levels. This can cause the loss of anatomic detail and tissue texture and frequent and inconvenient gain adjustments, potentially affecting the diagnosis. To mitigate these drawbacks, we employed tone mapping operators (TMOs) in HDR photography to create HDR US. We compared HDR US produced from three different popular TMOs (Reinhard, Drago and Durand) against conventional US using a simulated US phantom and in vivo images of patellar tendon pathologies. Based on visual inspection and assessments of structural fidelity, image entropy and contrast-to-noise ratio metrics, Reinhard and Drago TMOs substantially improved image detail and texture.

Application of High-Dynamic Range Imaging Techniques in Architecture: A Step toward High-Quality Daylit Interiors?

High dynamic range (HDR) imaging techniques are nowadays widely used in building research to capture luminances in the occupant field of view and investigate visual discomfort. This photographic technique also makes it possible to map sky luminances. Such images can be used for illuminating virtual scenes; the technique is called image-based lighting (IBL). This paper presents a work investigating IBL in a lighting quality research context for accelerating the development of appearance-driven performance indicators. Simulations were carried out using Radiance software. The ability of IBL to accurately predict indoor luminances is discussed by comparison with luminances from HDR photographs and luminances predicted by simulation in modeling the sky in several other more traditional ways. The present study confirms previous observations that IBL leads to similar luminance values than far less laborious simulations in which the sky is modeled based on outdoor illuminance measurements. IBL and these last methods minimize differences with HDR photographs in comparison to sky modeling not based on outdoor measurements.

Towards a new model of light quality assessment based on occupant satisfaction and lighting glare indices

This study looks at the effect of daylighting on human performance. It includes a focus on glare index combined with the actual feeling of users of the classroom as a way to assess indoor lighting quality. The main objective of this research is to understand the impact of daylighting from windows on the glare sensation and also to determine which glare index is the closest to human visual sensation under local daylighting conditions in Biskra, Algeria with highly luminous climate. The study used High Dynamic Range (HDR) photography, Evaglare and Aftab Alpha software to calculate the two glare metrics Daylight Glare, Index (DGI) and the Daylight Glare Probability (DGP). A survey was also used with 90 occupants under different lighting conditions (different configurations) in a design classroom. In order to link the mathematical model and the human assessment of glare, statistical regression analysis was used. We established a statistically compelling connection between daylighting and student performance.

Investigation of visual comfort metrics from subjective responses in China: A study in offices with daylight

Investigations have been conducted in two side-lit office spaces in the perimeter zone in a laboratory building in Canton, China under various sky conditions. High dynamic range photographs and sensor measured values with associated participants (Chinese, n = 19) evaluated visual comfort preference responses were collected to investigate against a set of visual comfort metrics. Tested against an evaluation dataset consisting of 476 measurements, correlations were analyzed of vertical eye illuminance (Ev), daylight glare index (DGI), daylight glare probability (DGP), luminance ratio and window/workplane luminance. Results showed that simple luminance metrics (e.g. workplane luminance) & vertical eye illuminance outperformed complex glare indices (DGP/DGI) & luminance ratio. Furthermore, metric thresholds were identified based on the results of this investigation.

Enhanced dynamic range x-ray imaging

X-ray images can suffer from excess contrast. Often, image exposure is chosen to visually optimize the region of interest, but at the expense of over- and underexposed regions elsewhere in the image. When image values are interpreted quantitatively as projected absorption, both over- and underexposure leads to the loss of quantitative information. We propose to combine multiple exposures into a composite that uses only pixels from those exposures in which they are neither under- nor overexposed. The composite image is created in analogy to visible-light high dynamic range photography. We present the mathematical framework for the recovery of absorbance from such composite images and demonstrate the method with biological and non-biological samples. We also show with an aluminum step-wedge that accurate recovery of step thickness from the absorbance values is possible, thereby highlighting the quantitative nature of the presented method. Due to the higher amount of detail encoded in an enhanced dynamic range x-ray image, we expect that the number of retaken images can be reduced, and patient exposure overall reduced. We also envision that the method can improve dual energy absorptiometry and even computed tomography by reducing the number of low-exposure (“photon-starved”) projections.

Experimental validation of ray tracing as a means of image-based visual discomfort prediction

Image-based visual discomfort analysis has strong potential to detect glare in order to predict occupant satisfaction with a space. However, in order to design buildings with occupant visual comfort in mind, architects need the ability to predict glare through renderings, rather than photographs. In this paper, we compare glare measurements from high dynamic range photographs and computer renderings of the same daylit space under 240 clear and 38 cloudy sky conditions in order to assess how accurately architects can predict glare though simulation. We use two ray tracing engines, Radiance and Accelerad, on scenes illuminated by a mathematically defined sky model or by recorded outdoor luminance levels from photographs. Depending on the scene, these methods predict daylight glare probability levels due to bright sources with between 93% and 99% accuracy and discomfort glare due to contrast with between 71% and 99% accuracy. Glare predictions are more sensitive to modeling and measurement accuracy than to simulation parameters, so accurate results can come from relatively fast simulations. Furthermore, with graphics acceleration, we are able to produce accurate glare predictions within minutes using Accelerad, achieving a speedup over Radiance of between 16 and 44 times.

Parallel Multiple-Bounce Irradiance Caching

Building designers rely on predictive rendering techniques to design naturally and artificially lit environments. However, despite decades of work on the correctness of global illumination rendering techniques, our ability to accurately predict light levels in buildings-and to do so in a short time frame as part of an iterative design process-remains limited. In this paper, we present a novel approach to parallelizing construction of an irradiance cache over multiple-bounce paths. Relevant points for irradiance calculation based on one or multiple cameras are located by tracing rays through multiple-bounce paths. Irradiance values are then saved to a cache in reverse bounce order so that the irradiance calculation at each bounce samples from previously calculated values. We show by comparison to high-dynamic range photography of a moderately complex space that our method can predict luminance distribution as accurately as Radiance, the most widely validated tool used today for architectural predictive rendering of daylit spaces, and that it is faster by an order of magnitude.

Evaluating a New Suite of Luminance-Based Design Metrics for Predicting Human Visual Comfort in Offices with Daylight

ABSTRACTA new suite of visual comfort metrics is proposed and evaluated for their ability to explain the variability in subjective human responses in a mock private office environment with daylight. Participants (n = 48) rated visual comfort and preference factors, including 1488 discreet appraisals, and these subjective results were correlated against more than 2000 unique luminance-based metrics that were captured using high dynamic range photography techniques. Importantly, luminance-based metrics were more capable than illuminance-based metrics for fitting the range of human subjective responses to data from visual preference questionnaire items. No metrics based upon the entire scene ranked in the top 20 squared correlation coefficients, nor did any based upon illuminance or irradiance data, nor did any of the studied glare indices, luminance ratios, or contrast ratios. The standard deviation of window luminance was the metric that best fit human subjective responses to visual preference on seven of 12 questionnaire items (with r2 = 0.43). Luminance metrics calculated using the horizontal 40° band (a scene-independent mask) and the window area (a scene-dependent mask) represented the majority of the top 20 squared correlation coefficients for almost all subjective visual preference questionnaire items. The strongest multiple regression model was for the semantic differential rating (too dim–too bright) of the window wall (adjR2 = 0.49) and was built upon three variables; standard deviation of window luminance, the 50th percentile luminance value from the lower view window, and mean luminance of the 40° horizontal band.

Removing Camera Shake via Weighted Fourier Burst Accumulation.

Numerous recent approaches attempt to remove image blur due to camera shake, either with one or multiple input images, by explicitly solving an inverse and inherently ill-posed deconvolution problem. If the photographer takes a burst of images, a modality available in virtually all modern digital cameras, we show that it is possible to combine them to get a clean sharp version. This is done without explicitly solving any blur estimation and subsequent inverse problem. The proposed algorithm is strikingly simple: it performs a weighted average in the Fourier domain, with weights depending on the Fourier spectrum magnitude. The method can be seen as a generalization of the align and average procedure, with a weighted average, motivated by hand-shake physiology and theoretically supported, taking place in the Fourier domain. The method's rationale is that camera shake has a random nature, and therefore, each image in the burst is generally blurred differently. Experiments with real camera data, and extensive comparisons, show that the proposed Fourier burst accumulation algorithm achieves state-of-the-art results an order of magnitude faster, with simplicity for on-board implementation on camera phones. Finally, we also present experiments in real high dynamic range (HDR) scenes, showing how the method can be straightforwardly extended to HDR photography.

Definition of the CIE standard skies and application of high dynamic range imaging technique to characterize the spatial distribution of daylight in Chile

The effectiveness and accuracy of a daylighting study relies on knowing the climate and light conditions available at a given place, to then evaluate the illuminance obtained during the project development process. This paper presents the methodologies of a research process, whose objective is providing realistic data to establish the foundations for future lighting studies in Chile. It starts with the revision of the metrics used to quantify daylight; later, the methodology used to collate the monthly frequencies on four types of standard CIE skies, starting from climate files, is presented. Finally, both the methodology and the results obtained are presented to measure and characterize the real skies, as well as the High Dynamic Range photographs (HDR) and, as complement of this information, the illuminances measurements of the sky dome to define the types of skies of six Chilean cities. From the study applied, it is possible to conclude that knowing the standard CIE skies of a place allows us to adjust and focus the daylighting study for predetermined and predominant skies. It is just as important to mention that the measuring techniques for the skies must be adjusted and calibrated for each studied location.