Luminance Ratio Research Articles

Lightness constancy: Ratio invariance and luminance profile

The term simultaneous lightness constancy describes the capacity of the visual system to perceive equal reflecting surfaces as having the same lightness despite lying in different illumination fields. In some cases, however, a lightness constancy failure occurs; that is, equal reflecting surfaces appear different in lightness when differently illuminated. An open question is whether the luminance profile of the illumination edges affects simultaneous lightness constancy even when the ratio invariance property of the illumination edges is preserved. To explore this issue, we ran two experiments by using bipartite illumination displays. Both the luminance profile of an illumination edge and the luminance ratio amplitude between the illumination fields were manipulated. Results revealed that the simultaneous lightness constancy increases when the luminance profile of the illumination edge is gradual (rather than sharp) and homogeneous (rather than inhomogeneous), whereas it decreases when the luminance ratio between the illumination fields is enlarged. The results are interpreted according to the layer decomposition schema, stating that the visual system splits the luminance into perceived lightness and apparent illumination components. We suggest that illumination edges having gradual and homogeneous luminance profiles facilitate the luminance decomposition process, whereas wide luminance ratios impede it.

Strength and variability of the backscroll illusion

When a walking person is presented in a movie, the background image appears to move in a direction opposite to that of the person’s locomotion. This study aimed to quantify the strength of this backscroll illusion and to examine interobserver and intraobserver variability. Stimuli were movie clips that presented a walking person in profile against a background of dynamic grating composed of two vertical sinusoidal gratings moving in opposite directions. Employing a motion-nulling method, we controlled the ratio of luminance contrasts of the component gratings to determine points that canceled the percept of unidirectional motion in the grating background. Results across 50 observers showed that the backscroll illusion disappeared when a luminance contrast of moving grating components consistent with a walker’s direction was about twice as high as that for the opposite motion direction. Intraobserver variability was relatively small. However, nulling points for individual observers were more variable under conditions with dynamically moving walkers than conditions presenting only a static picture of a walker. We speculated on the underlying mechanisms of the backscroll illusion in relation to similar phenomena.

Daylight for Energy Savings and Psycho-Physiological Well-Being in Sustainable Built Environments

Natural light is a vital force for human beings. Successful daylighting in buildings requires trade-offs and optimization between competing design aspects (e.g. light distribution, glare, solar gains, views, etc.), whilst also including consideration of façade layout, space configuration, internal finishes and choice/operation of shading devices. However, to design energy-sustainable built environments which are conducive to human health, these variables have necessarily to be related also with biological and behavioural factors such as metabolic rhythms, psychological stimulation and occupants’ preferences. Basing on a multidisciplinary review of existing literature, this paper looks at the relationship between quantitative physical measures of the luminous environment (e.g. horizontal and vertical illuminance, luminance ratio, correlated colour temperature), qualitative aspects of vision (e.g. uniformity, distribution), and psycho-physiological human response to natural light. The aim of the study consists in defining a framework to implement existing daylighting practices basing not solely on photopic requirements but also containing awareness of the demands for psychological and photobiological stimulation, so as to positively influence the health of occupants whilst enhancing energy savings.

On the search for an appropriate metric for reaction time to suprathreshold increments and decrements

Weber contrast, ΔL/L, is a widely used contrast metric for aperiodic stimuli. Zele, Cao & Pokorny [Zele, A. J., Cao, D., & Pokorny, J. (2007). Threshold units: A correct metric for reaction time? Vision Research, 47, 608–611] found that neither Weber contrast nor its transform to detection-threshold units equates human reaction times in response to luminance increments and decrements under selective rod stimulation. Here we show that their rod reaction times are equated when plotted against the spatial luminance ratio between the stimulus and its background (Lmax/Lmin, the larger and smaller of background and stimulus luminances). Similarly, reaction times to parafoveal S-cone selective increments and decrements from our previous studies [Murzac, A. (2004). A comparative study of the temporal characteristics of processing of S-cone incremental and decremental signals. PhD thesis, New Bulgarian University, Sofia, Murzac, A., & Vassilev, A. (2004). Reaction time to S-cone increments and decrements. In: 7th European conference on visual perception, Budapest, August 22–26. Perception, 33, 180 (Abstract).], are better described by the spatial luminance ratio than by Weber contrast. We assume that the type of stimulus detection by temporal (successive) luminance discrimination, by spatial (simultaneous) luminance discrimination or by both [Sperling, G., & Sondhi, M. M. (1968). Model for visual luminance discrimination and flicker detection. Journal of the Optical Society of America, 58, 1133–1145.] determines the appropriateness of one or other contrast metric for reaction time.

環境照度の違いがＣＲＴモニタの輝度比と超音波画像のコントラストに与える影響

Variation in the luminance ratio of a cathode ray tube(CRT)monitor and the ultrasonographic images at different levels of ambient light(0-150 lux)was investigated to obtain optimum ambient light in the ultrasonography suite. The maximum and minimum luminances of test patterns and ultrasonographic images were measured after three technicians independently optimized the brightness and contrast of the CRT monitor and ultrasonographic images at different levels of ambient light. Furthermore, the luminance ratio was calculated from the maximum luminance divided by the minimum luminance. When ambient light increased, it was difficult for the technicians to optimize the brightness and contrast settings of the CRT monitor to maintain a high luminance ratio at 0 lux. The luminance ratio decreased rapidly as ambient light increased up to 20 lux. However, the luminance ratio decreased gradually when ambient light was higher than 20 lux. It is necessary to take into consideration the ambient light to maintain a high luminance ratio of ultrasonographic images.

THE RANGE OF THE TOTAL-AMOUNT GLARE DETERMINED BY THE AVERAGE LUMINANCE OF THE VISUAL FIELD

The purpose of this paper is to identify the range of the total amount glare by using the average luminance of the visual field. Two series of experiments were carried out. In the first experiment , the pre-adaptation level equals the background luminance, and in the second, the pre-adaptation level is constant. The curve indicating the relationship between the luminance and glare sensation for the field with uniform luminance is obtained. The first experiment shows a significant difference in glare sensation, when the ratio of background luminance to source luminance is different, even when the average luminance of the visual field is the same. The second experiment shows that the effects of the ratio of background luminance to source luminance on glare sensation are not significant and that the average luminance can be used to predict glare sensation. The effect of the pre-adaptation level on glare sensation is also discussed.

The Perceptual Contrast of Impossible Shadow Edges

Luminance ratios along shadow edges remain the same even when they cross reflectance borders. According to Gilchrist (1988, Perception & Psychophysics 43 415-424) this so-called ratio-invariance property is a crucial factor in the perception of shadows. However, Soranzo and Agostini (2004, Perception 33 1359-1368) suggested that in some conditions (named 'impossible shadows'), a luminance pattern might still be perceived as a shadow even if the ratio-invariance property along its edge is violated. This can occur when an edge is collinear with another edge (contextual edge) which incorporates it, shares the same polarity, and generates a larger ratio. The hypothesis that impossible shadows are actually perceived as shadows is here tested by comparing the perceptual contrast of a luminance edge in the absence of a contextual edge (control condition) to that of both possible shadow edges (where the contextual and mediating edge share the same ratio) and impossible shadow edges (where the ratio of the contextual edge is larger rather than that at the mediating edge). We found that the perceived contrast of luminance edges shrinks in both possible and impossible shadow conditions rather than in the control condition. This evidence supports the hypothesis that a luminance pattern might be perceived as a shadow even when the ratio-invariance property is violated.

The dependence of luminous efficiency on chromatic adaptation

We investigated the dependence of luminous efficiency on background chromaticity by measuring 25-Hz heterochromatic flicker photometry (HFP) matches in six genotyped male observers on 21 different 1000-photopic-troland adapting fields: 14 spectral ones ranging from 430 to 670 nm and 7 bichromatic mixtures of 478 and 577 nm that varied in luminance ratio. Each function was analyzed in terms of the best-fitting linear combination of the long- (L) and middle- (M) wavelength sensitive cone fundamentals of A. Stockman and L. T. Sharpe (2000). Taking into account the adapting effects of both the backgrounds and the targets, we found that luminous efficiency between 603 and 535 nm could be predicted by a simple model in which the relative L- and M-cone weights are inversely proportional to the mean cone excitations produced in each cone type multiplied by a single factor, which was roughly independent of background wavelength (and may reflect relative L:M cone numerosity). On backgrounds shorter than 535 nm and longer than 603 nm, the M-cone contribution to luminous efficiency falls short of the proportionality prediction but most likely for different reasons in the two spectral regions.

Coronary artery anomalies detected by MSCT-coronary angiography in the adult.

Before coronary evaluation by modern imaging techniques was feasible, premorbid diagnoses of coronary artery anomalies (CAAs) were usually made fortuitously by invasive coronary angiography (ICA). However, this technique is limited by its invasive and projectional nature. Coronary magnetic resonance angiography (CMRA) and multi-slice computed tomography (MSCT) broadened clinical information by enabling visualisation of the coronary arteries in their anatomical environment. This case series visualises and reviews anomalous coronary artery from the opposite sinus (ACAOS) and coronary artery fistulae. All CAAs were detected by means of 64-slice dual source computed tomography after 1000 cardiac scans at the Erasmus MC, Rotterdam, the Netherlands. Eight ACAOS cases, one anomalous left coronary artery from the pulmonary artery (ALCAPA) and one congenital aneurysm of an aortic sinus were found. Seven out often detected CAAs were considered malignant whereas three CAAs of the ACAOS type (retroaortic path) were considered benign. Significant coronary artery disease was found in three out of eight ACAOS cases. In one of the ACAOS cases complete evaluation of the anomalous coronary artery was limited by motion artifacts. All five cases of right ACAOS were referred for MSCT because the right coronary artery could not be located by invasive angiography. All CAAs were easy to diagnose because of 3D imaging and high temporal and spatial resolution. High resolution made it possible to not only depict coronary artery abnormalities, but also to quantify luminal and vessel properties such as stenosis grade, aspects of plaque, anomalous vessel length, luminal area ratio and the asymmetry ratio. Because of its comprehensiveness, MSCT can be an effective imaging modality in patients suspected of coronary artery abnormalities caused by coronary artery disease, CAAs, or a combination of both. (Neth Heart J 2008;16:369-75.).

Electrophysiological measurement of macular pigment distribution using annular stimuli: implications for colour vision testing

Abstract Purpose To quantify macular pigment optical density (MPOD) and distribution using steady‐state VEPs and to optimize the koniocellular selectivity of large chromatic gratings. Methods Blue/Green (B/G) gratings were generated within 1 circular and 3 concentric annular fields (maximum radius 9°). All 4 fields were modulated simultaneously, each at a different temporal frequency. Onset‐offset VEPs were recorded as the luminance ratio between adjacent chromatic components within each field changed from 0.25 to 0.85 in 11 automated steps. Fourier analysis showed that the first harmonic was minimised at each subjects’ isoluminant ratio, as verified using flicker photometry. MPOD was computed at each retinal location relative to the most eccentric annulus. The stimuli were adjusted to compensate for the MP profile, allowing generation of a B/G grating that was isoluminant over the whole of the 18° stimulus area. The optimised B/G field was used to probe koniocellular function using onset‐offset VEPs and psychophysical temporal tuning characteristics. Results Results: MPOD values computed from VEP estimates of B/G isoluminance correlated with those derived from minimum flicker measurements (r=0.94, p<0.005, slope=0.80). Large B/G gratings comprising of concentric annuli and optimised to compensate for MP, elicted large chromatic‐specific onset VEPs of negative polarity. Temporal tuning characteristics to the same stimuli were low‐pass, in keeping with the low temporal resolution of the koniocellular system. Conclusion The steady‐state VEP can be used to determine MPOD at different retinal eccentricities. Macular pigment profiles may be used to optimise the koniocellular selectivity of large B/G stimuli.

Measurement of macular pigment optical density and distribution using the steady-state visual evoked potential

The purpose of this study was to specify isoluminance at different retinal eccentricities and to characterize macular pigment optical density (MPOD) and distribution using the steady-state visual evoked potential (VEP). Redgreen (B/G) gratings were generated within two circular stimulus fields (radius = 0.55 or 1.1 deg) and within four annular fields (maximum mean radius = 6.0 deg) on a color monitor. Temporal frequency was 15 Hz. Isoluminance was determined for each stimulus using minimum flicker photometry. Steady-state onset-offset VEPs were recorded to the same annular stimuli as the luminance ratio between adjacent chromatic components was changed from 0.25 to 0.85 in 11 automated steps (0.5 representing photometric isoluminance). Fourier analysis showed that the power of the first harmonic was minimized at the isoluminant ratio specific to each subject. Relative OD was computed by comparing the isoluminant ratio at any location with that for the most eccentric annulus. To compensate for the broadband characteristics of the monitor, OD values were corrected according to minimum flicker measurements made through known concentrations of carotenoid solution. MPOD was additionally measured using minimum motion photometry.There was high correlation between the isoluminant ratios determined by minimum flicker and VEPs for both R/G and B/G stimulation (r = 0.91, P < 0.005, slope = 1). Calibrated OD values computed from VEP estimates of B/G isoluminance correlated with those derived from minimum flicker (r = 0.96, P < 0.0005, slope = 0.85) and motion photometry (r = 0.94, P < 0.0005, slope = 0.88). OD values derived from B/G VEPs increased toward the fovea and corresponded closely with minimum flicker and minimum motion assessment of MP distribution profiles. The steady-state VEP can be used to determine isoluminance at different retinal eccentricities. MPOD and distribution can be measured by steady-state VEPs to B/G stimuli.

Bacterial cellulose membrane as flexible substrate for organic light emitting devices

Bacterial cellulose (BC) membranes produced by gram-negative, acetic acid bacteria ( Gluconacetobacter xylinus), were used as flexible substrates for the fabrication of Organic Light Emitting Diodes (OLED). In order to achieve the necessary conductive properties indium tin oxide (ITO) thin films were deposited onto the membrane at room temperature using radio frequency (r.f.) magnetron sputtering with an r.f. power of 30 W, at pressure of 8 mPa in Ar atmosphere without any subsequent thermal treatment. Visible light transmittance of about 40% was observed. Resistivity, mobility and carrier concentration of deposited ITO films were 4.90 × 10 − 4 Ohm cm, 8.08 cm 2/V-s and − 1.5 × 10 21 cm − 3 , respectively, comparable with commercial ITO substrates. In order to demonstrate the feasibility of devices based on BC membranes three OLEDs with different substrates were produced: a reference one with commercial ITO on glass, a second one with a SiO 2 thin film interlayer between the BC membrane and the ITO layer and a third one just with ITO deposited directly on the BC membrane. The observed OLED luminance ratio was: 1; 0.5; 0.25 respectively, with 2400 cd/m 2 as the value for the reference OLED. These preliminary results show clearly that the functionalized biopolymer, biodegradable, biocompatible bacterial cellulose membranes can be successfully used as substrate in flexible organic optoelectronic devices.

Effects of Luminance Ratio in a Liquid-Crystal Display on Discrimination of Separation by Elderly People

In a 2006 study the influence of spacing between vertical lines, exposure time, and number of vertical lines on visual performance was identified. The present study further explored the effects of luminance ratio on discrimination of separation. Analysis showed the luminance ratio of target vertical lines affected the visual performance of both younger and elderly people. The luminance ratio was greater, as was accuracy; however, accuracy might not become greater when the luminance ratio was too high. Further, the effect of the number of vertical lines on accuracy was significant, as was consistent with the 2006 study. Two interactive two-way effects were found, group x luminance ratio and luminance ratio x number of vertical lines.

SU‐GG‐I‐55: Comparison of Viewing Angle Performance and Observer Performance of Different Types of Liquid Crystal Displays

Purpose: Liquid crystal displays (LCDs) have a number of advantages over cathode ray tubes and conventional viewers based on film. However, it is nown that the performance of LCDs varies by viewing angle. The purpose of this study was to compare the angular performance and effects on observer performance in different types of LCDs, namely a general purpose LCD for personal computers and one especially designed for medical use. Method and Materials: Two types of LCDs were used in this study, an in‐plane switching (IPS) technology display especially designed for medical use and a general‐purpose twisted nematic (TN) technology display. We measured the luminance responses and contrast ratios for the two types of LCDs at different viewing angles (−60 degrees to 60 degrees). Furthermore, observer performance at various viewing angles in the horizontal direction for the medical display was also examined by eight observers based on contrast‐detail diagrams. Results: The two types of LCDs showed notable variations in luminance and contrast ratios as a function of the viewing angle. Results obtained in different directions (horizontal, vertical and diagonal directions) indicated different contrast ratios. Acceptable viewing angles in terms of the contrast ratio were much smaller in each direction than those for nominal viewing angles in the specifications provided by the manufacturers. Acceptable viewing angles of the medical‐grade display in the horizontal and vertical directions were broader than those of the general purpose display. There was no significant difference in observer performance between 0 and 40 degrees. On the other hand, our results showed a statistically significant difference in observer performance between 0 and 60 degrees. Conclusion: Contrast ratio and luminance response showed notable variations with different viewing angles and different types of LCD. More importantly, the detection of contrast‐detail materials of the LCD monitor was influenced by viewing angles.

A Subfield Coding Algorithm for the Reduction of Gray Level Errors Due to Line Load in a Plasma Display Panel

Displayed gray level of a plasma display panel (PDP) is often different from the intended target gray level due to the variation of the line load (the number of on-cells on a scan line). The conventional gray level generation algorithm does not take this difference into consideration and may deteriorate the displayed image quality. In this paper, luminance ratio variations that depend on the line load are formulated as the third-order regression equations for red, green and blue colors, respectively. Based on these equations, this paper proposes a new gray-level generation algorithm to reduce gray-level errors caused by the variation of the line load. The proposed algorithm performs the conventional algorithm three times with new weight vectors (i.e., the expected gray levels of subfields) adjusted based on the detected error of the previous run of the conventional algorithm. Simulations and experiments with a 50-in PDP show significant error reductions, including a 75.6% improvement of the mean absolute error evaluated with nine video sequences.

Resolution acuity for equiluminant gratings of S-cone positive or negative contrast in human vision

We measured resolution acuity separately for gratings with positive (+S) and negative (-S) S-cone contrast at different eccentricities in the temporal retina to determine if the resolution performance is limited by a single bipolar opponent mechanism. Gratings were modulated from an achromatic background in either the 90 or 270 deg direction in DKL space. Two additional directions were tested to examine the effects of increased lens yellowing and macular pigment absence in the periphery. The gratings used slow temporally rising hue (co-sinusoidal step) to aid selective S-cone ON or OFF stimulation. Resolution acuity was measured using different luminance ratios of the grating bars with the background around the equiluminant point. The data displayed a plateau where the acuity was minimal and mediated by the sparse S-cone pathway, with observer performance rising on either side. The acuity at the minimum was consistently higher for +S gratings than -S gratings beyond 10 deg eccentricity. Above the resolution limit, an aliased pattern was detected for both types of gratings, indicating that the resolution acuity was limited by the density of postreceptoral neurons. The results suggest that the resolution of gratings with opposite S-cone contrast polarity is limited by separate and rectified mechanisms of different distribution.

The comparative study of the performance of display devices in medical imaging system

Objective To analyze the effects of three different display devices on the interpretation of imaging data in medieal imaging system.Methods The luminance-meter L100 and the test patterns provided by the American Association of Physicists in Medicine(AAPM)were used to assess the performance of three display devices(the spherical,flat panel and liquid crystal display devices).The corresponding luminance response was compared with the reference criteria provided by AAPM Task Group 18(AAPM TG18).DR chest photography was taken on CDRAD2.0 contrast-detail phantom using the following experimental parameters:100 mA,80 kVp,and the exposure times for 6,10,12,and 16 ms.After each exposure,the surface dose of phantom was recorded and the image quality factor(IQF)was calculated.Statistical analysis of IQF was performed using ANOVA. Results The maximum luminance values in the spherical,flat panel and liquid crystal display devices were 170,59 and 231 cd/m2 and the luminance ratio was 257, 99 and 350 respectively.There was a significant difference in the IQF bewteen the liquid crystal display device and other two display devices.Conclusion The liquid crystal display device has the better image quality. Key words: Audiovisual aids; Diagnostic imaging; Evaluation studies

Hue-Shift Modeling and Correction Method for High Luminance Display

The human eye usually experiences a loss of color sensitivity when it is subjected to high levels of luminance, and perceives a discrepancy in color between high and normal-luminance displays, generally known as a hue shift. Accordingly, this paper models the hue-shift phenomenon and proposes a hue correction method to provide perceptual matching between high and normal-luminance displays. The modeling of the hue-shift phenomenon is determined by perceived hue matching experiments. To quantify the hue-shift phenomenon for the whole hue angle, 24 color patches with the same lightness are first created and equally spaced inside the hue angle for three lightness levels. These patches are then displayed one-by-one on both displays with the ratio of luminance between the two displays. Next, the hue value for each patch appearing on the high luminance display is adjusted by observers until the perceived hue for the patches on both displays appears the same visually. After obtaining the hue-shift values from the perceived hue matching experiment, these values are fit piecewise into seven sinusoidal functions to allow the shifted-hue amounts to be approximately determined for arbitrary hue values of pixels in a high luminance display and then used for correction. Essentially, the input red green blue (RGB) values of an image are converted to CIELAB values by a forward characterization model, and then, LCh (lightness, chroma, and hue) values are calculated to obtain the hue values for all the pixels. These hue values are shifted according to the amount calculated by the functions of the hue-shift model. Finally, the corrected CIELAB values are calculated from lightness, chroma, and corrected hue values, and after that, the output RGB values for all pixels are estimated by an inverse characterization model. For evaluation, an observer's preference test was performed using several test images with other hue shift results and the comparison results are shown with a z score.

Mathematical model of light propagation through liquid crystal display working under real conditions

AbstractThe main aim of our work is to present a theory of light propagation through a liquid crystal display worked out in our University. This theory takes into account real conditions of a display operation such as temporal coherence of light source, interference phenomena, spectral characteristics of refractive coefficients of individual layers and their complex forms, real directions of ordinary and extraordinary wavevectors into each layer, real directions of light polarization into anisotropic or dichroic layers etc. Therefore, our theory can be used to determine optical parameters of a liquid crystal display operating under real or at least quasi-real conditions. Contrary to other theories (e.g., Beremann or geometric optics approximation), this model is characterized by a small number of simplifications. Thus, it can be used to analyse a display operating in special conditions, such as military or out-door applications. In these cases, very high optical parameters (luminance and contrast ratio) aare needed. Application of even small simplifications conducts to significant changes of calculated parameters.

Effects of plasma loss on the luminous efficacy in a full high definition alternating current plasma display panel

We have investigated the effect of cell resolution change on the luminous efficacy of an alternating current plasma display panel with the three-dimensional plasma simulation. The luminous efficacy decreased and the minimum sustain voltage increased as the resolution increased from VGA to full high definition (HD) resolution, due to the increased particle loss to the enclosing surfaces. A simple increase of the Xe content to realize the high luminous efficacy resulted in a much enhanced luminous efficacy decremental ratio as the cell resolution increases because of the increased plasma loss and decreased electron heating efficacy. We are proposing some effective ways of realizing high luminous efficacy in the high resolution 50-in. full HD cell, such as the increase of the sustain electrode gap, the adoption of a segmented electrode in delta color arrayed, enclosed subpixel, type cell with 4:3 aspect ratio, and the optimization of the barrier rib height. Because the adoption of those geometrical parameters causes the reduced plasma loss, the transport efficacy of visible light from phosphor through the front plate in a full HD cell resulted in much increased improvement with the increase of Xe content.