UNC Charlotte Uptown Center University of North Carolina
Interim Glazing Analysis Gantt Huberman Architects 500 North Tryon Road Charlotte, NC 28202
Kieran Timberlake Associates 420 N 20th Street Philadelphia, PA 19130
Daylighting + Energy Performance Laboratory C e n t e r f o r A r c h i t e c t u r a l Te c h n o l o g y University of North Carolina Charlotte School of Architecture
Index Part 1 Methodology
Part 1.1 Methodology and Variables
Part 2 Analysis-Lab Spaces
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Part 2.1 HDR
Comparative Analysis- Glare
Glossary of Daylighting Terms
Lab Director
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Appendix A
Dale Brentrup Professor of Architecture
Lab Assistants
Rhonda Lowe Andrew Nagle Ben Futrell
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Methodology
A physical model was constructed of plywood and steel frame built for one interchangeable facade in order to create a test cell simulation. Tests were conducted consecutively and within a small window of time and facing south (Figure 1a) in order to best examine the effect of direct luminance on the performance of each glazing sample.
1.1 Tools and Metrics
Figure 1a. HDR Exposure Sequence of fenestration simulation High Dynamic Range (HDR) Photography provides luminance measurement mapping using the relative and absolute exposure differences from the file header of a series of original images (Fig1a). The thirty-two-bit output Radiance file produced contains floating points (single pixels) of measured luminance in candellas/m2. In this manner HDR photography accurately represents the wide range of intensity levels found in a real scene as experienced by the human eye. Through advanced tone mapping of bracketed images the resulting information can then be translated graphically as calibrated false color (Fig1 b-c) or isometric contour luminance images.
Figure 1.1b. Solarban 60 June 21, 12:00 2
Figure 1.1c. Solarban 60 June 21, 12:00
Figure 1.1d. OCG June 21, 12:00 pm 2
A sequence of digital photos were taken at multiple exposures from the rear of the space. The images were then combined to create a single HDR image which contains luminance values within individual pixels which are measured in cd/m2 (Figure 1.1b). The resulting false color images (Figure 1.1 b-d) represent the varied apparent brightness measured across the viewable exterior and diagram the changes in surface luminance values across the plane. Figure 2.1a represents the scaled key applicable to all false color images presented in this document. While a certain range of luminance is always desirable (Fig. 1.2b) the purpose of this study is to identify the effect of the screen printed areas of the glazing on luminance and glare. By utilizing High Dynamic Range (HDR) photography the amount of glare produced by each glazing sample can be determined. Comparing the HDR images of the two glazing types against a scenario without glazing, the amount of glare produced by the fritting can be separated from glare produced by the glass. While varying conditions of sky vault and season ultimately affect the data, all three samples were recorded within a small window of time and negligible divergence and it is important to note that the value here is primarily comparative. Given the low margin of deviation, the dynamic range within the simulation with no glazing serves as a standard by which to compare the glazing samples.
1.2 Design Performance Parameters
Glare is experienced when the luminance within the visual field is sufficiently greater than the ambient luminance Fig. 1.2c Visual Adaptation (MIT) to which the eye is visually adapted. Desired contrast ratios for avoiding glare vary for different portions of human view. Ratios of less than 3:1 are desirable Fig. 1.2d Visual Fields within the central or ergorama view. More often a broader view is of most consideration and a ratio less than 10:1 is desired within or between visual fields (figure 1.2d). That ratio is illustrated relative to units of luminance in cd/m2 in figure 1.2c.
While the same concepts apply, for the purpose of this study, contrast is examined on a micro level. More so than the inherent ratio of each individual scenario, as would normally Figure: 1.2a be the case, measurements of contrast here are valued in their comparison to one another. Blinds, adjacent tree cover, interior light shelves, exterior shading devices and deep illumination from bilateral apertures can all help to lower the apparent brightness of a space and create a uniform luminous environment that is conducive to occupant visual comfort.
665 cd/m2 Desired range or balanced luminance. 35 cd/m2
(exterior
(lowest
value )
Figure: 1.2b
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Glazing Analysis 2.1 Luminance and Glare Standards
Figure 2.1a. No Glass
Figure 2.1d HDR False Color No Glass 4
Figure 2.1b. Solarban 60 w grey frits
Figure 2.1e. HDR False Color Solarban 60 w grey frits
Figure 2.1c . Solarban 60 w white frits
Figure 2.1f. HDR False Color Solarban 60 w White frits 4
The false color images below (Figures 2.1g-i) reference readings from points that would normally represent comparative surface areas in a broad scale discussion of glare and visual comfort as described previously. Because the surface selections here are intended to isolate the variable of the glazing and would never occur in a true environment, the normal contrast values are not possible or relevant. In this case the values establish a comparative assessment of the affect of the glazing in general on the maximum and minimum luminance values. For instance, the max sky luminance without glazing is 10,006 cd/m2 (Figure 2.1g). While both glazing samples (Figure 2.1h and i) bear equivalent standards in every manner except the frit color, the measured point of maximum sky luminance of the grey fritting is 82% of the non-glazing sample (figure 2.1h) while the luminance of the white fritting is 102% (figure 2.1i). Any increase in luminance, while accounting for the addition of glazing itself, must be attributable to the creation of glare and or reflectance. Because the color and reflectance of surfaces is controlled, the increased reflectance and glare is attributable to the change in the color of the fritted glazing.
Figure 2.1g. No Glazing 5
Figure 2.1h. Grey Frit
Figure 2.1i. White 5
2.2 Glare Assessment 11,500 cd/m2
14,600 cd/m2
12,400 cd/m2
Contrast Ratio within Scene .0038:1
Contrast Ratio within Scene .016:1
201 cd/m2 50 cd/m2 Figure 2.2a HDR False Color No Glass
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44 cd/m2 Figure 2.2b. HDR False Color Solarban 60 w grey frits
Figure 2.2c. HDR False Color Solarban 60 w white frits
Figure 2.2d. Micro view of Frits and space between - Grey
Figure 2.2e. Micro view of Frits and the space between - White
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By utilizing High Dynamic Range (HDR) photography we are able to determine the amount of glare produced by each glazing sample. Comparing the HDR images of the two glazing types against a scenario without glazing, the amount of glare produced by the fritting can be separated from that produced by the glass. Figures (2.2a-2.2c). There is a significant difference between the performance of the glazing based on the color type of fritting. Readings taken directly from points across white fritted areas only, generally range from 2800 cd/m2-3500 cd/m2 -yielding an average of 3150 cd/m2- regardless of the values being emitted from the sky vault through the immediately adjacent glazing area. Readings taken directly from points across grey screened areas generally range from 550 cd/m2-800 cd/m2 -yielding an average of 675 cd/m2- T regardless of the values being emitted from the sky vault through the immediately adjacent glazing area. This micro measurement provides quantifiable data concerning the reflective quality of the color within variables of screening or “frits� and the proportion of glare attributable to that design variable. The approximate dynamic range of contrast values that exist across the plane from points that are not fritted in the white sample is .016:1. The approximate dynamic range of contrast values that exist across the plane from points that are not fritted in the gray sample .0038:1. The ratio of glare in the gray sample is much smaller at which coincides with the assessment of the micro level measurements. The grey fritting performs better as it is more opaque and less reflective. Thus reducing the average glare created in the adjacent environment which is directly proportionate to the degree of increased sky luminance. Therefore, the differences in glare are attributable directly to the color since it is the only inconsistent variable.
It is also important to note the minimum values within the overall scene of each sample. For instance, the smallest values of sky luminance through no glazing and the values of luminace through grey fritted glazing are much closer than the minimum value able to be found through the white fritted sample. Generally, a diffuse sky in itself creates greater levels of glare. However, the characteristics of the fritted areas will exacerbate the level of glare produced by direct clear sky. The test therefore suggests that this condition will be different from various points of viewing the window wall. The factors that determine the perception of glare include the size of the light source, its position in the visual field, and background brightness. Therefore it is important to consider how the design and materiality of interior objects vary those factors for the light source in question. A material’s hue is a component of its reflectance and in combination with either glazing type will become a design decision of greater than average importance. In this case, the characteristics of all interior objects and textiles may contribute to the experience of glare and diminish the apparent balance of illuminance. However, Overall the white frit produces a greater amount of glare.
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Figure 2.3a. No Glass
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Figure 2.3b. Solarban 60 w grey frits
Figure 2.3c . Solarban 60 w white frits
Figure 2.3d
Figure 2.3f
Figure 2.3e
Figure 2.3g
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2.3 Thermal/Solar Assessment
In order to isolate performance characteristics of fritted Solarban 60 (such as its potential ability to lower solar load and its contribution to building energy efficiency) against a comparable, high performance glazing such as Solarban 80, the following analysis quantifies solar heat gain through the assessment of TMY2 solar data for Charlotte, North Carolina. The analysis first measures the direct solar gain through both unshaded southeast and southwest fenestrations followed by an identical examination in terms of glazing scenarios but with the added variable of exterior sun shading. A comparison of fig 2.3d and 2.3f illustrates that the statistical solar data, in combination with the physical properties of the glazing types analyzed, suggests that the solarban 80 product would perform slightly better than Solarban 60 with frits. When exterior shading is added to assemblies of either orientation, the load profile of the unshaded glass is dramatically reduced (as in fig 2.3e and 2.3g). Regardless of the high performance glass types, exterior shading proves to be the greatest factor in creating the highest benefit for the building energy load profile.
Figure 2.3h . Solarban 60 w white frits vs... Solarban 80
The study assesses then, the solar load (direct and diffuse) per sq. ft. of glass. Solarban 80 has a lower heat gain coefficient and a comparable visual light transmittance to the fritted Solarban 60. A non-fritted glass scenario is a better choice, in terms of heat gain coefficient, compared to fritted glazing. To illustrate this, the following graphs convert the btu/ft2 per day load profile into cooling ton/hr reductions per square foot of glass. By quantifying the refrigerated ton/hr reductions two opportunities are illustrated 1) reducing the first cost associated with chiller and ventilation equipment 2) long term energy savings contributions that the design could achieve.
Figure 2.3i . Solarban 60 w white frits vs... Solarban
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Glossary Artificial Sky
An enclosure that simulates the luminance distribution of a real sky for the purpose of testing physical daylighting models
Daylight Factor DLF
The ratio of daylight illuminance at a given point on a given plane due to the light received directly or indirectly from a sky of assumed or unknown luminance distribution, to the illumination on a horizontal exterior reference plane due to an unobstructed hemisphere of the sky, expressed as a percentage. The daylight factor is the sum of the sky component, the external reflected component, and the internal reflected component.
Foot candle
The unit of illuminance measurement in English units. One foot candle equals one lumen per square foot. One foot candle of illuminance is received by a horizontal surface under and unobstructed sky having a uniform luminance of one foot-lambert.
Glare
The sensation produced by luminance within the visual field that is sufficiently greater than the ambient luminance to which the eye is adapted, visually causing annoyance, discomfort, or loss in visual performance and visibility.
Illuminance Light Shelf Overcast Sky Photometer
The density of the luminous flux incident on a surface, expressed in units or foot-candles (or lux). A horizontal shelf positioned (usually above eye level) to reflect daylight onto the ceiling and to shield either direct sunlight penetration or glare from the eye. (C.I.E. Standard Condition) A sky luminance distribution three times brighter near the zenith that at the horizon. An instrument for measuring photometric quantities such as luminance, luminous intensity, luminous flux and illuminance.
Luminance
or Photometric Brightness (apparent brightness of a surface). Luminous intensity of a surface. The luminous intensity in the given direction of an infinitesimal element of a surface perpendicular to the given direction.
H.D.R. Photography
A set of techniques that allows a greater dynamic range of exposures (the range of values between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows.
Radiance
Radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces.
Visual Light Transmittance (VLT)
The value of visual light transmittance through and specific to a glazing assembly
Appendix A: Glossary of Daylighting Terms