(Day)light Metrics. Dr.- Ing Jan Wienold. epfl.ch Lab URL: EPFL ENAC IA LIPID

Transcription

1 (Day)light Metrics Dr.- Ing Jan Wienold epfl.ch Lab URL:

2 Content Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

3 High School in New Jersey Union Carbide building, NY (SOM) How can we distinguish between good and bad design??

4 All is about comfort All energy we need in a building is related to comfort Heating, cooling: providing temperature Electric lighting: Providing light levels -> to design the building and to size the systems, we must know how much comfort we need -> metrics describe the amount of comfort

5 Thermal comfort well established Visual comfort upcoming

6 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

7 Photometric quantities lm/m 2 = lux lm lm/(m 2 sr)= cd/m 2 lm/sr = cd

8 Illuminance [lux]

9 Daylight design is challenging Sun is never at the same place (hourly, season) Huge difference in daylight availability between overcast and clear sky Orientation is important Shading and its usage hard to predict

10 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

11 Daylight provision metrics The metrics should answer how much daylight arrives in the space. The metrics should be able to distinguish between good and bad design. static climate based

12 Daylight factor Only for overcast sky Horizontal Daylight Factor DF [%] = (E point /E outside ) * 100% 2% Minimum 4-6% Satisfactory but it is a metric from the past!!!!!

13 Daylight factor Only for overcast sky literally outdated... was already proposed in the early 1900s when no sophisticated calculation methods were available.

14 Daylight factor Not taking care of sun means Palermo, Sicily -> no influence of orientation -> no influnce on climate -> time of day is irrelevant -> shading and it s usage is neglected Trondheim, Norway

15 Daylight factor Good for quick judgment for the daylight opening below 1% -> dark, only suitable for storage areas 1% to 2% -> low illumination, suitable for circulation areas 2% to 4% -> moderate, for living spaces 4% to 7% -> medium, for office work 7% to 12% -> high, for precision tasks over 12% -> very high, for exceptional light requirements

16 Daylight provision metrics static climate based

17 Climate based daylight provision metrics What does this mean? Takes into account climate Usually based on hourly timestep daylight calculations Summarized usually into one number

18 Climate based daylight provision metrics Daylight Autonomy Continuous Daylight Autonomy Zonal Daylight Autonomy Spatial Daylight Autonomy (sda) Temporal Daylight Autonomy Useful Daylight Illuminance Direct Sunlight Hours Maximum Daylight Autonomy Daylight Saturation Percentage

19 Climate based daylight provision metrics Daylight Autonomy DA Percentage of working hours when a minimum work plane illuminance is maintained by daylight alone DA = (hours without artificial lighting/ hours without artificial lighting possible) * 100% 50% Minimum >65% Good

20 Climate based daylight provision metrics Spacial Daylight Autonomy (sda 300/50% ) Reports a percentage of floor area that exceeds a specified illuminance level, e.g., 300 lux, for a specified amount of annual hours, e.g., 50 percent of the hours from 8:00 am to 6:00 pm Preferred threshold: sda 300/50% 75% of analysis area. Nominally acceptable threshold: sda 300/50% 55% of analysis area.

21 Climate based daylight provision metrics Useful Daylight Illuminance UDI Categorizes the (hourly) calculated illuminance values into three categories: 1. too low (typically <100lux) 2. full credit (typically between lux) 3. too high (>3000lux) UDI gives then the fraction of time of the full credit category

22 Climate based daylight provision metrics Useful Daylight Illuminance UDI BUT: Is an illuminance level >3000lux not useful?? This can be questioned and is under discussion!

23 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

24 Glare What is glare??

25 Daylight glare CIE Definition of glare: Condition of vision in which there is discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution or range of luminance, or by extreme contrasts Can be subdivided into 3 main categories: 1. Reflections or veiling glare -> Legibility of computer screens 2. Disability glare: impairs the vision, but not necessarily causing discomfort 3. Discomfort glare: Glare that causes discomfort without necessarily impairing the vision of objects

26 Disability glare

27 Disability glare Quantification (CIE) : Equation from Stiles- Holladay

28 Disability glare BUT: Unclear if applicable for daylight situations! Studies needed

29 Discomfort glare Discomfort = Subjective rating In most cases below disability glare Possible scaling: imperceptible, perceptible, disturbing, intolerable Indirect consequences (headaches, getting fatigue), often not direct measurable How to quantify?

30 Motivation Daylight glare metrics up to now German regulations: Using a fixed luminance value for the façade, independent of size ( reflex glare, not suitable for discomfort glare) Complex glare equations (like daylight glare index DGI. What causes more glare? This Wienold situation or this situation? J

31 Discomfort glare fixed luminance value makes no sense for discomfort glare! complex glare equations 3 4 b ł G = f Ł L L a s a 1 w P a s a L s : Luminance of source w s : Solid angle of source L b : Background luminance adaptation P: Position index 2

32 Solid angle definition a solid angle (symbol: W) is the two-dimensional angle in three-dimensional space that an object subtends a point is the seen size of an Area dimensionless [sr] steradian r A

33 Position index weights a glare source on its position depends on the horizontal and vertical displacement of the line of sight IES-Definition:

34 Position index The larger P the less influence has the glare source max influence in the line of sight (value =1) min influence at the top (value=16)

35 Discomfort glare for electric lighting Unified Glare Ratio L s : Luminance of source w s : Solid angle of source L b : Background luminance adaptation P: Position index

36 UGR Glare Criterion UGR Discomfort Glare Criterion 10 Imperceptible 13 Just perceptible 16 Perceptible 19 Just acceptable 22 Unacceptable 25 Just uncomfortable 28 Uncomfortable

37 Discomfort glare Daylight glare index developed with less than 10 subjects studies showed, that only low correlations between user perception and DGI exist DGI = 10log n L i= 1 b Ls Ws 0.07w 0.5 s L s

38 Discomfort glare Daylight Glare Probability DGP Combination of the vertical eye illuminance and a modified glare index equation DGP = c 2 s, i s, i 1 Ev + c2 log(1 + ) + a 2 1 i Ev Pi L w c 3 E v : vertical Eye illuminance [lux] L s : Luminance of source [cd/m²] w s : solid angle of source [-] P: Position index [-] c c c a = 5.87 = 9.18 = 0.16 =

39 Discomfort glare Daylight Glare Probability DGP How to calculate? -> Evalglare

40 Evalglare Detects glare sources Calculates solid angles from pixels/glare sources Calculates vertical illuminance from image Calculates various glare metrics (DGP, UGR, VCP, DGI, CGI, Ev, Lveil Can be used also for calculating average luminance of surfaces (window..) Field of view can be cut out

41 Discomfort glare how to evaluate on annual basis? Rendering of images for each hour is too time consuming! A fast and reliable method is needed

42 Discomfort glare how to evaluate on annual basis? gen_dgp_profile Calculating the vertical eye illuminance by the use of daylight coefficient method Hourly E v Calculation of a simplified image S a x Ev a (x) illuminance at x due to S a DGP = c 2 s, i s, i 1 Ev + c2 log(1 + ) + a 2 1 i Ev Pi L w c 3

43 Annual glare evaluation Glare maps

44 Annual glare evaluation Histogram 95% value A best class 95 % of office-time glare weaker than imperceptible B good class 95 % of office-time glare weaker than perceptible C reasonable class 95 % of office-time glare weaker than disturbing DGP limit Average DGP limit within 5 % band

45 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

46 Correlated Color temperature describes the color of a light source is defined as the Temperature [K] of an ideal black body radiator

47 Correlated Color temperature describes the color of a light source is defined as the Temperature [K] of an ideal black body radiator

48 Correlated Color temperature CCT can describe just a few possible colors in the real color space

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50 Color rendering index Describes how the color of an object appears The CRI is calculated from the color difference of 14 defined colors under a reference light source and the test light source

51 Color rendering index CRI 100: reference It should be greater than 70 for artificial light sources For glazings: It should be greater than 92

52 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

53 View to the exterior Importance relative frequency of occurrance 50% 40% 30% 20% 10% 0% Is the view to the outside important for you? 36% 39% 17% 8% 0-33% 33-66% 66-85% % not at all very much

54 View to the exterior Is view more important than glare? Relative frequency of occurrence 70% 60% 50% 40% 30% 20% 10% vor before Eingriff user setting nach after Eingriff user setting 0% imperceptible nicht wahrnehmbar noticeable wahrnehmbar disturbing störend intolerable inakzeptabel Degree of glare

55 View to the exterior How to quantify? -> So far no reliable method established German standard has following rules: Sum of width of all windows(glazing) at least 55% of façade Minimum height of window: 1.25m Upper level of glazing at least 2.20m Lower level of glazing not higher than 0.9m

56 View to the exterior German standard has following rules: 55% 0.9m

57 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

58 Legibility of computer screens Legibility -> Contrast Contrast: CR = L L H L L L L R Visible Contrast CR = L L H L + L + L R R L H

59 Reflections on computer screens from surround light sources and objects Superimposed on screen image Contrast on the screen might become lower than necessary to read Question: What it the necessary contrast in order to have no problems reading displayed text?

60 Reflections on computer screens Existing model ISO :2008 CR min.( = Kage LL ) Contrast is a function of Low state Luminance Based on very old experiments with other adaptation levels than in real office spaces Experiments show, that adaption of the model necessary In office environments a visible contrast of 4 is mostly sufficient

61 Daylight provision metrics Why do we need metrics? Luminous units, Light Levels Daylight Provision Glare: Electric lighting and Daylight Color temperature and Color rendering View to exterior Legibility of computer screens Glare in outdoor environments

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66 Natural glare situation

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70 Is it limited to solar panels? No! Every glazed or metal surface could cause severe glare!

71 Reflected glare by surfaces Glare effects are driven not by the type of application (e.g. solar panels) but by the surface properties! Also in nature and normal life glare situation can occur It has different dimensions: Traffic: It can cause severe problems for the driver/pilot of vehicles when reducing contrast and/or causing disability glare -> problems/slowing down in object recognition -> safety risk Discomfort: People perceive discomfort glare in their homes or in outdoor spaces

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74 Official calculation method in Germany to calculate and evaluate glare by solar panels (so far) Only PV panels Only for discomfort evaluation Based on a study of the stroboscopic effect of wind power stations Limits the amount of occurrence to 0.5h/day and 30h/year max. Every possible reflection is counted -> no real weather data is taken into consideration! The reflection properties are not taken into account, the PV is assumed as mirror!

75 Glare in the exterior No reliable metrics available yet! Trying to avoid high reflective surfaces, especially when traffic might be concerned

76 Conclusion Daylight metrics are important to prove a daylight design Daylight factor has very limited meaning Climate based metrics describe the annual daylight behavior better than DF Some of the metrics are still under development