Absolute Luminance Research Articles

Quality spaces: Mental and physical

Perceptual-role theories of mental qualities hold that we can discover the nature of a being’s mental qualities by investigating that being’s capacity to make perceptual discriminations. Many advocates of perceptual-role theories hold that the best explanation of these capacities is that mental quality spaces are homomorphic to the spaces of the physical properties that they help to discriminate. This paper disputes this thesis on largely empirical grounds, and offers an alternative. The alternative explains interesting patterns in our perception of color differences and similarities across edges, emphasizing the importance of visual discontinuities. Further, it provides a simple explanation of the intransitivity of indiscriminability. The homomorphism thesis can explain this intransitivity only with the addition of an independently unsupported empirical hypothesis. Also, in contrast to the homomorphism thesis, the alternative view is perfectly consistent with empirical data that suggest (a) that the visual system uses the dynamic allocation of a limited range of responses in order to discriminate a wider range of physical stimuli and (b) that we are much better at detecting changes in relative luminance than changes in absolute luminance. Moreover, it does all this while using a more efficient strategy for encoding sensory information.

范围限制的自适应亮度保持多阈值直方图均衡算法研究

In recent years, many histogram equalization algorithms have been proposed for the consumer electronics field. However, many of these algorithms are hard to realize. Even, for example, some algorithms may cause an effect on brightness saturation. Therefore, a range limited adaptive brightness preserving multi-threshold histogram equalization(RLAMHE) algorithm is presented in this paper. First, the input image is smoothed appropriately to obtain the number of its histogram peak points (N+1). Then the Otsu algorithm is extended by the N-threshold, and N segmentation thresholds of the image are obtained in this way, so that the image is segmented according to this threshold. In order to maximize the brightness of the input image, a range of the equalized image is recalculated according to the minimum Absolute Mean Brightness Error(AMBE) criterion of the input and the output image. Finally, all sub-images are equalized separately using the new equalization range. Test results show that the proposed algorithm is more efficient than other algorithms and can obtain sharper image details. Meanwhile, the overall brightness of the image is also ideal. Using this algorithm to process Lena graphs, the absolute mean luminance error is 0.416 4, which is obviously better than that obtained using RLBHE algorithm(0.629 5).

An Efficient Adaptive Fuzzy Switching Weighted Mean Filter for Salt-and-Pepper Noise Removal

An image degraded by noise is a common phenomenon. In this letter, we propose a novel adaptive fuzzy switching weighted mean filter to remove salt-and-pepper (SAP) noise. The process of denoising includes two stages: noise detection and noise elimination. In the first stage, pixels in a corrupted image are classified into two categories: original pixels and possible noise pixels. For the latter, we compute the maximum absolute luminance difference of processed pixels next to possible noise pixels to classify them into three categories: uncorrupted pixels, lightly corrupted pixels, and heavily corrupted pixels. In the second stage, under the assumption that pixels at a short distance tend to have similar values, the distance relevant weighted mean of the original pixels in the neighborhood of a noise pixel are computed. For a nonnoise pixel, retain it as unchanged; for a lightly corrupted pixel, replace it with the weighted average value of the weighted mean and its own value; and for a heavily corrupted pixel, change it to be the weighted mean. Experimental results show that compared to some state-of-the-art algorithms, our method keeps more texture details and is better at removing SAP noise and depressing artifacts.

A neural response-based model to predict discomfort glare from luminance image

It is well known that one of the problems of the current method for discomfort glare evaluation, called the unified glare rating, is the non-uniform luminance of the glare source. This paper addresses this issue by considering the spatial contrast of luminance as a measure of non-uniformity. An image-based metric is proposed to evaluate discomfort glare by modeling the neural response of human vision. The model takes an absolute luminance image as input and predicts visual discomfort based on the spatial distribution of the luminance of the stimulus and the background. The developed model was tested to predict subjective glare ratings based on an experiment conducted using non-uniform LED sources with symmetric and asymmetric patterns of LEDs, and its performance was compared with the unified glare rating. As expected, the unified glare rating predictions correlated well with the subjective glare evaluations of luminaires with symmetric patterns of LEDs (as they appear less non-uniform) but not for those with asymmetric patterns. Results showed that the developed model, named the Neural Response-based Glare Model, gave similar performance to unified glare rating for symmetric patterns but outperformed UGR for asymmetric patterns of LEDs.

Effects of absolute luminance and luminance contrast on visual discrimination in low mesopic environments.

Recent research revealed considerable decline in visual perception under low luminance conditions. However, systematic studies on how visual performance is affected by absolute luminance and luminance contrast under low mesopic conditions (<0.5cd/m2) is lacking. We examined performance in a simple visual discrimination task under low mesopic luminance conditions in three experiments in which we systematically varied base luminance and luminance contrast between stimulus and background. We further manipulated eccentricity of the stimuli because of known rods and cones gradients along the retina. We identified a "deficiency window" for performance as measured by d' when luminance was < 0.06cd/m2 and luminance contrast as measured by the luminance ratio between stimulus and background was below < 1.7. We further calculated performance-based luminance as well as contrast efficiency functions for reaction times (RTs). These power functions demonstrate the contrast asymptote needed to decrease RTs and how such a decrease can be achieved given various combinations of absolute luminance and luminance contrast manipulations. Increased eccentricity resulted in slower RTs indicative of longer scan distances. Our data provide initial insights to performance-based efficiency functions in low mesopic environments that are currently lacking and to the physical mechanisms being utilized for visual perception in these extreme environments.

Ubiquitous luminance sensing using the Raspberry Pi and Camera Module system

In this paper, the authors have calibrated a Raspberry Pi and Camera Module (RPiCM) for use as an absolute luminance sensor. The spectral response of the RPiCM chip as well as linear mapping to the standard CIE-XYZ colour space have been measured, calculated and presented. The luminance values are anchored to absolute luminance measurements. Further, by using high dynamic range imaging techniques making use of different shutter speeds in a sequence of images, the measurement of luminance values from approximately 10 to 50,000 cd/m2 is possible. Lens correction for vignetting is also addressed, while pixel point spreading is ignored. This measurement goes beyond a single point measurement, economically and accurately allowing each of the arrays within the RPiCM chip to act as an individual luminance meter over the entire field of view of the camera system. Applications and limitations of the embedded camera system are discussed. An Energy Plus model is constructed as a motivational application of a simple one room, one window space and simulated for a year using weather files from around the world. These simulations highlight the need for spatial luminance-based sensing within the built environment to counteract the experience of discomfort glare by building occupants.

Sky Based Light Metering for High Dynamic Range Images

AbstractImage calibration requires both linearization of pixel values and scaling so that values in the image correspond to real‐world luminances. In this paper we focus on the latter and rather than rely on camera characterization, we calibrate images by analysing their content and metadata, obviating the need for expensive measuring devices or modeling of lens and camera combinations. Our analysis correlates sky pixel values to luminances that would be expected based on geographical metadata. Combined with high dynamic range (HDR) imaging, which gives us linear pixel data, our algorithm allows us to find absolute luminance values for each pixel—effectively turning digital cameras into absolute light meters. To validate our algorithm we have collected and annotated a calibrated set of HDR images and compared our estimation with several other approaches, showing that our approach is able to more accurately recover absolute luminance. We discuss various applications and demonstrate the utility of our method in the context of calibrated color appearance reproduction and lighting design.

Modelling the Mach bands illusion by means of a diffusion model.

First, we criticize the validity of the principle of lateral inhibition. Second, on the basis of illusory phenomena and stabilized retinal images, we point out that the retina does not code the absolute luminance; the retina forwards a relative luminance sketch towards higher levels of the visual system. However, at the level of conscious processing the perceptual counterpart of absolute luminance, brightness, is available. Therefore, it is reasonable to assume that a reconstruction process is carried out by the visual system, which recovers the inner representation that corresponds to the retinal light distribution from the coded relative luminance sketch. We provide an illustrative description of a computational model of this reconstruction process. The basis of the reconstruction is a mathematically provable theorem, according to which if image P is produced from image I by Laplacian filtering, and then P is used as the sources and sinks of a homogeneous linear diffusion process, then the equilibrium of the diffusion will be identical to the original image I. We have illustrated this by a one-dimensional heat diffusion example, and by a series of test tubes connected to each other, also in one dimension. Brightness illusions are considered as a side effect of this diffusion-based reconstruction process. If the diffusion process deviates from the principle of homogeneous linearity, then the result of the reconstruction will deviate from the original image I. We showed a concrete illustration of this with regards to the Mach bands illusion: here we violated the principle of homogeneous linearity by means of inserting a small vertical tube serving as a serial resistance between each test tube and the horizontal connecting tube. This violation resulted in a change of water level in the source and the sink test tubes corresponding to the Mach bands illusion.

Luminance meter for photopic and scotopic measurements in the mesopic range

This paper presents a design and realization of a dual-channel luminance meter for simultaneous measurement of luminance with photopic and scotopic weightings. Such measurements are useful in mesopic conditions, i.e. when the luminance is in the range of 0.005–5 cd m−2. The instrument is a spot luminance meter with two spectrally weighted channels. The collected light is detected with silicon detectors and a computer-controlled dual-channel switched-integration amplifier. The instrument is characterized for relative spectral responsivity against a calibrated spectroradiometer using a radiance source based on an integrating sphere with input from a monochromator. An absolute luminance responsivity calibration is made against a sphere-based luminance standard at a level close to the high end of the mesopic range. The standard uncertainty in luminance responsivity calibration is 0.3% for the photopic channel and 0.6% for the scotopic channel. In addition, characterization measurements were carried out for the instrument’s linearity, stray light sensitivity and polarization dependence. The results show very good noise performance, allowing fast measurements over the whole mesopic range. The noise equivalent power was measured to be approximately 20 fW Hz−1/2, equal to a noise equivalent luminance of 30 µcd m−2 Hz−1/2. Estimated uncertainty of measurements for typical light sources is 2.2% (k = 2) at the lowest luminance levels of the mesopic range.

Characteristics of Optical Diffusers for Light-Emitting Diodes Backlight Unit Prepared by Melt-Extrusion Process

Using extrusion compounding followed by compression molding processes, polycarbonate-based optical diffusers with uniform dispersion of diffusing particles could be prepared for application in direct-lit LED backlight unit. Inorganic porous silica and organic silicone microsphere particles were employed as diffusing agents. The inclusion of diffusing particles up to 3 wt % substantially improved the luminance uniformity with respect to both location and viewing angle, and the effect was shown to be more prominent for the silicone particles. Alternatively, inorganic silica particles could yield diffusers with enhanced absolute luminance and thermal resistance property. The thermo-mechanical property of the elastic modulus was revealed to be improved upon addition of diffusing particles of silica and silicone with cross-linked structure.

Mechanisms underlying global stereopsis in fovea and periphery

To better understand the pooling properties underlying global stereopsis we examined the relationship between carrier luminance spatial frequency and modulator disparity spatial frequency. Thresholds for detecting global sinusoidal disparity corrugations of spatially band-pass noise were measured as a function of modulator disparity spatial frequency for both centrally and peripherally located stimuli using a standard 2-IFC task. We found a characteristic relationship that depended on modulator disparity spatial frequency. At high modulator disparity spatial frequencies (>1c/d), there is an optimal ratio of around 2.6, whereas at low modulator disparity spatial frequencies, there is an optimal absolute carrier luminance spatial frequency (i.e., 3c/d). In the periphery, vision is restricted to modulator disparity spatial frequencies below 1c/d and, as a consequence, following the above rule, there is an optimum absolute carrier luminance spatial frequency that reduces in spatial frequency with increasing eccentricity. This finding is consistent with there being more than one channel processing global stereo that is subsequently confirmed using a 2×2 AFC detection/discrimination paradigm. Furthermore, because of the different carrier/modulator relationships in central and peripheral vision, peripheral global stereo cannot be simply related to central global stereo by a scaling factor and thus cannot be simply due to cortical magnification, as originally thought.

Development of new daylight glare analysis methodology using absolute glare factor and relative glare factor

Many glare analysis methods have been developed and utilized for predicting and quantifying discomfort glare issues inside a building. However, the existing methods have not been able to accurately evaluate glare issues when extremely bright glare sources are involved. As luminance values of glare sources get extremely high, the impact of the absolute luminance value begins to dominate the impact of the contrast ratio. This type of glare should be classified as absolute factor glare which is different from relative factor glare in terms of glare analysis. There are many existing luminance thresholds and contrast ratios that address these two factors, but they are not consistent. Due to the inconsistency and inaccuracy issues, the existing glare analysis methods have been barely utilized in design practice. Therefore, it is necessary to develop a more accurate methodology that separates the absolute glare component from the relative glare component. This approach can increase the accuracy of glare evaluation and simplify glare evaluation procedures. This paper makes clear definitions of the absolute and relative glare factors and also suggests three different luminance ranges for absolute glare factor, relative glare factor, and no-glare based on the results of this preliminary study.

Determinants of filled/empty optical illusion: Influence of luminance contrast and polarity.

Subjective estimates of lengths or areas in the visual field depend on the visual contents of the estimated space (filled/empty or Oppel-Kundt illusion). We studied the dependence of this phenomenon on the presentation mode (white on black vs. black on white background), and on the figure/ground contrast. We found, as expected, overestimation of the filled part of the figure for both contrast polarities. The expansion effect was found to be an increasing function of the absolute luminance contrast, and was consistently higher for the negative (luminant figures on a dark background) than for the positive polarity. The contrast factor contributes from one-fifth to one-third of the total effect. Possible interpretations in terms of known sensory phenomena (irradiation, lateral interactions) or higher, integrative functions are discussed.

A New Color Breakup Measure Based on Color Difference Between Fields and Contrast to the Surrounding

Field-sequential-color displays can have low power consumption and high spatial resolution, but can suffer from annoying color breakup. Various approaches to suppress color breakup have been presented in recent literature. However, a suitable color breakup measure is necessary to compare the performances of those approaches, and is also helpful for quick evaluation of algorithms and parameter settings. A perception experiment was carried out to investigate the perceived color breakup with different field color combinations, different light intensities, and different backgrounds. The results indicate that the color difference and the relative contrast between fields have a significant effect on the perceived color breakup, rather than the absolute luminance value, which is different from existing metrics for color breakup visibility. Based on this new insight, a computational metric for color breakup visibility is proposed and discussed, which has a high correlation with the subjective scores (R2 = 0.945).

Evidence for brightness constancy in the mongolian gerbil

Event Abstract Back to Event Evidence for brightness constancy in the mongolian gerbil Josephine Henke1, Christian Garbers1, Christian Leibold1, 2, Thomas Wachtler1, 2 and Kay Thurley1, 2* 1 Ludwig-Maximilians-University, Munich, Department Biology II, Germany 2 Bernstein Center for Computational Neuroscience Munich, Germany The mongolian gerbil (Meriones unguiculatus) retina is composed of a combination of two cone photoreceptors that are maximally sensitive at a wavelength of 360 nm and 493 nm, respectively, and one rod photoreceptor with a maximal sensitivity at 501 nm [1]. It is the only known mammal that has a rod receptor with a peak sensitivity at longer wavelength than all the cone receptors. Gerbils therefore exhibit photopic vision with a peak sensitivity at shorter wavelength than that of the scotopic mechanism [2]. Since gerbils can be trained to visual discrimination tasks, their visual capabilities can be analysed in behavioral experiments [1,2]. However, so far little is known about visual processing in these animals. In our behavioral experiments we used a virtual reality (VR) setup with a spherical treadmill that is surrounded by a 360° toroidal screen onto which the virtual environment is projected. This design permits a fixation of the animal such that it can freely rotate around its vertical body axis. The VR setup offers a highly controlled environment for sensory testing. Visual stimuli were presented as a two-alternative choice task using a virtual y-maze in which the animal had to run to the end of the chosen arm to receive a reward. First we trained gerbils to discriminate between bright and dark walls at the ends of the maze arms. In a second set of experiments, we trained the gerbils on the brightness of a central spot relative to its local background. One group of animals had to choose the side with the relative brighter spot and the other group the side with the relative darker spot. Despite differences in the absolute luminance of the stimuli the animals chose the ones with the correct relative brightness. These results are an indication of brightness constancy in the mongolian gerbil.

Analysis of sky luminance experimental data and comparison with calculation methods

Data of sky luminance measured in Osaka (Japan) and in Arcavacata di Rende (Italy) during a year were analysed and compared with the values predicted by the Perez, Igawa and CIE calculation methods. The best predictions of absolute, relative and zenith luminances were obtained by the CIE method. If the measured luminances in a locality are not available, only by the Perez and the Igawa method is possible to predict the sky luminance distribution under different meteorological conditions. Of these two methods, the Igawa method appears to be more accurate for the prediction of the absolute luminance, and the Perez method for the predictions of the relative luminance. It should be stressed that, before comparison with the calculation models, the sky luminances measured in each scanning have to be corrected in order to make the horizontal illuminance obtained by integration of the luminances on the sky vault equal to the value of the horizontal illuminance measured by photometers. If such a correction is not made owing to the absence of this latter measurement, the errors of the predictions obtained by the models become larger, but they can be reduced by applying a new methodology proposed by the authors.

Implementation and Optimization of an Enhanced PWD Metric for H.264/AVC on a TMS320C64 DSP

A common method for selecting the best prediction mode based on block matching algorithm is to compare, for each source block, the associated distortions among the available prediction candidates. The human visual perception is sensitive to luminance contrast rather than absolute luminance values. In fact, the human eyes ability to detect the magnitude difference between an object and its background depends on the background luminance average value. The Perceptually Weighted Distortion (PWD) is a new distortion measure that can produce better image quality. In this paper, we propose to add a new feature to the PWD by introducing another diagonal component that yields to a significant quality improvement. The enhanced PWD metric actually outperforms the original PWD and the SAD metric, according to the experimental results, especially in the aspect of reducing block artifacts. An increase in terms of implementation complexity will be noticed as a result of this contribution. Therefore, optimized implementation of the Enhanced PWD exploiting the C64 DSP-Core assets will be presented. In fact, Standard Assembly (SA) is used to implement the different Enhanced PWD functions in order to exploit efficiently the C64 internal architecture and resources. Experimental results show more than 85% improvement in terms of cycle cost compared to C code.

Empirical Validation of Daylight Simulation Tool with Physical Model Measurement

Problem statement: In recent years, daylighting simulation tools have been increasingly used by many architects, engineers and researchers to evaluate the day lighting performances of building design. Most of these tools employ CIE sky models for simulation. However, the accuracy and applicability of these tools for tropical sky are doubtable. The aim of this study was to validate the computer simulated result with scaled physical model results measured under real tropical sky. Approach: Daylighting model was constructed using scaled physical model to be tested under real sky measurement. The same model was configured in Desktop Radiance 2.0 to perform day lighting simulation experiments. All the measurements were carried out under intermediate and overcast tropical sky conditions in Malaysia; while related CIE sky conditions were used for simulations. Results: Due to the CIE sky conditions are very dissimilar from the actual tropical sky; simulated absolute value results such as external illuminance, absolute work plane illuminance and surface luminance recorded high mean differences from the measured results, with 81.63; 71.06 and 49.71%, respectively. However, relative ratios such as Daylight Factor (DF) yielded mean difference of 26.06% and luminance ratio was 29.75% only. The average mean difference was 44.37%. Conclusion/Recommendations: To compare the performances, relative ratios such as DF and luminance ratio showed better accuracies. For future research, validation on other parameters can be performed such as orientations, angle of the overhang, glazing, window sizes, colors, environment settings and electric lighting.

The Effect of Luminance Distribution Patterns on Occupant Preference in a Daylit Office Environment

New research in daylighting metrics and developments in validated digital High Dynamic Range (HDR) photography techniques suggest that luminance based lighting controls have the potential to provide occupant satisfaction and energy saving improvements over traditional illuminance based lighting controls. This paper studies occupant preference and acceptance of patterns of luminance using HDR imaging and a repeated measures design methodology in a daylit office environment. Three existing luminance threshold analysis methods [method1: predetermined absolute luminance threshold (for example, 2000 cd/m2), method2: scene based mean luminance threshold and method3: task based mean luminance threshold] were studied along with additional candidate metrics for their ability to explain luminance variability of 18 participant assessments of ‘preferred’ and just disturbing' scenes under daylighting conditions. Per-pixel luminance data from each scene were used to calculate Daylighting Glare Probability (DGP), Daylight Glare Index (DGI), and other candidate metrics using these three luminance threshold analysis methods. Of the established methods, the most consistent and effective metrics to explain variability in subjective responses were found to be; mean luminance of the task (using method3; adjr2 = 0.59), mean luminance of the entire scene (using method2; adjr2 = 0.44), and DGP using 2000 cd/m2 as a glare source identifier (using method1; adjr2 = 0.41). Of the 150 candidate metrics tested, the most effective was the ‘mean luminance of the glare sources’, where the glare sources were identified as 7* the mean luminance of the task position (adjr2 = 0.64). Furthermore, DGP consistently performed better than DGI, confirming previous findings. ‘Preferred’ scenes never had more than ∼ 10 percent of the field of view (FOV) that exceeded 2000 cd/m2. Standard deviation of the entire scene luminance also proved to be a good predictor of satisfaction with general visual appearance.

Correlation of fMRI responses to absolute luminance changes in visual cortex

Correlation of fMRI responses to absolute luminance changes in visual cortex