Top Lighting Research Articles

Assessing visual comfort for active building enclosures: a novel simulation workflow for environmentally active top lighting cells

This study examines the impact of top lighting cells in a low-latitude city. It hypothesizes that an environmentally active control system enhances indoor illuminance without causing glare. It proposes a parametric procedure based on three simulation stages using UDI/UDIavg, Image-based, and Imageless DGP. The first stage is a simplified approach for evaluating image-based simulations considering representative dates. The second stage uses the image-based results for selecting the most contributing motion parameters through a One-way Variance-based Sensibility Analysis. The third expands the system's complexity for grid-based year-round simulations. Results indicate tilt motion as the most contributing factor for daylight control. Furthermore, the cells supplement daylight indoors, rendering a minimal increase of approximately 30% of the hours within the adequate illuminance range compared to the base case with no zenithal opening. The system ultimately achieve 100% of the hours in compliance with the selected daylight thresholds without significantly increasing glare.

Intra-canopy LED lighting outperformed top LED lighting in improving tomato yield and expression of the genes responsible for lycopene, phytoene and vitamin C synthesis

Greenhouses located at high latitudes and in cloudy areas often experience a low quality and quantity of light, especially during autumn and winter. This low daily light integral (DLI) reduces production rate, quality, and nutritional value of many crops. This study was conducted on Sakhiya RZ F1 tomato plants to evaluate the impact of LED lights on the growth and nutritional value of tomatoes in a greenhouse with low daily light due to cloudy weather. The treatments included LED growth lights in three modes: top lighting, intra-canopy lighting, and combined top and intra-canopy lighting. The results showed that although the combined top and intra-canopy lighting reached the maximum increase in tomato yield, exposure to intra-canopy LED lighting alone outperformed in tomato fruit yield increase (28.46%) than exposure to top LED lighting alone (12.12%) when compared to no supplemental lighting during the entire production year. Intra-canopy exposure demonstrated the highest increase in tomato lycopene (31.3%), while top and intra-canopy lighting exhibited the highest increase in vitamin C content (123.4%) compared to the control. The LED light treatment also had a very positive effect on the expression of genes responsible for metabolic cycles, including Psy1, LCY-β, and VTC2 genes, which had collinearity with the increase in tomato fruit production.

Research on Lighting Schemes for College Gymnasiums Based on Dynamic Light Environment Simulation

The purpose of this study is to explore and optimize natural lighting schemes for college gymnasiums through dynamic light environment simulation. The study used Daysim software, combined with Rhino & Grasshopper for modeling and parameter optimization, to evaluate different lighting schemes through a multi-objective optimization approach. The results show that the effective indoor illuminance of the top lighting scheme at DA300lx, UDI100-2000lx, aDGP(0.4), DA580lx, and UDI300-890lx are 91.50%, 88.44%, 90.58\%, 92.56%, and 52.89%, respectively, which show good lighting effects. The side lighting scheme also shows excellent lighting performance in different configurations. In addition, the side + top synergistic light harvesting scheme achieved efficient light environment utilization while ensuring visual comfort. This study suggests that the top and side synergistic lighting scheme can effectively enhance the natural lighting effect of the gymnasium. This study provides a scientific basis for the lighting design of college gymnasiums and is of great significance in guiding the actual engineering design.

Side Lighting of Red, Blue and Green Spectral Combinations Altered the Growth, Yield and Quality of Lettuce (Lactuca sativa L. cv. "Yidali") in Plant Factory.

A plant factory with artificial lighting (PFAL) usually uses top lighting for cultivation. The light from the upper part of the canopy cannot penetrate the entire lettuce canopy, however, resulting in uneven vertical spatial light in the canopy, and accelerating the senescence of both the bottom and side leaves of the plant canopy. Therefore, in this study, the performance of lettuce in hydroponics was investigated upon supplemental side lighting with different spectral LEDs in a PFAL. A set of short-term side lighting treatments, including no side lamps (CK), red (R), blue (B), red + blue (RB), and red + blue + green (RGB) LED lamps (150 μmol·m-2·s-1, respectively), was employed for an additional 2 h per day after normal top lighting for 6 days before harvest. The results showed that the lettuce canopy was relatively loose and had a large crown size under side lighting compared with CK. Side lighting, irrespective of spectral qualities, significantly increased the fresh weight, and the R, B, RB, and RGB treatments increased the shoot fresh weight of lettuce plants by 34%, 19%, 31%, and 34%, and increased the fresh weight of leaf layer 2 by 50%, 17%, 44%, and 48%, respectively. The side lighting of different spectral qualities had a significant impact on the nutritional quality of the first row of lettuce at the edge of the top lighting illuminated area. Treatment B significantly promoted the chlorophyll content of leaf layer 3; the soluble sugar contents from leaf layer 1, 2, and 3; the starch contents in leaf layers 2 and 3; and the content of phenolics in the leaf layers 3; and significantly reduced the nitrate content in leaf layers 2 and 3. RGB significantly increased soluble sugar content by 91%, and the starch content in leaf layer 1, as well as the leaf chlorophyll and flavonoid content of leaf layer 3, while R had opposite effect completely. RB significantly increased the leaf chlorophyll content of leaf layer 3 and the nitrate content in leaf layer 1, but the overall effect was lower than that of RGB. In summary, side lighting of any type could effectively improve lettuce yield, solve the problem of inconsistent lettuce plant size caused by the edge effect of top lighting, and affect the nutritional quality of lettuce. B and RGB performed best. There was spatial response diversity of lettuce plants to side lighting spectral qualities.

Counting Murine Corpora Lutea to Determine the Number of Oocytes Ovulated.

One can determine if and how many oocytes have been ovulated in a female mouse by counting the number of corpora lutea (CL) on the ovaries during the process of preimplantation embryo collection. A simple method of harvesting the ovaries and observing with a dissecting microscope and top lighting is provided along with a description of how to recognize CL. As the embryos rarely, if ever, cross the uterotubal junction, this provides a measure of the maximum number of embryos expected to be recovered from each oviduct or uterine horn, a number that can be valuable in analyzing early lethal mutations.

Top and Side Lighting Induce Morphophysiological Improvements in Korean Ginseng Sprouts (Panax ginseng C.A. Meyer) Grown from One-Year-Old Roots.

Nowadays, not only the roots, but also leaves and flowers of ginseng are increasingly popular ingredients in supplements for healthcare products and traditional medicine. The cultivation of the shade-loving crop, ginseng, is very demanding in terms of the light environment. Along with the intensity and duration, light direction is another important factor in regulating plant morphophysiology. In the current study, three lighting directions-top (T), side (S), or top + side (TS)-with an intensity of 30 ± 5 μmol·m-2·s-1 photosynthetic photon flux density (PPFD) were employed. Generally, compared with the single T lighting, the composite lighting direction, TS, was more effective in shaping the ginseng with improved characteristics, including shortened, thick shoots; enlarged, thick leaves; more leaf trichomes; earlier flower bud formation; and enhanced photosynthesis. The single S light resulted in the worst growth parameters and strongly inhibited the flower bud formation, leading to the latest flower bud observation. Additionally, the S lighting acted as a positive factor in increasing the leaf thickness and number of trichomes on the leaf adaxial surface. However, the participation of the T lighting weakened these traits. Overall, the TS lighting was the optimal direction for improving the growth and development traits in ginseng. This preliminary research may provide new ideas and orientations in ginseng cultivation lodging resistance and improving the supply of ginseng roots, leaves, and flowers to the market.

Supplementary Top LED Lighting Improved Physiological Parameters and the Postharvest Life of Cut Rose Flowers

Supplementary Top LED Lighting Improved Physiological Parameters and the Postharvest Life of Cut Rose Flowers

The Influence of Multi-Storey Surrounding Development on a Daylighting of Interiors in Top-Lit Premises: An Experimental Study

To ensure a required daylight in premises with top system of outdoor illumination through roof lighting installations, it is necessary to take into account the photometrical effects of surrounding development. For the time being such a concept in lighting engineering design is adopted only for side lit premises through windows, though the dense urban development in modern megalopolises offers nearly the same problem for buildings with overhead daylighting system. This paper presents a brief description of experimental case studies, based on the scale model method, as well as on the photometric studies of a real premise with skylights. The studies were conducted in both cases under CIE standard overcast sky with changing geometrics and photometrical parameters of model elements. The results of the studies showed the great photometric influence of opposite-standing objects on the values of daylight factor (D) in premises with top lighting system and dependence of this influence on size, position, and structural type of skylights, as well as on size and finishing of the model parts and on distance between skylights and opposite-standing building.

Consequences of intra-canopy and top LED lighting for uniformity of light distribution in a tomato crop.

In the past decade, the potential of positioning LED lamps in between the canopy (intra-canopy) to enhance crop growth and yield has been explored in greenhouse cultivation. Changes in spatial heterogeneity of light absorption that come with the introduction of intra-canopy lighting have not been thoroughly explored. We calibrated and validated an existing functional structural plant model (FSPM), which combines plant morphology with a ray tracing model to estimate light absorption at leaflet level. This FSPM was used to visualize the light environment in a tomato crop illuminated with intra-canopy lighting, top lighting or a combination of both. Model validation of light absorption of individual leaves showed a good fit (R2 = 0.93) between measured and modelled light absorption of the canopy. Canopy light distribution was then quantified and visualized in three voxel directions by means of average absorbed photosynthetic photon flux density (PPFD) and coefficient of variation (CV) within that voxel. Simulations showed that the variation coefficient within horizontal direction was higher for intra-canopy lighting than top lighting (CV=48% versus CV= 43%), while the combination of intra-canopy lighting and top lighting yielded the lowest CV (37%). Combined intra-canopy and top lighting (50/50%) had in all directions a more uniform light absorption than intra-canopy or top lighting alone. The variation was minimal when the ratio of PPFD from intra-canopy to top lighting was about 1, and increased when this ratio increased or decreased. Intra-canopy lighting resulted in 8% higher total light absorption than top lighting, while combining 50% intra-canopy lighting with 50% top lighting, increased light absorption by 4%. Variation in light distribution was further reduced when the intra-canopy LEDs were distributed over strings at four instead of two heights. When positioning LED lamps to illuminate a canopy both total light absorption and light distribution have to be considered.

COMPARISON OF DIFFERENT ROOF TYPES IN TERMS OF LIGHTING CONDITIONS IN AN INDUSTRIAL HALL

The indoor climate of industrial buildings is a function of production technology and requirements for the creation of an optimal artificial material environment. Currently, we consider daylight not only as a source of illumination, but also as an aesthetic element of a building or a way of reducing energy consumption. Light in a closed space allows a person to obtain basic visual information (perception) and perform visual tasks. Top lighting schemes can provide increasingly more useful illumination from smaller apertures than side lighting when they capture and diffuse sunlight. Sunlight is roughly 10 times brighter than light from the sky or clouds. A combined lighting system (top lighting and side lighting) ensures a better light distribution in industrial buildings. In this study, we present a comparison of daylight factors for different types of skylights. Specifically, a saddle skylight in the hall and three other types of skylights were created and simulated. In all the cases, the models of skylights were prepared and simulated using RADIANCE. Additionally, a comparison of simulation results obtained with RADIANCE was conducted to quantify the lighting climate. Overall, saddle roof was considered as the best choice for daylight in an industrial hall.

Visual measurement of valve opening area with improved subpixel edge location

The opening area error of the gas valve is mainly caused by machining, assembling errors and transmission clearances. Accurate measurement of the opening area is important for the production quality inspection of gas valves. This paper proposes a novel solution with a specific measurement system. Due to the reflective effect of the valve surface, the valve opening’s saliency is enhanced by the combination of top lighting and camera exposure. Then, a partial image is used to extract the single closed edge of the valve opening. To obtain the precise edge position, a subpixel location based on the rotating area effect is applied. Finally, the area computation based on the minimum circumscribed circle and the maximum inner circle of the edge is provided. The experiment results prove the system reliability, and the general significances of the proposed algorithms.

Side Lighting Enhances Morphophysiology by Inducing More Branching and Flowering in Chrysanthemum Grown in Controlled Environment.

Light is one of the most important factors that influence plant growth and development. This study was conducted to examine how lighting direction affects plant morphophysiology by investigating plant growth parameters, leaf anatomy, epidermal cell elongation, stomatal properties, chloroplast arrangement, and physiological changes. In closed-type plant factory units, the rooted cuttings of two chrysanthemum (Chrysanthemum morifolium Ramat.) cultivars, ‘Gaya Glory’ and ‘Pearl Egg’, were subjected to a 10 h photoperiod with a 300 μmol∙m−2∙s−1 photosynthetic photon flux density (PPFD) provided by light-emitting diodes (LEDs) from three directions relative to the plant including the top, side, and bottom. Compared to the top or bottom lighting, the side lighting greatly enhanced the plant growth, improved the leaf internal structure and chloroplast arrangement, induced small stomata with a higher density, and promoted stomatal opening, which is associated with an increased stomatal conductance and photosynthetic efficiency. It is worth noting that the side lighting significantly enhanced the induction of branching and flowering for both cultivars., The plants grown with side lighting consistently exhibited the greatest physiological performance. We conclude that the lighting direction had a profound effect on the morphophysiological characteristics of chrysanthemum, and that side lighting dramatically promoted their growth and development, especially in their branching and flowering.

Photosynthetic Efficiency and Yield of Cucumber (Cucumis sativus L.) Grown under HPS and LED Lighting in Autumn-Winter Cultivation.

The objective of this study was to evaluate the effect of the supplemental lighting of cucumber with sodium pressure lamps (HPSs) and light-emitting diodes (LEDs) on photosynthetic efficiency and yield in autumn–winter cultivation. Cucumber plants of the ‘Svyatogor’ F1 midi-cucumber parthenocarpic type cultivar were grown on mineral wool mats in three compartments, differing only in the type of light, i.e., (1) HPS top lighting (HPS) in the first compartment, (2) HPS top lighting and LED panel interlighting (HPS + LED) in the second compartment and (3) LED top lighting and inter-row LED panels (LED) in the third compartment. The photosynthetically active radiation was the same in each compartment. The study showed that the highest commercial yields of cucumber could be achieved under LED light (top and inter-row). The chlorophyll content in the leaf blade of younger leaves was higher in plants under LED lighting. This type of lighting also had a positive effect on the gas exchange of plants (net carbon assimilation, stomatal conductance, transpiration). LED and HPS + LED lighting increased the chlorophyll a fluorescence parameters, such as Fs, Fm’ and vitality index (PI), in both younger and older leaves, which also increased the fruit yield in the tested combinations.

Effects of supplemental lighting using HPS and LED lamps with different light spectra on growth and yield of the cucumber (Cucumis sativus L.) during winter cultivation in greenhouse

Abstract The aim of the experiment was to assess the effects of supplemental lighting of cucumber grown in greenhouse using lamps with differentiated light spectra: sodium lamps – high-pressure sodium (HPS) and light emitting diodes (LEDs). Plants (cucumber ‘Pacto’ F1) were grown in two greenhouse compartments with five light treatments: I – HPS + LED (top light with HPS LEDs as interlighting), II – only HPS as top light, III – LED R (LED chips on board (COB) type with an increased level of red band), IV – LED W (LED COB type, white), V – LED B (LED COB type with an increased level of blue spectrum). Light treatments: HPS + LEDs and HPS were grown in one greenhouse compartment and the other three light treatments: LED R (red light supplementation), LED W (without additional supplementation) and LED B (red spectrum supplementation) in the second compartment in analogous climatic conditions. The LED lamps using COB technology are known to be a very efficient source of light. Plants were cultivated from December 2018 to March 2019 in mineral wool slabs Grotop Master (100 × 15 × 10) with four plants on one mat. They were illuminated for 18 h (from 5 am to 11 pm), setting the threshold value (on and off) at 130 W. The plants were drip-irrigated with a complete nutrient solution. The irrigation was controlled based on a weighting system. The assessment of the effect of lighting on early yield and quality of cucumbers was completed after 8 weeks of cropping. It was shown that it was possible to obtain 3.59 kg from one plant during the 8-week period of evaluation by illuminating plants with sodium lamps (HPS), while using HPS and LEDs as additional illuminated inter-rows 3.89 kg. The yield of plants illuminated by LED lamps varied depending on the variant of the spectrum used and was respective for LED R, LED W and LED B, 3.30 kg, 3.90 kg and 3.25 kg. The obtained results indicated that the yield of cucumber ‘Pacto’ F1 grown with HPS lamps at top lighting and at the same time using interlighting with LED lamps was similar to LED W lamps (i.e. without additional supplementation in the range of red (LED R) and blue (LED B) light). Due to good results of LED lamps (type COB for top lighting or as interlighting) used for the cucumber supplemental lighting and high energy efficiency of LEDs, the promising future for that type of lamps compared with traditionally used HPS during winter cultivation in a greenhouse was demonstrated.

Lighting Direction Affects Leaf Morphology, Stomatal Characteristics, and Physiology of Head Lettuce (Lactuca sativa L.).

Plants are exposed to numerous biotic and abiotic stresses, and light is one of the most important factors that influences the plant morphology. This study was carried out to examine how the lighting direction affected the plant morphology by investigating the growth parameters, epidermal cell elongation, stomatal properties, and physiological changes. Seedlings of two head lettuce (Lactuca sativa L.) cultivars, Caesar Green and Polla, were subjected to a 12 h photoperiod with a 300 μmol·m−2·s−1 photosynthetic photon flux density (PPFD) provided by light emitting diodes (LEDs) from three directions: the top, side, and bottom, relative to the plants. Compared with the top and side lighting, the bottom lighting increased the leaf angle and canopy by stimulating the epidermal cell elongation in leaf midrib, reduced the leaf number and root biomass, and induced large stomata with a low density, which is associated with reduced stomatal conductance and carbohydrate contents. However, the proline content and quantum yield exhibited no significant differences with the different lighting directions in both cultivars, which implies that the plants were under normal physiological conditions. In a conclusion, the lighting direction had a profound effect on the morphological characteristics of lettuce, where the plants adapted to the changing lighting environments.

Energy consumption analysis of glass house using electrochromic window in the subtropical region

PurposeElectrochromic window (ECW) has been gradually applied to building engineering in recent years. However, empirical study of this technology used in green building design is still lacking. This study aims to verify the lighting and energy-saving performance of ECW under a specific geographical environment.Design/methodology/approachThe meditation pavilion of Jinwan new materials research institute in Zhuhai is taken for research object. Three kinds of sun-shading techniques, namely, ECW, Low-E window and ordinary glass window, with same specifications are selected as the building facade structure for simulation. Day lighting analysis, sun-shading performance and annual energy consumption are separately simulated in the same environment by the Autodesk Ecotect Analysis software. The energy-saving performance of ECW is obtained by comparisons.FindingsResult shows that the shading performance of ECW is much better than ordinary window and Low-E window. When ECW is used in the east, west or top lighting interfaces of a building, about 40% of the total solar radiation can be reduced during daytime in summer. Taking the ordinary glass window as a basic reference, ECW can save about 90% of the annual energy consumption of the glass house. ECW can effectively reduce the annual refrigeration energy consumption of buildings in the subtropical region.Practical implicationsReasonable use of ECW in the subtropical region can effectively reduce the annual energy consumption of buildings.Originality/valueIt is a precedent study to analyze the lighting performance and energy consumption of a glass house with ECW. The energy-saving characteristics and beautiful appearance of ECW shall make it a future green building technology.

Influence of Roof Windows Area Changes on the Classroom Indoor Climate in the Attic Space: A Case Study

Windows are a complex part of building design and provide a considerable benefit, including to school buildings. For the evaluation of the daylighting conditions prevailing in classrooms, the daylight factor (DF) was considered as the most appropriate parameter for indicating the quantity of admitted daylight. The DF values and CIE overcast sky were calculated using Velux Daylight Visualizer 3 software. The task of the paper is to compare various roof window openings in relation to the level of daylight in the attic, looking to optimize the use of the attic for teaching. The indoor air temperature has a general influence on comfort in the interior, in addition to daylight. In winter, the situation is not critical. The thermal insulation properties of packaging structures are sufficient. The situation is worse in summer, due to the fact that the heat-storage properties are undersized and there is excessive overheating of the indoor air. Four variants of roof windows and their influence on the overall microclimate in the attic are compared. The variant without roof windows is a suitable solution with regard to minimum overheating, but the worst situation for daylight. In order to receive even more light from the window (by moving windows to the top of the roof), we can use variant 2. Based on a combination of daylight calculations and summer temperature, a graphical dependence on window size prediction in terms of top and combined lighting is derived. This was hypothesized without shading the windows. Of course, the shading elements of these windows or cooling are expected in the summer. Finally, the energy required for cooling is compared depending on the size of the windows and achievement of the permissible temperature.

Continuous and Intermittent Light at Night, Using Red and Blue LEDs to Suppress Basil Downy Mildew Sporulation

Lighting from red and blue light-emitting diodes (LEDs) is common for crop production in controlled environments. Continuous application of red or blue light at night has been shown to suppress sporulation by Peronospora belbahrii, the causal organism of basil downy mildew (DM), but the suppressing effects of intermittent applications of red and blue LEDs have not been thoroughly researched. This study examined the effects of red (λmax = 670 nm) and blue (λmax = 458 nm) LED top lighting, at two photosynthetic photon flux densities (PPFD = ≈12 and ≈60 µmol·m−2·s−1), using continuous (10-hour) nighttime and two intermittent nighttime exposures, to suppress basil DM sporulation. The two intermittent treatments consisted of one 4-hour exposure and three 1.3-hour exposures spaced 3 hours apart. Continuous nighttime treatments with blue or red LED top lighting at ≈60 µmol·m−2·s−1 were able to suppress basil DM sporulation by more than 99%. At a given nighttime dose of light that did not completely suppress sporulation, continuous lighting was more effective than intermittent lighting, and for these partially suppressing doses, red LEDs were not significantly different from blue LEDs for suppressing sporulation. The present study showed that horticultural lighting systems using red and blue LEDs to grow crops during the day can also be used at night to suppress basil DM sporulation by up to 100%.

Mineral nutrients needs of cucumber and its yield in protected winter cultivation, with HPS and LED supplementary lighting

The objective of the study was to evaluate yield, mineral nutrients uptake and nutrient demands of cucumber plants in winter cultivation with HPS and LED supplementary lighting. The experiment was carried out in the three variants: 1) HPS - top light only; 2) HPS + LED - top light + 2 lines of LED inter-row light; 3) LED + LED - top LED + 2-line LED inter-row light. In these variants the same PAR intensity level, of about 320 μmol m−2 s−1 (PPFD), was provided. Midi-cucumber parthenocarpic cultivar ‘Svyatogor’ F1, of high shade tolerance and 18–22 cm fruit lenght, was grown for 17 weeks during winter, in mineral-rock slabs. The plants were trained on a single stem up a string, according to the ‘high wire system’ method. Number and weight of every-day harvested fruit were determined. On the basis of monitoring, content of macro- and micro-elements in drip and drain solutions that was undertaken during every two-week period and constant monitoring of total daily drip irrigation quantity with nutrient solution per one plant, as well as total daily drain of the nutrient solution, the daily uptake of nutrient solutions and nutrients per one plant was calculated. The nutrient status of the plants was determined by leaf samples analysis performed for the younger, upper 3rd–5th leaves, as well as for the older, 10th–12th, lower leaves. Cucumber plants were characterized by stable yield and showed high fruit yield of very good quality. The highest yield was found for the plants grown with supplementary lighting with LED + LED. Leaves from the plants grown with the standard HPS top lighting showed faster senescence than the leaves of the plants grown with inter-row lighting. This is evidenced by a higher concentration of Ca, Mg, S, Mn and B ions, together with a lower concentration of K ions, in 10th–12th leaves in the plants grown with the standard HPS top lighting, compared with the plants grown with the HPS + LED lighting, as well as with the LED + LED lighting. The plants grown with the inter-row lighting showed a higher nutritional demands than the plants grown with the standard HPS top lighting. Parallelly used LED top lighting resulted in a higher and more stable yield, compared to standard HPS lighting conditions.

Design schemes’ universalization in calculation of daylight factor under side and roof lighting system in buildings

The article describes the methods of contemporary daylight factor’ (D.F.) design either with top or side natural lighting systems in buildings. The different methods of design, with different design formulas are based on preliminary determination of the type of lighting system used. But this division is rather conditional and not accurate. Sometimes the source of natural light (say, a window in a wall), which traditionally is judged as a source of a side light, is placed higher, than a monitor or skylight on a roof, which usually are determined as a sources of a top light. The same situation can be seen in determination of an angle of observation of a sky vault portion, observed from a design point within the premise considered. Thus, taking into account the above mentioned statements, one can assume, that with no strict division of external natural illumination under the overcast sky conditions, adopted as a basic theory in contemporary domestic «Codes and Regulations on Natural Lighting of Interiors», it is necessary to offer an universal design formula. This one should be based on standard formulas with respect to specific points, typical either to elements of window, monitor or skylight structures.