A Holistic Approach for Improving Visual Environment ... - Science Direct

Available online at www.sciencedirect.com

ScienceDirect Procedia Environmental Sciences 38 (2017) 372 – 380

International Conference on Sustainable Synergies from Buildings to the Urban Scale, SBE16

A Holistic Approach for Improving Visual Environment in Private Offices Iason Konstantzosa,b,*, Athanasios Tzempelikosa,b b

a Lyles School of Civil Engineering, Purdue University, 550 Stadium Mall Dr., West Lafayette, Indiana 47907 USA Center for High Performance Buildings, Ray W. Herrick Laboratories, Purdue University, West Lafayette, Indiana USA

Abstract Visual comfort is one of the main priorities in designing working and living environments. An efficient design of an office space however should also take into account its energy performance, and the connection to the outdoors. The latter is a poorly studied but critical factor connected to the overall visual satisfaction in the interior. While new designs offer freedom to the architects in terms of orientations, window-to-wall ratios and high performance glazing systems, existing spaces require more targeted and effective retrofit solutions, mostly connected to shading devices and automatic control systems or even the potential adjustment of the positional layout in the interior. Glare from daylight is a common issue in building perimeter zones with large facades that highly affects productivity and occupant comfort, therefore its mitigation should be considered as a priority. Then, maximizing the lighting energy performance or the degree of connection to the outdoors should be secondary decisions connected to the use of the space. Several indicators have been developed to quantify the degree of visual discomfort, and Daylight Glare Probability (DGP) is widely accepted as an appropriate index. However, in this study, due to limitations of the applicability of DGP in cases with the sun visible through roller shades, an alternative criterion is being used, based on thresholds for the direct and total vertical illuminance on the eye. In compliance with IES Standard LM-83-12, the continuous daylight autonomy is used to assess the lighting energy performance of the space, while for the connection to the outdoors, the newly developed Effective Outside View index is extended in order to quantify the case of dynamic shading systems. The triple visual environment criterion allows a holistic evaluation of the visual environment in private offices with roller shades. © by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2017 2017Published The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SBE16. Peer-review under responsibility of the organizing committee of SBE16. Keywords: Daylight glare; visual comfort; lighting energy use; connection to the outdoors; roller shades; office layout

* Corresponding author. Tel.: +1-765-409-9504 E-mail address: [email protected]

1878-0296 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SBE16. doi:10.1016/j.proenv.2017.03.104

Iason Konstantzos and Athanasios Tzempelikos / Procedia Environmental Sciences 38 (2017) 372 – 380

1. Introduction Visual comfort in office spaces has been the main focus of building science research regulations attempts and control and façade configuration designs over the past few years. Maintaining comfortable and desirable conditions within office spaces leads for the occupants to be more productive, desire to spend more time in their workstations, and even leads to improve health. Although open plan offices are gaining popularity nowadays, private offices, a common practice of the past still remain the main trend in older buildings, compensating poor space utilization and isolation of employees with benefits as the complete freedom of positioning and the potential for personalized comfort for the occupants. A successful design of an office should provide comfortable conditions, taking also into account the lighting energy performance and the connection to the outdoors1. Visual comfort has been studied mostly associated with discomfort glare. Daylight Glare Probability or DGP2 is the most recent index used to evaluate glare from daylight, and it is extracted by experimental data in private office spaces involving human test subjects focusing their view to a specific task area. Chan et al.3 suggested an alternative criterion based on the direct and total parts of vertical illuminance on the eye to assess discomfort glare for the cases of direct sunlight penetrating the room through shading fabrics. The positional aspects of discomfort glare (distance from the window and view direction) have been discussed in several studies; Konis4 investigated the occupants’ preferences in side-lit open plan offices, while Jakubiec and Reinhart5 stated that rotated views can reduce or eliminate discomfort glare. In a simulation study, Chan et al.6 stated that wall-facing directions lead to less uncomfortable conditions over the year, while Konstantzos and Tzempelikos1 investigated open plan offices with fully applied shading devices to state that side wall facing could allow fabrics of high visual clarity to be used to improve outside view without creating issues of discomfort. Energy efficiency in terms of lighting energy use is studied through daylighting design metrics. The most widely used is Daylight Autonomy, defined as the percentage of annual office hours when the interior the illuminance is higher than a pre-defined level. In order to obtain a more realistic daylighting amount due to the values below the set point, continuous Daylight Autonomy or cDA7 has been suggested, accounting for the values lower than the desired threshold. IES Standard LM-83-128 suggests a desirable set point of 300 lux. The connection to the outdoors, in terms of amount and clarity of view is not adequately studied; Konis4 found that occupants in perimeter zones left a portion of the window unshaded for most of the time to maintain adequate outdoor view, despite the occurrence of visual discomfort. Other studies 9,10 pointed the qualitative relationship between sensation of glare and outside view, while Hellinga and Hester11 presented a computational method to assess outdoor view quality. Konstantzos and Tzempelikos1 proposed the Effective Outside View, a geometrical metric to quantify the connection to outdoors for the case of windows with fully applied shading fabrics. This study applies a triple criterion to select the most effective design visual environment approach for a private office space with respect to visual comfort, lighting energy performance and connection to the outdoors. A strategy of maximizing lighting energy performance and connection to the outdoors while keeping visual comfort as a constrain constitutes a straightforward decision making process, which can be used either in existing buildings, in terms of retrofitting and positional layouts or also optimize the design of new spaces, in terms of orientations, façade configurations, control methods or even spatial layouts according to the specific needs and functions of the space. 2. A suite of metrics for the holistic evaluation of the visual environment 2.1 Visual Comfort Autonomy (VCA) The Visual Comfort Autonomy or VCA1 is defined as the portion of working hours when a person in a specific position and under a selected viewing direction is under comfortable conditions. There is some controversy regarding the use of DGP2 in the cases of direct sunlight penetrating the room through roller shade fabrics, stated by Konstantzos et al.12 and the common experience that roller shade fabrics of low openness lead to comfortable conditions for most of the time; as the solar corona’s luminance reaches extreme values, even when observed through low-openness fabrics, the calculations of the contrast term of DGP inflate the predicted glare level compared to everyday practice, especially for the cases of tight fabrics. To account for these inconsistencies, given that the objective of this study is to retrofit existing spaces with the application of shading fabrics, an alternative

373

374


discomfort criterion3 is applied for the VCA calculations. The latter states that in order to expect visually comfortable conditions, (i) DGPs should be lower than 0.35 13, to account for the overall brightness related glare, and (ii) the direct part of vertical illuminance on the eye should be lower than 1000lux, as a modification of the IES Standard LM-83-128 to account for the peak contrast-induced glare due to sun being visible from the interior. As DGPs is a linear function of the total vertical illuminance on the eye, the above criterion is translated to a double illuminance (Direct and Total) criterion. The validity of the criterion, especially concerning the direct illuminance discomfort threshold was recently evaluated using experiments with human subjects at Purdue University, focused on cases with the sun visible through shading fabrics of different openness. It has to be mentioned however that in spite of the lack of respective validation studies, the 1000 lux threshold for the direct vertical one eye illuminance is already mentioned in literature14. In order to constitute a seating layout as usable, VCA must be more than 95% throughout the annual working hours. Therefore, only seating layouts that comply with that constrain should be further evaluated for the secondary objectives (lighting energy savings potential and connection to the outdoors). The decision about which of the latter two should be a priority belongs to either the architect or the building manager. 2.2 Lighting Energy Performance In order to account for daylighting provision and lighting energy use reduction, the annual index of continuous Daylight Autonomy7 is used. This metric is more suitable for obtaining light energy use for offices with light dimming control systems, a common practice nowadays. A threshold of 300 lux on the work plane is used, complying with IES recommendations8. For the case of positioning alternatives in a private office, the cDA300 for each possible seating position is evaluated (spatial variation), while for the case of an open plan office, where the overall evaluation of the office layout is of the essence, the average cDA of all points of the usable grid is used. 2.3 Connection to the outdoors The Effective Outside View or EOV1 has been recently proposed in order to quantify the connection to the outdoors in terms of amount of view and quality of view for the case of fully applied shading. As quality of view depends on a variety of highly subjective parameters, including scenery, location, orientation etc., the authors decided to include as the sole quality parameter the clarity of view, quantifiable by the View Clarity Index (VCI)15. The latter (Eq. 1) associates the two commonly available fabric properties (openness factor and visible transmittance) with the level of visual clarity while looking towards the exterior through a specific fabric. 1.1

VCI

§ OF · 1.43 (OF )0.48 0.64 ¨ ¸ 0.22 0 d VCI d 1 © Tv ¹

(1)

, where OF is the openness factor and Tv is the normal total visible transmittance of the fabric as provided by the specifications. According to the Effective Outside View definition, the amount of view is defined as the portion of the occupant’s visual field that is covered by a window, corrected by the clarity of view through the particular fully applied shading fabric. In that scope, and in order to provide a sense of measure against visual field having a full connection to the exterior, the projected solid angle of the visible part of the window (for each position and view direction of interest) is normalized with the overall solid angle of the human visual field, ΩFOV (approximated as a circular cone with a half-angle of 78o). To make calculations more efficient, the window is discretized into rectangular fragments to approximate the total solid angle of the window as the sum of the respective solid angles of the fragments (Figure 1). An algorithm rejects all window fragments that extend beyond the field of view of 78 o and includes the rest in equation 2, which gives the Effective Outside View for cases of fully applied shading, considering both the amount and the clarity of outdoor view for any seating position, view direction and shading fabric.

375


EOV

¦:

iFOV

VCI

Ai cos Ti VCI

¦

: FOV

iFOV

2S (1 cos 78 ) Di

2

(2)

,where Ai is the area of each visible window fragment i, θi is the angle between the normal to the window and the line connecting the eye and the fragment, and Di is the distance between the eye and the fragment as shown in Fig. 1.

Fig. 1. Calculation of differential solid angles and projected solid angle of each window segment in the direction of the observer.

The EOV index was originally developed to quantify the connection to the outdoors for the case of open plan offices with fully applied shading, based on the assumption that when hosting multiple subjects, an optimally selected fully applied shading fabric with a relatively high view clarity would be more effective. However, when it comes to private offices, personalized comfort becomes a priority and dynamic shading, either manual or automated is of the essence. In that scope, the EOV can be modified with equation 3 in order to cover partly shaded windows. EOV (t )

¦:

iSA

(t ) VCI s ¦ :iUA (t ) VCI g

(3)

: FOV

, where SA is associated with the shaded area and UA with the unshaded area of the visible part of the window, respectively. EOV is time dependent in cases of controlled shades. In all other cases, it keeps constant over time for a single position. For that case (as well as the case of fully unshaded fenestration), the view clarity of the glass 15 is considered to be equal to 1. While this assumption can be considered to be reasonable for often used glazing systems (both clear, tinted or coated), it has to be modified for light-redirecting systems and translucent materials. 5

a

5

0.90 0.80

0.10

0.20

0.20

4

0.70

4

0.70

3

0.40 0.30

0.40

0.60 0.80 3

Facade

0.30

Facade

0.60

0.50

0.70

2

0.40 0.30

0.50 0.60

0.50

2

0.20 0.10

1 3

2

1

5

4

3

2

1

b

0.10

0.20

0.0

1

1

0.40

2

2

0.30

3

4

5

5

4

3

2

1

5

4

3

2

1

0.10

0.30 3

0.20

Facade

4

Facade

5

1

0.20

4

0.10

5

0.0

Fig. 2. Annual Effective Outside View spatial distribution for controlled and for fully open shades respectively for (a) viewing direction towards the window and (b) viewing direction towards the side wall. The two axes indicate a 5x5 grid of the room.

376


Differences in shading controls in terms of dominant opened portions can be reflected in clearer way when investigated in a longer term basis. For that reason, an annual evaluation is suggested to draw safer conclusions about the differences between control strategies. In that scope, for the case of a private office with specific seating positions of interest, the Effective Outside View can be evaluated on an annual level using the average value of the index over the annual working hours for each position, as shown in equation 4. The strict consideration of working hours is essential, as in most control strategies roller shades are returned to either fully closed or fully open positions when the space is unoccupied. Consideration of these states in annual basis comparison would mask the actual differences by including more than 4,000 annual hours with identical shading positions. EOVan

1 nw.hours

¦

nworking hours 1

EOV (t )

(4)

It has to be noted that due to the high dependence of solid angle to the distance, EOV is rapidly decreasing when moving away from the window. This leads to reduced values even for relatively small distances from the window, especially in the side wall facing layout. However, it is still to be investigated whether the proximity to the window plays a significant role to subjects through experimentation. EOV is developed to serve as an effective quantification index for the connection to the outdoors. The absolute level of the sensation of connection to the outdoors is yet to be investigated with human subjects. For that reason, the authors suggest for the values of EOV to be used in a relative level to find the better of two different configurations. The impact of shading controls on EOV can be clearly reflected in Figure 2, for (a) facing the window and (b) facing the side wall, for a controlled and a fully open shades case respectively. 3. Using the suite of metrics as a triple visual environment criterion3.1 Simulation methodology A hybrid ray-tracing and radiosity daylighting model with a glare module is used for the calculations of VCA and continuous daylight autonomy for the suggested configurations. The model consists of four main parts, the first simulating the exterior lighting conditions (direct and diffuse illuminance on the exterior of the window), based on TMY3 weather files, the second simulating the properties of the complex fenestration system (glazing and dynamic shading), the third calculating the interior mapping for luminances and illuminances over a specified grid, and the fourth finally calculating the glare index for each position and viewing direction of interest. The diffuse sky illuminance distribution is calculated using the model by Perez et al.16. For the interior illuminance and luminance distributions calculations, the hybrid ray tracing and radiosity module17 was implemented. The detailed beam-diffuse and off-normal properties of the roller shades were calculated using the semi- empirical method introduced by Kotey et al.18. Ray tracing is used to capture the sun’s position and the directly lit areas in the interior. Then, the radiosity method uses the initial exitances obtained above to apply the interreflections of the interior surfaces and calculate the final luminance and illuminance distribution in the interior for the desired a grid of positions, while all surfaces densely discretized in order for the glare module to accurately identify the glare sources which will be taken into account in the equations. The model has been validated with experiments6. For the calculations of the EOV index, a separate 3D geometry model was used, discretizing the window as described in section 2.3 and calculating the effective outside view. For the cases of dynamic shading, an additional input was the shade position for each time step. 3.2 Shading controls Four different shading control approaches are evaluated for this study: x

A fully open strategy (Case I): An open shades (or no shades) case is the baseline for any evaluation, as it considers the present conditions without applying any type of shading; if it is possible to achieve comfortable conditions only by changing the layouts, then this can be the most cost efficient approach, while by definition it will at the same time maximize energy performance and connection to the exterior.

377


x x x

A fully closed strategy (Case II): Closed shades is the most efficient method of mitigating glare, having obvious disadvantages in terms of low daylight availability and connection to the outdoors. Case III represents a typical shading control industry standard, based on protecting the work plane area from direct light. The shading position is calculated as a function of the solar profile angle and the sunlight penetration length in the room.19 Case IV uses an advanced shading algorithm which combines the approach of Case III with adjusting the shading position to prevent high illuminances (over 2000 lux) and at the same time maximize daylight in cases of cloudy sky. A mounted sensor on the window is used in order to calculate the “effective” illuminance (Eeff), which is then plotted against work plane illuminance for a targeted position (simulating the entire year) to determine a threshold19. For effective transmitted illuminances below this threshold (Eesp), the shades position is determined by the control described in Case III; otherwise, the shades will move to a lower position to avoid excessive amounts of daylight on the work plane. The lowest shade position hsh (portion of unshaded window) to avoid high illuminances is obtained by:

Esh ( H hsh ) Eg hsh

Eesp H

(5)

where H is the window height, and Eg and Esh are the illuminances transmitted through the unshaded and shaded window parts respectively. The illuminance threshold applied in this study was 6500 lux. 3.3 A private office case study In this case study, the visual environment in an existing perimeter office space in Chicago will be investigated, in terms of fabrics, shading controls and positioning in the room. Seating layout C 1.05m

Seating layout B

3.50m

3.50m

3.50m

2.50m

2.50m

2.50m

Seating layout A

1.75

1.50m

3.50m

3.50m

3.50m

Fig. 3. Three different layouts investigated for this case study

This is a common case where perimeter spaces with fixed characteristics (orientation, window to wall ratio, glazing type etc.) need to be revisited in order to improve visual comfort and lighting energy performance potential. A typical 3.5mx3.5mx3.5m high private perimeter office space is investigated, facing towards the south and having a 60% window to wall ratio. The glazing is assumed to be a regular double-clear module, often used in existing buildings. In the analysis, three typical seating layouts will be investigated: one facing the window from a distance of 2.50m, one facing the side wall but having part of the window within the visual field, and one facing the wall without having the window within the visual field (Figure 3). A layout facing the back wall was considered to be out of the scope of the study, as, while facing the back of the room solves the discomfort glare problems, it entirely eliminates any view outside. A shading fabric (OF of 4.2% and visible transmittance of 5%) was used on the windows.

378


4. Results and Discussion 4.1 Results The results for the three indices are shown in Table 1, for a south and north orientation, in order to point out the fundamental differences regarding the suggested retrofit solutions. Table 1. Simulation results South window

North window

Case I

VCA

cDA

EOV

VCA

cDA

EOV

Layout 1

0.43

0.90

0.15

0.65

0.90

0.15

Layout 2

0.57

0.90

0.08

0.95

0.90

0.08

Layout 3

0.79

0.90

0.03

1.00

0.90

0.03

Case II

VCA

cDA

EOV

VCA

cDA

EOV

Layout 1

0.98

0.29

0.10

1.00

0.21

0.10

Layout 2

1.00

0.40

0.05

1.00

0.21

0.05

Layout 3

1.00

0.38

0.02

1.00

0.20

0.02

Case III

VCA

cDA

EOV

VCA

cDA

EOV

Layout 1

0.74

0.79

0.13

0.66

0.90

0.15

Layout 2

0.87

0.81

0.07

0.95

0.90

0.08

Layout 3

1.00

0.81

0.03

1.00

0.90

0.03

Case IV

VCA

cDA

EOV

VCA

cDA

EOV

Layout 1

0.98

0.68

0.11

1.00

0.62

0.12

Layout 2

1.00

0.70

0.07

1.00

0.63

0.07

Layout 3

1.00

0.69

0.02

1.00

0.63

0.03

Table 1 shows that for each orientation, although more than one layout and control approach can be acceptable in terms of the main constrain (VCA), the methodology can at the same time provide information about the other two important design factors, making the final decision more efficient. For the south orientation, it is apparent from the results of Case I that a shading system is necessary, even for a side wall facing layout. Although Case II (fully closed) can provide comfortable conditions for all layouts, it leads to very dark conditions (therefore increased electric lighting usage), while the connection to the outdoors is also reduced compared to Case I. Using the Advanced control (Case IV) for the shades and applying the seating Layout 1 will give at the same time comfortable conditions, acceptable lighting energy performance and only slight loss for the connection to the outdoors compared to the fully open window, therefore it can be considered to be the optimal choice. For the North orientation however, it can be seen that even Case I (no shading) could be acceptable when using Layouts 2 and 3 (facing the side wall). This gives an example of how just adjusting the layout, can give significantly improve comfort without any installation cost. As the case study building is located in the northern hemisphere, direct sunlight exposure during the winter is not an issue, therefore the standard control approach (Case III) operates identically to Case I. This holistic approach is useful for cases of renovations as well as for new designs, where more parameters can be altered, such as WWR, glazing systems, room geometry, orientations etc. Having a complete image of the visual environment of a space allows better prioritizing and more efficient handling of the needs of the spaces and the budget.


4.2 Discussion- Limitations The methodology also introduces some limitations, the solution to which are part of the future work of the authors. x The permitted annual amount of discomfort hours (here assumed to be 5%) has not been extensively studied; Jakubiec and Reinhart14 present a value connected to direct vertical illuminance exceeding 1000 lux and discomfort, however this is not based on an annual test, but in a long exposure in the workstations. Long term experiments with human subjects are needed in order to define a specific annual threshold. x The EOV index is a strictly geometrical quantification of the amount of outdoor view corrected with the clarity of view, for fixed or dynamic fenestration systems. While VCI is extracted by human subjects experimentation, the acceptable ranges of the amount of view are still unclear; the extent in which the distance from a window affects the satisfaction of occupants if the whole window is still within the visual field is yet to be investigated, along with the extent of compromise a partly shaded window can cause to the sensation of connection to the outside, even with a very high VCI fabric. It has to be noted that for very clear fabrics (VCI close to 1), the importance of the shading control gets not effectively captured by EOV, as the shaded and the unshaded portions are almost counted as identical. All the above remarks have yet to be investigated with human subjects, being an important part of the future work of the authors. x The study does not take into account possible reflections of the sun on the screens in the cases of side viewing directions. This effect is highly dependent on the type and positioning of the screen, while different subjects choose to have their screens right in front of them or at a slight angle, while others may prefer to use their laptops instead of regular monitors, leading to a multitude of possible scenarios that extend beyond the scope of the study. As a general recommendation, non-reflective screens should be used in all side wall seating layouts, independent from the shading controls used, as a significant amount of the solar direct light will still penetrate the room, except the case of using very low openness fabrics. Acknowledgements This work is supported by Alcoa Foundation and Lutron Electronics Co Inc. References 1. Konstantzos, I, Tzempelikos, A., Design recommendations for perimeter office spaces based on visual performance criteria, CISBAT International Conference, Lausanne, Switzerland, September 2015. 2. Wienold J., Christoffersen J.: Evaluation methods and development of a new glare prediction model for daylight environments with the use of CCD cameras. Energy and Buildings 38(7), pp. 743-757, 2006. 3. Chan Y.-C., Tzempelikos A., Konstantzos I.: A systematic method for selecting roller shade properties for glare protection. Energy and Buildings 92, pp. 81-94, 2015. 4. Konis K.: Evaluating daylighting effectiveness and occupant visual comfort in a side-lit open-plan office building in San Francisco, California. Building and Environment 59, pp. 662-677, 2013. 5. Jakubiec J.A., Reinhart C.F.: The adaptive zone – a concept for assessing discomfort glare throughout daylit spaces. Lighting Research and Technology 44(2), pp. 149-170, 2012. 6. Chan Y.-C., Konstantzos I., Tzempelikos A.: Annual daylight glare evaluation for typical perimeter offices: simulation models versus fullscale experiments including shading controls. Proc. of ASHRAE Annual Conference, Seattle, Washington, 2014. 7. Rogers, Z. Daylighting Meric Development Using Daylight Autonomy Calculations in the Sensor Placement Optimization Tool. Boulder: Architectural Energy Corporation, 2006. 8. IESNA: IES Standard LM-83-12. Approved method: IES Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE), New York, 2012. 9. Aries M.B., Veitch J.A., Newsham G.R. Windows, view, and office characteristics predict physical and psychological discomfort. Journal of Environmental Psychology 30 (4), pp. 533-541, 2010. 10. Tuaycharoen N., Tregenza P. View and discomfort glare from windows. Lighting Research and Technology 39 (2), 185-200, 2007. 11. Hellinga H., Hordijk T.: The D&V analysis method: A method for the analysis of daylight access and view quality. Building and Environment 79, pp. 101-114, 2014.

379

380

Iason Konstantzos and Athanasios Tzempelikos / Procedia Environmental Sciences 38 (2017) 372 – 380 12. Konstantzos I., Tzempelikos A., Chan Y-C.: Experimental and simulation analysis of daylight glare probability in offices with dynamic window shades. Building and Environment 87, pp. 244-254, 2015. 13. Wienold J.: Dynamic simulation of blind control strategies for visual comfort and energy balance analysis. Proc. of IBPSA 2007 Conference, Beijing, pp. 1197-1204, 2007. 14. Jakubiec J.A., Reinhart C.F.: A concept for predicting occupants’ long term visual comfort within daylit spaces. Leukos 2724, 1-18, 2015 15. Konstantzos I., Chan Y-C., Seibold J., Tzempelikos A., Proctor R.W., Protzman B.: View Clarity Index: a new metric to evaluate clarity of view through window shades. Building and Environment, in press, 2015. 16. Perez, R., R. Seals, P. Ineichen, R. Stewart and D. Menicucci. 1987. A new simplified version of the perez diffuse irradiance model for tilted surfaces. Solar Energy 39(3): 221-231. 17. Chan Y.-C., Tzempelikos A.: A hybrid ray-tracing and radiosity method for calculating radiation transport and illuminance distribution in spaces with venetian blinds. Solar Energy 86(11), pp. 3109-3124, 2012. 18. Kotey N.A., Wright J., Collins, M.R.: Determining off-normal solar optical properties of roller blinds. ASHRAE Transactions 117 (1), 10 pages, 2009. 19. Tzempelikos, A. and Shen, H., 2013, Comparative control strategies for roller shades with respect to daylighting and energy performance. Building and Environment 67: 179-192.