Diffuse Illumination Research Articles

Estimation of direct, diffuse, and total FPARs from Landsat surface reflectance data and ground‐based estimates over six FLUXNET sites

AbstractThe fraction of photosynthetically active radiation (PAR) absorbed by green elements (FPAR) is an essential climate variable in quantifying canopy absorbed PAR (APAR) and gross and net primary production. Current satellite FPAR products typically correspond to black‐sky FPAR under direct illumination only, but the radiation transfer and vegetation absorption processes differ for direct and diffuse PARs. To address this, the present study developed a new approach to estimate direct, diffuse, and total FPARs, separately, from Landsat surface reflectance data. Field‐measured direct and diffuse FPARs were first derived for crops, deciduous broadleaf forests, and evergreen needleleaf forests at six FLUXNET sites. Then, a coupled soil‐leaf‐canopy radiative transfer model (SLC) was used to simulate surface reflectance under direct and diffuse illumination conditions. Direct, diffuse, and total FPARs were estimated by comparing Landsat‐5 Thematic Mapper (TM) data and simulated surface reflectances using a lookup table approach. The differences between the Landsat‐estimated and the field‐measured FPARs are less than 0.05 (10%). The diffuse FPAR is higher than the direct FPAR by up to 19.38%, whereas the total FPAR is larger than the direct FPAR by up to 16.07%. The direct APAR is higher than the diffuse APAR under clear‐sky conditions, but underestimates the total APAR by −277.72 µmol s−1 m−2 on average. The approach described here can be extended to estimate direct, diffuse, and total FPARs from other satellite data and the obtained FPAR variables could be helpful to improve modeling of vegetation processes.

A 0.53 THz Reconfigurable Source Module With Up to 1 mW Radiated Power for Diffuse Illumination in Terahertz Imaging Applications

This paper presents a high-power 0.53 THz source module with programmable diversity to adjust the brightness and the direction of light to obtain the desired diffuse lighting conditions in THz imaging applications. The source module consists of a single SiGe BiCMOS chip which operates an array of 16 source-pixel incoherently. Each source pixel consists of a primary on-chip ring-antenna and two triple-push oscillators locked 180° out-of-phase. The module provides a total radiated power of up to 1 mW (0 dBm) with 62.5 μW (-12 dBm) per source pixel on average and an EIRP per pixel of 25 dBm. The circuit layout is scalable in size and output power. The chip consumes up to 2.5 W from a 2.4 V supply and 3.2 mW from a digital 1.2 V supply respectively. The module includes a secondary silicon lens, is programmable through a CPLD, and supplied from a USB port. The THz radiation can be recorded with a CMOS 1 k-pixel THz video camera and represent an all silicon solution for real-time active THz imaging.

A formula for human average whole-body SARwb under diffuse fields exposure in the GHz region

A simple formula to determine the human average whole-body SAR (SARwb) under realistic propagation conditions is proposed in the GHz region, i.e. from 1.45 GHz to 5.8 GHz. The methodology is based on simulations of ellipsoidal human body models. Only the exposure (incident power densities) and the human mass are needed to apply the formula. Diffuse scattered illumination is addressed for the first time and the possible presence of a Line-of-Sight (LOS) component is addressed as well. As validation, the formula is applied to calculate the average whole-body SARwb in 3D heterogeneous phantoms, i.e. the virtual family (34 year-old male, 26 year-old female, 11 year-old girl, and 6 year-old boy) and the results are compared with numerical ones—using the Finite-Difference Time-Domain (FDTD) method—at 3 GHz. For the LOS exposure, the average relative error varies from 28% to 12% (resp. 14–12%) for the vertical polarization (resp. horizontal polarization), depending on the heteregeneous phantom. Regarding the diffuse illumination, relative errors of −39.40%, −11.70%, 10.70%, and 10.60% are obtained for the 6 year-old boy, 11 year-old girl, 26 year-old female, and 34 year-old male, respectively. The proposed formula estimates well (especially for adults) the SARwb induced by diffuse illumination in realistic conditions. In general, the correctness of the formula improves when the human mass increases. Keeping the uncertainties of the FDTD simulations in mind, the proposed formula might be important for the dosimetry community to assess rapidly and accurately the human absorption of electromagnetic radiation caused by diffuse fields in the GHz region. Finally, we show the applicability of the proposed formula to personal dosimetry for epidemiological research.

Investigation of Useful Daylighting during a Day

Annual availability of daylight depends on exterior luminous conditions. If sunny sky is presented the interiors are illuminated by direct sunlight and diffuse skylight or only by skylight in dependence on the orientation and cloudiness. Under sunless situations only skylight is source of daylighting. The minimum illuminance expressed by Daylight Factor is required for performance of visual tasks by the Slovak standards. Traditionally, the overcast situation with exterior illuminance 5000 lx represents this condition. According to recent studies the occurrence of overcast sky in a year is not dominant in Europe and new metrics are looking for. It was published that minimum 100 lx and adequate of 300 – 500 lx is sufficient for the healthy and comfortable indoor environment. Direct sunlight can produce glare and visual discomfort. Bratislava daylight measurements show that such situations can occur often during a year.The typical daily diffuse and global illuminance courses were selected from Bratislava measurements. The periods in the day of useful daylighting, periods of exterior illuminance below 5000 lx and direct irradiance above 120 W.m-2 when sunshine can cause glare and overheating were identified in a classroom. Paper presents achieved results, their evaluation and discussion.

Mapping Layers of Clay in a Vertical Geological Surface Using Hyperspectral Imagery: Variability in Parameters of SWIR Absorption Features under Different Conditions of Illumination

Hyperspectral imagery of a vertical mine face acquired from a field-based platform is used to evaluate the effects of different conditions of illumination on absorption feature parameters wavelength position, depth and width. Imagery was acquired at different times of the day under direct solar illumination and under diffuse illumination imposed by cloud cover. Imagery acquired under direct solar illumination did not show large amounts of variability in any absorption feature parameter; however, imagery acquired under cloud caused changes in absorption feature parameters. These included the introduction of a spurious absorption feature at wavelengths > 2250 nm and a shifting of the wavelength position of specific clay absorption features to longer or shorter wavelengths. Absorption feature depth increased. The spatial patterns of clay absorption in imagery acquired under similar conditions of direct illumination were preserved but not in imagery acquired under cloud. Kaolinite, ferruginous smectite and nontronite were identified and mapped on the mine face. Results were validated by comparing them with predictions from x-ray diffraction and laboratory hyperspectral imagery of samples acquired from the mine face. These results have implications for the collection of hyperspectral data from field-based platforms.

Clear sky solar illuminance using visual band irradiance

Solar illuminance may be quantified by applying relevant efficacy functions to available full spectrum ground irradiance. Alternatively, illuminance may be determined by applying the Photopic function to ground level irradiance spectra obtained from the terrestrial irradiance spectrum adjusted using relevant atmospheric absorption and scattering coefficients. The Photopic function has finite values in the wavelength range of 400 nm to 700 nm and is concentrated around a mean wavelength of 555 nm with irradiance at wavelengths close to 555 nm contributing to the major portion of illuminance. Calculation of ground level direct, diffuse and hence global illuminance is simplified as absorption of irradiance in the atmosphere by water vapour and uniformly mixed gases is negligible and may be ignored. It is shown that due to the small variations in the overall irradiance over wavelengths in the visible bandwidth combined with the concentration effect of the Photopic function in this wavelength range illuminance may be calculated, with good accuracy, using constant extinction functions for both direct and diffuse illuminance. Due to the concentration of illuminance by the Photopic function global illuminance data were also correlated with a simplified description of illuminance attenuation through the atmosphere based on the Beer-Lambert-Bouger Law using a single constant effective extinction coefficient which accounts for all extinction processes under clear sky conditions over the visible range of wavelengths. Constants used in extinction functions for solar irradiance due atmospheric aerosols were obtained by fitting experimental data to analytical descriptions of atmospheric extinction while published constants were used for ozone and Rayleigh scattering. The analytical descriptions of global illuminance with solar elevation were compared with experimental data collected at Johannesburg over both summer and winter clear sky conditions. Correlations of measured and calculated global illuminance data for the method based on extinction of various atmospheric components was 4.47 % and 4.49 % for the method based on the Beer-Lambert-Bouger Law, both normalised using the terrestrial illuminance constant. While measurements were made at a specific site in Gauteng, the methods used to correlate the data are general and location independent but local climatic conditions may need to be taken into account to quantify the extinction coefficients for specific areas.

Application of Perez Daylight Efficacy Model for Kolkata

The availability of daylight on any plane of a building and the passive heat gain due to the corresponding incident solar radiation is correlated by daylight luminous efficacy. The present study was carried out to evaluate daylight efficacy on exterior horizontal plane for Kolkata using Perez Model an established analytical model and Indian measured solar radiation database. The analytical model was selected according to the sky conditions throughout the year. The estimated efficacy models were used to predict horizontal global and diffuse illuminances. The predicted global and diffuse illuminance was validated with the measured illuminance values using standard statistical tools. The estimated Root Mean Square Deviation (RMSD) and Mean Bias Deviation (MBD) between experimental and simulated values of illuminance are +7 % and ±4.5 % respectively. An analytical model of luminous efficacy for Kolkata was also developed from the measured values. From the study, it has been found that diffuse luminous efficacy varies from 114–138 lm/W whereas global luminous efficacy varies from 98–112 lm/W in a year. It is also observed that sky clearness index and diffuse irradiance ratio have significant effect on the variations of luminous efficacy in different seasons.

Transparent, 3-dimensional light-collected, and flexible fiber-type dye-sensitized solar cells based on highly ordered hierarchical anatase TiO2 nanorod arrays

Two kinds of hierarchical anatase TiO2 structures are synthesized by a facile hydrothermal method in this report. A new transparent, 3D light-collected, and flexible fiber-type dye-sensitized solar cell (FF-DSSC) with such hierarchical TiO2 structures is developed. The conversion efficiency of the FF-DSSC based on a TiCl4-treated TiO2 nanorod array (hierarchical structure I) exhibits about 4 times higher than that based on a HCl-treated TiO2 nanorod array, and further rises to 4.4% when the TiCl4-treated TiO2 nanorod array is treated in a mixed solution of (NH4)2TiF6 and H3BO3 three times (hierarchical structure II). The obvious enhancement in conversion efficiency can be ascribed to the dye adsorption promotion benefiting from their hierarchical structures. Beyond the attractive conversion efficiency, the new designed FF-DSSC possesses several advantages including good flexibility, excellent stability, and 3D light-collection. The conversion efficiencies of the FF-DSSCs can still keep 85%–90% even the FF-DSSCs are bent for 1000 times. The maximum power outputs of the FF-DSSCs characterized by Diffuse Illumination Mode using home-made Al reflector exhibit about 3 times higher than that done by Standard Illumination Mode due to 3D light-collections. The FF-DSSCs based on highly ordered hierarchical anatase TiO2 nanorod arrays hold great promise in future energy harvest.

Dynamic responses in retinal vessel caliber with flicker light stimulation in eyes with diabetic retinopathy.

This study investigated the responses of retinal vessels to flickering light in diabetic patients with various stages of diabetic retinopathy (DR). This cross-sectional observational study evaluated adult subjects with diabetes mellitus. The Dynamic Vessel Analyzer (DVA) was used to measure retinal vascular dilatation in response to diffuse illuminance flicker. Diabetic retinopathy was graded from retinal photography. There were 279 subjects in total, with a mean age of 59.9 ± 9.2 years. The majority were male (73%) and the mean HbA1c level and mean duration of diabetes were 7.7% ± 1.4% and 13.9 ± 10.4 years, respectively. After adjustments for age, sex, smoking, duration of diabetes, HbA1c, hypertension, and hyperlipidemia, the responses of both retinal arterioles and venules to flicker stimulation decreased continuously with increasing stages of diabetic retinopathy (P = 0.008 and <0.001, respectively). Subjects with reduced arteriolar dilation responses were more likely to have any DR (odds ratio, OR, 1.20, [95% confidence interval (CI), 1.01-1.45], P = 0.045, per SD decrease). Subjects with reduced venular dilation responses were more likely to have any DR, moderate DR, or vision-threatening DR (OR: 1.27 [1.04-1.53], P = 0.02; OR: 1.27 (1.06-1.49), P = 0.007; and OR: 1.51 (1.14-1.50), P = 0.002; per SD decrease, respectively). The responses of retinal arterioles and venules to flickering light are reduced in subjects with DR, and decrease progressively with more severe stages of DR.

Assessment of the Effect of Distance and Duration of Illumination on Retinal Pigment Epithelial Cells Exposed to Varying Doses of Brilliant Blue Green

To assess the cytotoxicity of varying concentrations of Brilliant Blue Green (BBG) on human retinal pigment epithelial (HRPE) cells exposed to metal halide surgical endoilluminator (SE) at varying distances of illumination. HRPE (ARPE-19) were exposed to 2 concentrations (0.25 and 0.5 mg/mL) of BBG and illuminated with SE for 1, 5, and 15 min. Illumination (measured with light meter) was positioned at varying distances (1 and 2.5 cm) to mimic surgical distance of illumination. Cell viability (WST-1 assay) as well as structural changes in cells was quantified. At 1 cm distance of illumination, 0.25 mg/mL BBG decreased viability of HRPE to 89.6%±4.3%, 83.9%±10.9%, and 38.9%±5.1% of controls after 1, 5, and 15 min of exposure, respectively. Similarly, 0.5 mg/mL BBG at 1 cm distance of illumination reduced the viability of HRPE to 93.7%±2.8%, 59.6%±16%, and 34.7%±3.5% of controls. At the distance of 2.5 cm, HRPE showed improved viability; cells exposed to 0.25 mg/mL BBG maintained 98.85%±3.3%, 95.31%±7.12%, and 62.07%±3.0% of viability compared with controls after 1, 5, and 15 min of exposure. Morphometric evaluation of RPE cells showed increased width (swelling) of HRPE. BBG at its clinically used concentration (0.25 mg/mL) during vitreoretinal surgery is safe and not toxic to HRPE for up to 5 min under focal illumination (1 cm) and up to 15 min under diffuse illumination (2.5 cm) from the commonly used SE. RESULTS of our study are useful in establishing safety parameters for the use of BBG dye in vitreoretinal surgery.

The simultaneous occurrence and relationship of sunlight and skylight under ISO/CIE standard sky types

In daylight science the availability of sunlight and skylight at different times has been studied because of the desire to use daylight in both exterior and interior spaces. Exterior skylight illuminances under overcast skies were adopted as a standard for window design in the past. Current ISO/CIE sky types are standardised as relative luminance patterns normalised to the zenith. In this paper, the zenith luminance in candela/square metre and the resulting diffuse illuminance in lux are determined for all sky types. Furthermore, the proportions of sunlight and skylight under different levels of turbidity are found and documented. Efforts to harmonise electric lighting and daylighting standards need to analyse the data on available daylight in physical units in order to ensure energy savings that respect human requirements as well as providing information suitable for computer-aided design.

37.3: Viewing Direction Measurements with Hemispherical Diffuse Illumination on E‐Paper Displays

AbstractViewing direction measurements on reflective e‐paper displays under hemispherical diffuse illumination are an essential part of characterizing reflectance under common usage conditions such as outdoor shade, or indirect indoor lighting. This paper proposes a hemispherical diffuse illumination geometry based on splitting a sampling sphere into two hemispheres to allow the light measuring device to be pointed at the display at different angles. First results on e‐paper agree well with conventional sampling sphere data, show the effects of top layer and surface reflection, and were used to predict the viewing direction distribution of ambient contrast ratio.

37.1: OLED Displays under Ambient Illumination, an Implementation of IEC 62341‐6‐2

AbstractThis contribution introduces a first implementation of the measurement setup according to IEC 62341‐6‐2 for evaluation of the effect of ambient illumination on OLED display contrast and chromaticity. A sampling sphere is providing hemispherical diffuse illumination, with and without specular components, in the d/8‐geometry, and a point‐light source is providing directional illumination. From the spectral reflectance factors thus obtained contrast and chromaticity for arbitrary illuminants (i.e. spectra) and illuminance levels can easily be calculated.

Arctic canopy photosynthetic efficiency enhanced under diffuse light, linked to a reduction in the fraction of the canopy in deep shade

We investigated how radiation conditions within a tundra canopy were linked to canopy photosynthesis, and how this linkage explained photosynthetic sensitivity to sky conditions, that is total radiation and its diffuse fraction. We measured within canopy radiation at leaf scales and net CO2 exchanges at canopy scales, under varied total irradiance and diffuse fraction, in Alaskan shrub tundra. Normalised mean radiation profiles within canopies showed no significant differences with varied diffuse fractions. However, radiation density distribution was non-normal, being more unimodal under diffuse conditions and distinctly bimodal under direct sunlight. There was a nearly three-fold increase in the proportion of the canopy in deep shade under direct illumination, compared to diffuse conditions. Under diffuse conditions the canopy had higher light-use efficiency (LUE), resulting in up to 17% greater photosynthesis. The enhancement in LUE under diffuse illumination was not related to differences in the mean light profiles, but instead was due to significant shifts in the density distribution of light at leaf scales, in particular a reduced fraction of the canopy in deep shade under diffuse illumination. These results provide unique information for testing radiative transfer schemes in canopy models, and for better understanding canopy structure and trait variation within plant canopies.

The effects of surface gloss and roughness on color constancy for real 3-D objects

Color constancy denotes the phenomenon that the appearance of an object remains fairly stable under changes in illumination and background color. Most of what we know about color constancy comes from experiments using flat, matte surfaces placed on a single plane under diffuse illumination simulated on a computer monitor. Here we investigate whether material properties (glossiness and roughness) have an effect on color constancy for real objects. Subjects matched the color and brightness of cylinders (painted red, green, or blue) illuminated by simulated daylight (D65) or by a reddish light with a Munsell color book illuminated by a tungsten lamp. The cylinders were either glossy or matte and either smooth or rough. The object was placed in front of a black background or a colored checkerboard. We found that color constancy was significantly higher for the glossy objects compared to the matte objects, and higher for the smooth objects compared to the rough objects. This was independent of the background. We conclude that material properties like glossiness and roughness can have significant effects on color constancy.

A method for mapping monthly average hourly diffuse illuminance from satellite data in Thailand

This paper presents a method for mapping monthly average hourly diffuse illuminance from satellite data. The calculation of monthly average hourly diffuse illuminance starts with the estimation of monthly average hourly global illuminance from MTSAT-1R satellite data using an improved satellite-based illuminance model. Next, a diffuse fraction model is developed from ground and satellite-based data which is then used to extract diffuse illuminance from the satellite-derived global illuminance. To assess the performance of the method, modeled diffuse illuminance obtained from this method is compared with that obtained from measurements at four stations in Thailand. There is good agreement between calculated and the measured values of monthly average hourly diffuse illuminance, with the root mean square difference and mean bias difference of 9.7% and −1.4% respectively. The model is used to map monthly average hourly diffuse illuminance for the country. The maps reveal the diurnal and seasonal variations in response to a range of factors including cloud cover, zenith angle and monsoonal effects.

Impact of the New CEN Minimum Illuminance Criterion on the Working Plane Size

The aim of this study is to present a comparison of daylight evaluation after requirements of Slovak standard STN 73 0580 and after the new CEN criterion 300 lux median diffuse horizontal illuminance in three rooms with different plans and different size of windows. Minimum values of illuminance in lux calculated on the border of the working plane were investigated. This study shows how this new CEN criterion influences the length of working plane and size of its part usable for performance of visual tasks.

Tree species identification in aerial image data using directional reflectance signatures

Tree species identification using optical remote sensing is challenging. Modern digital photogrammetric cameras enable radiometrically quantitative remote sensing and the estimation of reflectance images, in which the observations depend largely on the reflectance properties of targets. Previous research has shown that there are species-specific differences in how the brightness observed changes when the viewing direction in an aerial image is altered. We investigated if accounting for such directional signatures enhances species classification, using atmospherically corrected, real and simulated multispectral Leica ADS40 line-camera data. Canopy in direct and diffuse illumination were differentiated and species-specific variance-covariance structures were analyzed in real reflectance data, using mixed-effects modeling. Species classification simulations aimed at elucidating the level of accuracy that can be achieved by using images of different quality, number and view-illumination geometry. In real data, a substantial variance component was explained by tree effect, which demonstrates that observations from a tree correlate between observation geometries as well as spectrally. Near-infrared band showed the strongest tree effect, while the directionality was weak in that band. The gain from directional signatures was insignificant in real data, while simulations showed a potential gain of 1–3 percentage points in species classification accuracy. The quality of reflectance calibration was found to be important as well as the image acquisition geometry. We conclude that increasing the number of image observations cancels out random observation noise and reflectance calibration errors, but fails to eliminate the tree effect and systematic calibration inaccuracy. Directional reflectance constitutes a marginal improvement in tree species classification.

Automated Vertical Blinds for Daylighting in Tropical Region

The use of automatically controlled interior Venetian blinds can provide view and penetration of daylighting through window. In this study, an automatic system of vertical blinds affixed behind windows situated on north-west facades of a laboratory room were developed in order to investigate its daylighting performance. The system controls automatically the blind slats to shade out beam sunlight. Simulation using reference calculation algorithms and sky luminance distribution models and utilizing measured beam sunlight and diffuse daylight from the sky or skylight produces results that agree well with results from experiments. Moreover, using records of beam sunlight and diffuse skylight illuminance, simulation results show that the automated vertical blind and electric lighting system, when used in a room with the same configuration as that of the experimental room, can utilize daylight to save more than 24% of electric lighting energy in every month of the simulated year while maintaining illuminance at the workstation at the required level.

A three-dimensional topographic survey based on two-dimensional image information

A riverbed topographic survey is one of the most important tasks for river model experiments. To improve measurement efficiency and solve the riverbed interference problem in traditional methods, this study discussed two measurement methods that use digital image-processing technology to obtain topographic information. A new and improved approach for calibrating camera radial distortion, which comes from originally distorted images captured by our camera, was proposed to enhance the accuracy of image measurement. Based on perspective projection transformation, we described a 3D reconstruction method based upon multiple images, which is characterized by using an approximated maximum likelihood estimation method (AMLE) considering the first-order error propagation of the residual error to compute transformation parameters. Moreover, a theoretical derivation of 3D topography according to grey information from a single image was carried out. With the diffuse illumination model, assuming that the ideal grey value and topographic elevation value are positively correlated, we derived a novel closed formula to explain the relationship of 3D topographic elevation, grey value, grey gradient, and the solar direction vector. Experimental results showed that our two methods both have some positive advantages even if they are not perfect.