APPLICATION OF VIDEOPHOTOMETER IN THE EVALUATION OF DGI IN SCHOLASTIC ENVIRONMENT

Transcription

1 , Volume 6, Number 2, p.82-88, 2005 APPLICATION OF VIDEOPHOTOMETER IN THE EVALUATION OF DGI IN SCHOLASTIC ENVIRONMENT L. Bellia, A. Cesarano and G. Spada DETEC, Università degli Studi di Napoli FEDERICO II, P.le Tecchio, 80, Napoli, Italy (Received 23 February 2005; Accepted 24 June 2005) ABSTRACT This work deals with visual comfort for interior scholastic environment and videographic technique applied in the verification procedures reported in the Italian Standard Code UNI (March 2000) [1,2]. Several aspects of glare due to daylighting and concerning all parameters used to define the DGI (Daylight Glare Index) have been examined. Each parameter, both geometric and photometric, was acquired with the videophotometer, the setup of which has been carried out previously. In this way, DGI calculation is very easy and fast and allows to formulate various considerations about the glare source classification in daylighted indoor environment. Finally, the advantages obtained using the videophotometer and how this kind of instrument will support in the future verification procedures and lighting design in buildings are pointed out. 1. INTRODUCTION Daylighting is receiving increased consideration as a light source for buildings [2,3]. It takes significant advantages as psychological well-being, eye health, colour rendering and energy saving [4]. A correct daylighting design has to take the necessary steps to the control of the illuminance level and uniformity on the work plane, luminance ratios, glare [5,6] and contact to the world outside. There are different types of glare: disability glare and discomfort glare [7]. Disability glare is the masking effect caused by light scattered in the ocular media which produces a veiling luminance over the field of view. Discomfort glare is the cumbersomely distracting effect of peripheral light sources in the field of view. A particular kind of discomfort glare is Dazzling glare, it dims the vision, especially by intense light coming from wide surfaces (sunlit book, sky or sandy desert), it is probably located in the iris sphincter, however this last case is usual in the interior daylighted environment. Today the technological features regarding to the CCD sensors (Charge Coupled Device) allows application in architectural field. In particular this innovative instrument can be applied in the visual comfort evaluation for interior environments. The new CCD sensors are characterized by wide dimensions, linear response curve, high quantic efficiency and low cost. So, the videophotometer represents a potential support device for the verification of prescriptive regulations regarding illumination: in fact the simultaneous measure of luminance values in numerous points enables the rapid, straightforward verification of prescribed standards. This work analyses a case study about the verification in school environment, in particular it deals with the problem of the glare due to daylighting in the school rooms. 2. GLARE EVALUATION IN THE ITALIAN STANDARD CODE FOR DAYLIGHTED SCHOLASTIC ENVIRONMENT The new European Standard EN Light and lighting - Lighting for work places Part 1 Indoor work places [8], available in Italy from October 2004, lacks about the glare evaluation due to daylight. Therefore it has been considered the Italian Standard Code UNI (March 2000) [4], which provides to the glare control through DGI (1) evaluation. = 10log n DGI G (1) i i= 1 G i is the constant of glare and it is given by the equation (2): Gi = 0.48 L b Ls Ω ω L w (2) where L s is the average luminance of each glare source in the field of view [cd/m 2 ], Ω is the solid angle subtended by each glare source [sr], L b is the average luminance of the background excluding the glare source [cd/m 2 ], ω is the solid angle subtended by the window [sr], and L w is the average luminance of the window [cd/m 2 ]. 82

2 Major factors influencing DGI evaluation are the brightness of light sources, the apparent area of the light sources as seen from a particular point of view and background luminance level. Glare sources are divided into three generic areas: sky, obstructions and ground. The Standard Code is lacking in further indication about the splitting of the window area in different glare sources. Table 1 summarizes DGI limits values for activities and spaces. Work places in scholastic environment are verified if DGI is less or equal to 21. In the criteria of glare evaluation, shown in Table 2, this is considered an acceptable level of glare [1]. Table 1: DGI limits values for activities and spaces ACTIVITY/SPACE DGI Laboratories 21 Reading room 21 Classroom 21 Computers 21 Drawing room 21 Music room 23 Libraries 21 Gyms 23 Table 2: Criteria of glare evaluation CRITERIA OF GLARE GI DGI EVALUATION Just perceptible Acceptable Irritable Intolerable VIDEOGRAPHIC TECHNIQUE The videophotometer (Fig. 1), the set up of which was carried out in October 2003 in the Illumination Technology Laboratory at DETEC in the University of Naples FEDERICO II [9], is composed of six fundamental parts. The main component is the videocamera JAI equipped with a sensor CCD 1280 x 1024 pixel. The videographic system is assembled with a lens of focal length 6 mm. This lens allows an acquisition field of 70 degrees on the horizontal plane and 60 degrees on the vertical plane; consequently a field of view of 180 degrees needs three acquisitions on vertical plane and three on the horizontal plane, so nine acquisitions are necessary. Finally, the system is composed by a photopic filter, a frame grabber, a personal computer and a specific image analysis software. Fig. 1: Videophotometer Videographic technique requires both spatial and photometric calibration. By means of spatial calibration, it is possible to give a geometric correspondence among the apparent surfaces and pixel area. In this way, the solid angle subtended by the glare source, is appraisable based on the pixel composing the image of the area of interest. Photometric calibration instead, defines the photometric values and in particular the luminance of the surfaces. Photometric calibration needs specific calibration procedures which require the following principal steps: zenith calibration, correction for vignetting characteristics, correction for background influence, correction for spectral response and bracketing technique. Luminance range goes from 0 to 40,000 cd/m 2 but this range can be widened using special neutral density filters. The luminance videophotometer allows a close net of measures necessary for a correct evaluation of the average luminance values, above all in the scenes with high level of luminance variability. 4. THE SITE The room examined for the case study is located in the historic centre of Naples, it is the School of Architecture of the University of Naples FEDERICO II. The object of the study is a reading room with two work places and a vertical window in one wall, glazed with glass having a transmittance factor of 80%. The room is used for the consultation of the texts of the library, then visual task in it concerns writing and reading activities. Curtains and daylight control screens are not available in the room. Geographic direction of the window is southwest and the sky, during the survey, can be considered clear sky. Dimensions and geometric ratios about the room are as below: 83

3 - room area = m 2 - window area = 5.46 m 2 - A w /A r = room height = 3 m - room factor = 0.7 Different kinds of surfaces are visible through the window, they are the sky, the floor of the balcony and the surrounding buildings, each representing a light source which would be considered a glare source. In the interior of the room the reflection factor of the ceiling is 90% and of the walls and the floor is 80% [10]. The study was conducted during the spring period when the solar arc is quite high on the horizon. Fig. 3 shows the video-camera setup to acquire the image corresponding to the cone which has direction right-up. Brightness and apparent size of light sources and background average luminance values were acquired by means of videophotometer. Luminance range of the videophotometer, even if not wide, was enough for the experimental tests. 5. THE CASE STUDY DGI is evaluated with the aid of the videographic technique, using a different procedure in comparison with the traditional. The field of view of the observer was considered as a cone with a central angle of 180 degrees, so nine acquisitions were enough to acquire the whole field. Fig. 2 shows the reading room and the cones of acquisition on the horizontal and vertical plane. Fig. 3: Acquisition (cone right up) A A - A Desk 2 (up) (left) (centre) (right) (centre) Desk 1 (down) A Acquisitions (left - up) (centre - up) (right - up) (left - centre) (centre - centre) (right - centre) (left - down) (centre - down) (right - down) Fig. 2: Acquisitions (horizontal end vertical plane) 84

4 The image acquired using the video-camera (Fig. 4) placed in the workplace (Fig. 5) is separated into basic elements. It means of 4624 squared elements (Fig. 6) each of them is composed by 15 x 15 pixel. Each pixel corresponds to a luminance value and a known solid angle; for what concerns the solid angle, from the spatial resolution verify, angular resolution loss with increasing angle fro m the optical axis is observed. This problem known as geometric compression at the edges of the image, is in countertendency with the solid angle decrease from the optical axis to the edges of the image. In this way it is possible to use a solid angle average value for every pixel. Fig. 5: Way in which the instrument is placed The value of luminance instead, derives from a correspondence function between the grey level of the pixel and luminance itself. This function has been obtained by means of a calibration procedure [9]. The classification of the external light sources, is based on the luminance range. Four specific classes are defined and reported below: - Class 1: the luminance range is between 8350 and 8000 cd/m 2 (Fig. 7) - Class 2: the luminance range is between 8000 and 6440 cd/m 2 (Fig. 8) - Class 3: the luminance range is between 6440 and 4100 cd/m 2 (Fig. 9) - Class 4: the luminance range is between 4100 and 980 cd/m 2 (Fig. 10). The choice of the classes is based on the area extension and on the luminance uniformity. Using the photometric calibration, a luminance map (Fig. 11) in false colour is carried out with extreme simplicity. Each colour corresponds to a specific range of luminance. In this way the classification is very easy and fast, but it remains the problem of how to identify the glare sources. Fig. 6: Image splitted in squared elements Fig. 7: Class 1: Sky ( cd/m 2 ) Fig. 4: Image captured by the video-camera Fig. 8: Class 2: Floor ( cd/m 2 ) 85

5 Fig. 9: Class 3: Obstruction ( cd/m 2 ) Fig. 10: Class 4: Obstruction ( cd/m 2 ) Fig. 11: Luminance mapping in false colour Italian Standard Code suggests, as previously mentioned, a division in three areas: sky, obstructions and ground. In this way, every source is seen as a part of a whole, but very often the threshold between each source is not very clear. In fact it occurs that some parts of obstructions, characterized by complex geometry and very variable reflection factor, have got characteristics values of luminance more similar to other types of surfaces belonging to the sky and ground. In this work four different cases of division between the glare sources, based on the luminance levels, are tested. In case one (Table 3), every class corresponds to a single glare source; in the second case (Table 4), first and second classes are merged in a single glare source; in the third case (Table 5) also third and fourth classes are merged. Finally all the classes are merged in a single, wide glare source in case 4 (Table 6). As shown in Table 7, Italian Standard Code results are not verified from case one to case three, but DGI estimated value regarding the last case is less than the limit value established by the Code, then is verified. 86

6 Table 3: 1 st kind of splitting between the several glare sources: case 1 MEASURE CLASS 1 CLASS 2 CLASS 3 CLASS 4 Pixel [n.] Solid angle [sr] Range of luminance [cd/m 2 ] Luminance av [cd/m 2 ] Gi DGI Table 4: 2 nd kind of splitting between the several glare sources: case 2 MEASURE CLASS 1+2 CLASS 3 CLASS 4 Pixel [n.] Solid angle [sr] Range of luminance [cd/m 2 ] Luminance av [cd/m 2 ] Gi DGI Table 5: 3 rd kind of splitting between the several glare sources: case 3 MEASURE CLASS 1+2 CLASS 3+4 Pixel [n.] Solid angle [sr] Range of luminance [cd/m 2 ] Luminance av [cd/m 2 ] Gi DGI Table 6: 4 th kind of splitting between the several glare sources: case 4 MEASURE CLASS Pixel [n.] Solid angle [sr] 0.39 Range of luminance [cd/m 2 ] Luminance av [cd/m 2 ] 5474 Gi DGI Table 7: Abstract of the four cases CASE DGI Class 1 + Class 2 + Class 3 + Class Class (1 + 2) + Class 3 + Class Class (1 + 2) + Class (3 + 4) Class ( ) CONCLUSIONS The application of the normative prescriptions needs easy and clear procedures and, mainly, suitable instruments qualified to give, in short time, a trustworthy answer. The Italian Standard Code today establishes, with regard to the luminance measurement, the use of instruments which field of measure is less than or equal to 1 about small surfaces and 10 about wide surfaces (UNI 10380) [11]. It considers only the traditional instruments measure field and limits the use of innovative instruments, even if with analogous performances with regard to the limit of uncertainty. This work shows some benefits which can result using the videographic technique. On one hand there is the saving of time, on the other hand there is a greater availability of data. Both aspects are very interesting but the second point is fundamental for 87

7 the final result. Results obtained from division between the several glare sources are very important. A rational division, which is not very clear in the Italian Standard Code, is based on the homogeneous luminance areas. Scene with high level of luminance variability needs a close net of measures for a correct evaluation of the average luminance values. A number of measures not suitable for wide surfaces can distort the average values, then to invalidate the verification. Also very short surfaces but with high luminance value can represent a problem and require a more close net of measures with additional measurements. Another important element in DGI evaluation is the changeable aspect of daylight, it is suitable to survey the luminance values at the same time in the whole area of interest, to the end of avoiding any change of illuminance due to the path of the sun all along the solar arc or to the weather. Well, the videophotometer, creditable to its characteristics, can be a smart tool not only for the DGI evaluation, but generally for the study of wide scenes and when the luminance values are characterized by great variability (complex surfaces). From the obtained results it can be inferred that DGI value can change strongly, on changing division in different areas (Table 7). In such way the same window can be verified or not as a consequence of the choice of division in more or less homogeneous areas. Consequently, the geometrical discretization of the source represents a critical point in the DGI model. The authors think that, in order to define a right criterion of division of the glare source for daylight, more experimental research is needed. At present a comprehensive discomfort glare model from daylight has not been validated, despite the fact that daylight is a very real part of our everyday lives while electric light for interiors wellestablished models are available for different sources [12]. ACKNOWLEDGEMENTS This work has been carried out with National Research Program Lighting and daylighting and their integration to the ends of the visual comfort and energy saving co-funded by the Italian Ministry for Universities and Scientific and Technological Research. REFERENCES 1. Norma Italiana UNI 10840, Luce e illuminazione, Locali scolastici, Criteri generali per l illuminazione artificiale e naturale, Marzo (2000). 2. S. Ranasinghe and R. Mistrick, A study of photosensor configuration and performance in a daylighted classroom space, Journal of the Illuminating Engineering Society, Summer, pp (2003). 3. CIE 16/1-1970, Daylight (1970). 4. W. Wu, A review of the development of daylighting in schools, Lighting Research and Technology, Vol. 35, No. 2, pp (2003). 5. CIE , Discomfort glare in interior lighting (1995). 6. CIE , CIE Collection on glare (2002). 7. J.J. Vos, Reflections on glare, Lighting Research and Technology, Vol. 35, No. 2, pp (2003). 8. European Standard EN 12464, Light and lighting Lighting for work places Part 1 Indoor work places, October (2004). 9. L. Bellia, A. Cesarano, F. Minichiello, S. Sibilio and G. Spada, Calibration procedures of a CCD camera for photometric measurements, Instrumentation and Measurement Technology Conference, Vail, CO, USA, Maggio (2003). 10. CIE , Practical methods for the measurement of reflectance and transmittance (1998). 11. Norma Italiana UNI 10380, Illuminazione di interni con luce artificiale, Maggio (1994). 12. M.L. Eble-Hankins and C.E. Waters, VCP and UGR glare evaluation systems: a look back and a way forward, Leukos, Vol. 1, No. 2, Ottobre 2004, pp (2004). From the above mentioned considerations, the authors retain that the glare models for daylight need further studies. 88