Retro-reflective Research Articles

A comparative study on the optical performance of retro-reflective coatings before and after the aging process

Retro-reflective (RR) materials are largely included among the strategies to mitigate Urban Heat Island (UHI) phenomenon in the urban spaces thanks to their optical directional properties. As materials potentially used in the building exterior envelope, RR materials are subjected to outdoor aging and soiling processes, which could alter their optical behaviour. In this perspective, the investigation focuses on the characterization of the optical performance of several types of RR plaster coatings after outdoor aging and soiling processes. The RR plaster coatings were previously developed and studied, before undergoing the outdoor exposure. The RR samples were developed covering the support with a reflective white paint and glass beads. Five different microsphere diameter ranges and three different microsphere superficial density ranges have been employed. In this paper, the RR plaster coatings were tested in terms of optical properties after outdoor aging and soiling and results were compared with data of the same plaster coatings pre-aging. Generally, all RR aged samples show a lower solar reflectance in the Vis region with respect to the pre-aging conditions. The RR behaviour is qualitatively maintained after aging and soiling. The RR sample made of glass beads of 200–300 μm diameter range could represent the best solution both in pre- and post-conditions and both in terms of solar reflectance and retro-reflective behaviour, regardless of the density of microspheres used.

Investigating the influence of reflective materials on indoor thermal environment and solar reflectance in buildings

This in-depth study explores the intricate dynamics of reflective materials, emphasizing their impact on the indoor thermal environment and urban heat island (UHI) mitigation. Examining diffuse highly reflective (DHR), general reflective (GR), and retro-reflective (RR) materials on a simplified building model during summer days, the research utilizes outdoor experiments to analyze air and surface temperatures, as well as solar radiation. Prioritizing key metrics—mean radiant temperature (MRT), operative temperature (OT), and solar reflectance (ρ)—the study uncovers nuanced distinctions in DHR, GR, and RR materials. Solar reflectance calculations consistently show higher values for DHR and RR materials compared to GR material, highlighting reflectance's pivotal role in influencing surface temperatures and indoor thermal environment. When evaluating the impact of exterior wall materials on building temperatures, RR material with a 76% reflectance performs similarly to DHR material (82%). Notably, with a 6% lower reflectance in RR, the temperature contrast between external and internal walls is only about 1.5 °C at its maximum, underscoring RR's effectiveness as an outer wall material for UHI mitigation and building energy conservation, surpassing both DHR and GR materials.

The effect of a protective layer on the optical behavior of retro-reflective paintings for building envelopes

Retro-reflective (RR) materials, applied to building envelopes, constitute an option to tackle Urban Heat Island phenomenon, thanks to their capability to reflect the sunlight predominantly towards the solar incidence direction. RR coatings are obtained with the deposition of glass beads on traditional diffusive materials. During their lifetime, outdoor aging and soiling affect their optical behavior. In addition, without a proper protection, some glass beads could detach from the paint and disperse in the environment. Hence, a necessity arises for the application of a safeguarding stratum on RR materials to avert the separation of glass beads in RR coatings following the aging process. In this paper, five RR samples were produced, employing a highly reflective paint as foundation, RR glass beads and a protective layer. A diffusive sample, without glass beads, was made for comparison. Samples underwent spectrophotometric and angular distribution analyses. The effect of the protective layer on the optical behavior was assessed comparing the results with those obtained for the same RR materials without the protective layer. RR samples with a protective layer exhibit a higher reflectance with respect to the same RR sample without a protective layer. In the near-infrared (NIR) region, a lower reflectance occurs for all RR samples with a protective layer. A less concentrated angular distribution of the reflected light was observed for all RR samples with the addition of a protective layer.

Analysis of performance properties of retro-reflective woven fabrics made with retro-reflective yarns and natural yarns

Retro-reflective (RR) materials have been the subject of recent interest largely due to their ability to ensure personal safety on the road. Their use limits vehicle–pedestrian collisions, especially at night-time. Studies have mainly developed RR textiles for aesthetics and reported on their appearance performance accordingly. This study, however, investigates the mechanical and functional properties of RR fabrics produced with RR yarns and natural yarns. The results reveal that the woven fabrics have radiative cooling effects to transmit body heat to the environment, and a good ultraviolet protection rating (15–35) with a transmittance rate below 6% depending on the weave structure. The contact angle measurements reveal that the surface of these fabrics is hydrophobic or has low wettability. The total hand value for the fabric shows their suitability for men’s dress shirts in the winter or thin women’s dresses.

Neural network models for predicting urban albedo of urban surfaces with different reflection directional properties

The urban albedo is regarded as an important indicator for mitigating the urban heat island (UHI) phenomenon. The goal of this paper is to create neural network models that forecast the urban albedo of various reflection directional characteristic urban surfaces by simply inputting the sun’s position (altitude and azimuth) and solar radiation. In this study, two urban models with diffuse highly reflective (DHR) and retro-reflective (RR) urban coatings are created, and their urban albedo for two sunny days is calculated using standard (ASTM E1918A). Using the measured urban albedo for one sunny day, two predictive neural network models, Gaussian process (NNGP) and hyperbolic tangent function (NNTanH) are developed (August 1, 2021). The two developed neural network models are used to forecast the urban albedo for another sunny day (July 19, 2021). In the case of DHR urban coatings, the NNTanH model is more accurate with higher R-squared (R2) and lower root mean squared error (RMSE) than the NNGP model, and there is no significant difference between the two neural network models. However, it is demonstrated that the NNGP model is more accurate than the NNTanH model in predicting urban albedo in the case of RR urban coatings, with higher R2 and lower RMSE.

The impact of outdoor aging and soiling on the optic features of glass beads retro-reflective coatings

The latest research suggests the use of retro-reflective (RR) materials as innovative coating solutions for building envelopes and urban surfaces. This study aims at investigating the performance of RR materials after outdoor aging and soiling during summer 2021 in Perugia, Italy. Different glass beads RR coatings were realized with three substrate typologies, characterized by various roughness, and they were investigated in terms of their optic performance. Also diffusive (DIFF) coatings were made for comparison purposes. Variations in terms of solar reflectance and relative angular reflection distribution were compared with the same coatings in pre-aging conditions. Pre-aging optic data were presented and discussed in a previous work by the Authors. In all cases, the aged samples exhibit lower global reflectance values compared to their original conditions: the RR coating over a smooth pine wood substrate shows the highest reduction value equal to 14.4%. A stronger relative RR component was found for all the RR samples in post-aging conditions. Future developments may involve the identification of an optimal protective layer to be applied on RR materials to prevent the detachment of glass beads in RR post-aging coatings.

Can retro-reflective materials replace diffuse highly reflective materials for urban buildings’ wall to improve outdoor thermal comfort?

In recent years, there have been numerous studies on the application of retroreflective (RR) materials to exterior building walls to relieve the urban heat island (UHI) phenomenon and save building energy consumption instead of diffuse highly reflective (DHR) materials. Experimental measurements were undertaken to investigate the impact of applying DHR and RR materials to building exterior walls on the outdoor thermal environment. DHR and RR walls were tested with three canyon aspect ratios (H/ D) of 0.5, 1.0, and 1.5, respectively, with seven environmental indexes recorded and compared, including air temperature (Ta), relative humidity (RH), sol-air temperature (SAT), operative temperature (OT), standard effective temperature (SET*), change in operative temperature (COT), and real-time solar reflectance (ρ). The results show that the RR wall is more effective in improving the outdoor thermal environment, where SAT decreased by 4.5 °C, SET* decreased by 0.5 °C, and COT decreased by 1.2 °C, on average, while it can also improve the real-time solar reflectance (ρ) by 12% on average compared to the DHR wall. Moreover, it gets more effective in scenarios with a higher canyon aspect ratio.

An investigation towards the optimum design of retro-reflective materials as building envelopes for the enhancement of optical performance

Retro-reflective (RR) materials may represent an advanced solution for building envelopes, as they could contribute to Urban Heat Island mitigation. This study could bring novelty to the current state of the art on this topic, as it proposes to investigate the optimum design of glass beads RR materials for building application. To this aim, the impact of glass beads superficial density and diameter on the optical performance of RR materials has been further explored. In particular, 15 RR glass beads samples were developed, using a highly reflective paint as substrate and varying both glass beads superficial density and diameter. Also a diffusive sample, without RR glass beads, was made for comparison. Three different glass beads superficial density ranges and five diameter ranges were investigated. Samples were tested through spectrophotometric and angular reflection distribution analysis. All RR samples exhibited an overall lower solar reflectance compared to the diffusive sample, whereas RR samples with 200 ÷ 300 µm diameter exhibited the highest RR capability. By reducing glass beads superficial density, generally a reduction of RR performance was observed.

Spectral properties of retro-reflective materials from experimental measurements

Retro-reflective (RR) materials have received much attention due to their retro-reflective optical properties, which has essential differences with the diffuse reflection of traditional highly-reflective materials. As a guide to determine the performance improvement and the building applications of RR materials, the spectral characteristics are measured and related to the typical glass-bead and prism RR materials. The integrating sphere is used to measure the global reflectivity of the two RR materials, while a new instrument is developed to study the angular distribution and the retro-reflectivity of two RR materials at different incident angles. The results show that the global reflectivity is found to be 68.08% and 51.44% for the glass-bead and Prism RR materials, respectively, but the retro-reflectivity of the prism RR material is 12.56% higher than that for the glass-bead RR materials. The RR materials display the greatest different for the retro-reflectivity under the different radiation wavelengths. The variation trend of the retro-reflectivity is distinct for different wavelength ranges: the higher the wavelength, the lower the retro-reflectivity. Increasing the incident angle could reduce the retro-reflective property of the RR materials, and the incident angle had a high impact on radiation wavelengths less than 1200 nm. With the incident angle increased from 0° to 80°, the retro-reflectivity is reduced by 40.58–48.60% for the 800 nm wavelength, 35.40–45.13% for the 900 nm, 23.43–26.01% for 1000–1200 nm, and 8.64–13.71% for 1300–1600 nm.

Glass beads retro-reflective coating for building application: albedo assessment in urban canyon configurations

Materials for urban pavements and building coatings have a direct impact on urban thermal environment: low-reflective and highly absorptive materials, in fact, could contribute to the exacerbation of the Urban Heat Island (UHI) phenomenon, which negatively affects building energy consumptions and indoor and outdoor thermal comfort. These effects could be worsened especially in Urban Heat Canyon configurations, where the inter-building effect becomes more relevant. As regards UHI mitigation strategies, high-reflective (HR) and retro- reflective (RR) materials have been studied for their application as urban coatings, in order to provide a cooling effect inside urban canyons. In this framework, this study aims at comparing the albedo performance of HR and RR materials applied on a small scale urban canyon facility. To this aim, an in-field experimental campaign was conducted in two days during summer 2020. Different canyon coating combinations and height/width (H/W) ratios were investigated and compared in terms of albedo. Results show that an albedo worsening occurs when the RR coating was applied to the canyon’s surfaces, because of the lower global reflectance of the RR with respect to the HR. Nevertheless, the application of RR coating on walls, coupled with HR paint on pavement, lead to higher albedo values of up to 4.7%, with respect to the opposite mixed coating combination. Future developments may concern the investigation of the optimum design of a glass bead RR material for building application, by studying the correlation between its optical capabilities and the dimensions and density of the glass beads.

Evaluation of outdoor thermal comfort under different building external-wall-surface with different reflective directional properties using CFD analysis and model experiment

Recently, urban heat island (UHI) has become more intense due to the increase in artificial constructions in cities. In addition, in many office buildings in big cities in Japan, except for indoor workplaces, some outdoor workplaces that surround a building are provided to improve people productivity. Therefore, a comfortable outdoor thermal environment is considered as an important topic. As one of the countermeasures for mitigating the UHI phenomenon, highly reflective (HR) materials are applied to external-wall surfaces of buildings to increase the solar reflectivity of the external-wall surface to finally reduce the heat emitted from the building external walls. These applied HR materials mainly possess diffuse HR (DHR) directional properties. In recent years, retro-reflective (RR) materials have been studied to replace the traditional DHR materials for possible application in building facades. The present study aims to use the computational-fluid-dynamics (CFD) analysis to predict the outdoor thermal environment, including air temperature (Ta), wet-bulb globe temperature (WBGT), and standard effective temperature (SET*) under two different building-facade types (DHR and RR properties). To further analyze the effect of DHR and RR building facades on the thermal environment, in addition to evaluating the three thermal-environment indicators ( Ta , WBGT, and SET*), this study evaluates two outdoor radiation-environment indicators, including sol-air temperature (SAT) and change operative temperature (COT) using an actual facade-model experiment. The results show that the RR building facade are more effective in reducing WBGT, SET*, SAT, and COT. • Determined the reflective directional property of RR materials using our developed optical apparatus. • Simulated urban blocks with different reflective directional building facades (DHR and RR). • Used computational-fluid-dynamics analysis to evaluate the outdoor thermal environmental factors. • Implemented a facade experiment to investigate the effect of DHR and RR facades on outdoor environment. • Demonstrated that RR facade was more effective in reducing the environmental factors, WBGT, SET*, SAT, and COT.

Optimized Cool Coatings as a Strategy to Improve Urban Equivalent Albedo at Various Latitudes

This research study aimed to investigate the influences of angular-selective retro-reflective (AS-RR) and retro-reflective (RR) materials on the urban equivalent albedo (αeq). Full ray tracing solar analyses were conducted through the Monte Carlo-based numerical model validated in a previous work. Different geometry scenarios with different patterns of urban density were modelled. AS-RR and RR materials were alternately applied to the street and to the most irradiated façade. AS-RR materials were proposed to enhance the αeq of the urban environment particularly during summer. Solar analyses were reiterated for three latitudes (i.e., Oslo, Milan, Cairo). RR pavements and façades were capable of increasing the αeq throughout the year. However, implementing an angular-selective behavior allowed for a reduction of the mitigation potential of RR materials during the winter season. In their best application, RR and AS-RR materials enabled higher αeq in summer (122%) with negligible effects during the winter (7%). Finally, the study highlighted the need for exploiting numerical models capable of conducting full ray tracing solar analyses when investigating materials whose optical properties depend on the angle of incidence of the sunrays (such as RR materials).

The Effect of the Substrate on the Optic Performance of Retro-Reflective Coatings: An In-Lab Investigation

Retro-reflectivity is a promising surface capability, which has attracted the interest of researchers for building applications in order to counteract Urban Heat Island (UHI) effects. This work aims at studying the impact of the substrate material on the optic performance of retro-reflective (RR) coatings. Three types of substrate materials were investigated: smooth pine wood panels, rough plywood panels, and smooth acetate sheets. The RR coating samples were made by firstly adding a high reflective white paint onto the substrate material and a homogeneous RR glass beads layer on the top. As a reference case, also diffusive samples, without RR beads, were developed. Samples have been tested through a spectrophotometric and an angular reflectivity analysis. Results show that, despite a lower global reflectance of the RR samples with respect to the diffusive ones, the glass beads coating provides a good retro-reflective capability to all the diffusive samples. Additionally, the roughest RR sample exhibited the highest RR capability of up to 16%, with respect to the other smoother samples. Future developments may involve the optimum design of RR coatings, in terms of their optic performance by varying the substrate materials and roughness, the glass beads density and dimension.

Application of retro-reflective materials in urban canyon at different geographical locations

Retro-reflective materials (RRMs) reflect incoming radiation toward the incident direction. The present paper estimates the cooling potential (as the decrease of energy kept inside the canyon) of retro-reflective (RR) colored plasters at different latitudes, longitudes and canyon's orientations, through the use of an analytic model developed in MATLAB R2019b, in 8 different geographical locations. The energy reflected outside the canyon with respect to the incident energy (%) is reported for 5 combinations of RR/diffusive materials and for aspect ratios from 1 to 5. Results show that at latitudes near the Equator, from March to September, RRMs have no sensible effect in the energy circulating inside the canyon, with South-oriented façade. At low latitudes, better values are obtained in narrow canyons with East-oriented façade (+30.6%, aspect ratio equal to 5). Appreciable positive effects occur in other months, especially for narrow canyons (in January, increase in the energy sent outside the canyon with respect to diffusive materials: +29.5%). At middle latitudes, RR use is suggested especially for narrow canyons. At high latitudes, the effect of RRMs is stronger for narrow canyons during summer, while in other months it does not depend on the canyon's geometry. RRMs behavior is not affected by longitude. Generally, RRMs have a significant cooling effect in cities at middle/high latitudes while seem not to bring benefits with respect to diffusive materials in cities like Abu Dhabi or Singapore. Deeper insight should be devoted to the evaluation of the heating demand penalties during cold seasons.

Effects of retro-reflective and angular-selective retro-reflective materials on solar energy in urban canyons

The study deals with the application of retro-reflective (RR) and angular-selective retro-reflective (AS-RR) coatings on surfaces (i.e. the street ground and the south-exposed façade) within the urban canyon. Solar analyses were conducted at different latitudes (Oslo, Milan, and Cairo) by varying the urban canyon height-to-width ratio (H/W) to determine the variation in solar irradiation absorbed by north and south façades. Both summer and winter conditions were considered, and up to five material patterns (one reference case and four enhanced scenarios) were investigated for each combination of latitude and height-to-width ratio values. A validated Monte Carlo-based numerical model was used to conduct full-ray tracing analyses and to simulate the behavior of these coatings. The outcomes allowed the development of guidelines for the adoptions of RR and AS-RR materials in different scenarios. It was demonstrated that RR and AS-RR materials applied to the street performed better in low-density urban environment (H/W ≤ 0.5) with a consequent increase of the solar energy gains on the north façade by up to 15%. Employing RR and AS-RR materials on the south-exposed façade showed greater effectiveness on high-density urban canyon (H/W ≥ 2.0) and reduced by up to −8% the solar irradiation absorbed by the façade.

Analytical model to evaluate the reflective directional characteristics of retro-reflective materials

In order to mitigate the urban heat islands (UHI) phenomenon, instead of diffuse reflective (DR) building coating material, retro-reflective (RR) coating material is proposed to be applied to external wall surface of buildings. The reflective directional characteristics of RR materials can be usually investigated through optical measurement. In addition to the method of optical experiment, there are other methods of analytical models predication. This study deals with an optical experiment and two analytical models to evaluate the angular distribution of reflection intensity for RR materials. A total of three RR materials, including prism type plate, capsule type plate and glass bead type plate are created, and their angular distribution of reflection intensity were evaluated by using methods of optical experiment and two analytical models (original analytical model; anisotropic body of rotation of the normal distribution function (AND) model). In order to propose an appropriate analytical model of evaluating the reflective directional characteristics of RR materials, regarding the angular reflection intensity obtained by optical experiment as a real value, the results evaluated by two analytical models are compared in this study. It is shown that it is better to apply the AND model to evaluate the reflective directional characteristics of RR materials, compared to original analytical model.

Optic-energy and visual comfort analysis of retro-reflective building plasters

This paper presents the results of the characterization of new retroreflective (RR) plasters for outdoor applications. The paint was obtained through deposition of four different types of RR microspheres on a traditional plaster for exterior applications. The plasters were characterized through a spectrophotometric, directional and colorimetric analysis. All the types of microspheres provide strong RR behavior for incident light directions from 0° to 60° with respect to the surface normal. However, the aluminum coating, with and without fluoropolymer coating, affects negatively the global reflectance and change completely the tiles’ original color, making the application of the aluminum RR tiles very limited. Glass and barium microspheres instead improve the optic properties of the original plaster, with negligible effects on the color. An analytic model was used to evaluate the amount of energy that is reflected beyond the canyon by the RR materials. It was found that at 42°N latitude, the best performing configurations are the combination of RR façade and RR pavement, for glass spheres. Two full-scale vertical surfaces, covered with RR plaster with glass microspheres and diffusive plaster respectively, were developed to investigate the visual comfort of pedestrians, in particular glare. Results show that the Daily Glare Probability (DGP) and superficial temperature for the RR wall is always lower than those for the diffusive wall.

Experimental assessment of the combined effect of retroreflective façades and pavement in urban canyons

Retro-reflective (RR) materials represent an opportunity to reduce UHI phenomenon due to their ability to reflect most of the incident energy towards the same incoming direction. The RR behaviour depends on the angular distribution of the incident energy: for small incident angle the radiation is reflected mainly backward in the incoming direction; for large angles RR materials reflect the radiation symmetrically with respect to the surface normal. To overtake the limit of RR materials of being mainly retroreflective only for small angles of incidence, the investigation is focused on the optic interaction between vertical and horizontal RR surfaces. Therefore, the aim of this paper is to investigate the synergic effect of RR pavements and façades on a physical model in which urban canyon layouts with H/W equal to 1 and 2 are represented. To evaluate the global cooling effect of RR pavements and façades, a new parameter, called equivalent albedo, is introduced and calculated over the canyon’s ceiling. The results show that a canyon with RR vertical surfaces and RR pavement reflects outward more radiation than a canyon with RR vertical surfaces and diffusive pavement due to the interaction of optical properties of the materials.

Exploiting selective angular properties of retro-reflective coatings to mitigate solar irradiation within the urban canyon

The combination of urban sprawl and densification in built areas increases the number of interbuilding solar reflections. This contributes to over warming in urban environments – a phenomenon known as the urban heat island (UHI) effect. During the last decades, the exploitation of cool materials, such as high-reflective (HR) and retro-reflective (RR) coatings, has been proposed to mitigate UHIs. In this context, this work is a part of the wider research focused on the investigation of a new concept for surface treatments, which consists of RR materials enhanced with an angular-selective behavior. This research study aims at evaluating the application of the selective angular properties to assess solar irradiance impinging urban canyons’ surfaces in different climatic contexts, from the Arctic to the Persian Gulf area. First, the geometry configurations receiving the largest solar energy contribution were defined by varying the canyon’s orientation and height-to-width ratio. Second, yearly and seasonal solar analyses were conducted considering traditional and angular-selective RR materials applied to the façade and the street. The outcomes show that angular-selective RR materials can activate the mitigation potential of these materials only during the warm season, when the risk of UHI is higher without hindering solar energy contribution in cold months. In fact, optimized RR turns out to be efficient at all latitudes considered and reduced the yearly solar irradiation impinging canyon’s surfaces by up to 20%, 30%, and 50% in Oslo, Milan, and Cairo, respectively.

Performance of Retro-Reflective Building Envelope Materials with Fixed Glass Beads

Urban heat islands (UHI) are growing in size and intensity, which is partly attributable to the large amount of anthropogenic waste heat. Moreover, heat emitted from building exterior walls accounts for a large portion of the total anthropogenic waste heat. Thus, strategies and technologies for preventing the emission of heat from building exterior walls are being pursued by researchers worldwide. Amongst these technologies, the potential of use of retro-reflective (RR) materials instead of diffuse highly reflective (HR) materials applied to building envelopes for UHI mitigation is being studied widely. However, RR materials haven’t been applied to building envelopes in practice due to their unproven weather resistance. In order to develop RR materials with high weather resistance for application to building envelopes, two types of micro glass beads with different refractive indices (1.5, 1.9) and five different colors of base layers were evaluated in this study. Their RR performance was measured by optical experiment and compared to two types of RR sheets commercially available in Japan. The results showed that the glass bead RR samples with a refractive index of 1.9 had much higher retro-reflectivity (better RR capacity) compared to those with a refractive index of 1.5.