Discomfort and Disability Glare in the Visual Environment

Abstract

It is important to understand recent lighting and fenestration history and its development. Before the era of on-tap electrical energy supply for lighting, when artificial lighting sources were inefficient, fenestration was used as a means of providing working illuminance in addition to its many other well-documented functions. Daylighting technology developed firstly from a need to ease natural light into buildings in order to illuminate tasks and interior spaces. High ceiling heights with tall windows were favored and roof lighting, to provide working illuminance, became extensively used in industrial premises. With the advent of the fluorescent lamp, and its rapid rise to prominence during the 1950s, buildings were freed from many configuration constraints and the use of fenestration, as a means of providing working illuminance, diminished. Within a further decade, artificial lighting levels soared, especially in the USA. Levels of 2,000 lux were not uncommon as the philosophy that more light is better propagated rapidly. Controversy abounded concurrently with these developments. One area of controversy was associated with the trend toward seemingly unnecessary ever higher interior illuminances. The lighting industry was blamed for producing a body of opinion which favored higher illuminance and, hence, higher financial returns to the industry. Then the 1970s energy crisis materialized. It was already known that the then popular deep office building, illuminated almost totally by artificial lighting, was a prime offender in the excess energy consumption league. The high lighting levels used in these buildings came under immediate attack, resulting in large-scale delamping, which resulted in widespread but scarcely acknowledged complaints about difficulties due to observed fenestration brightness.

Appendix 12

12.1.1 Comparison of Changing Glare Situations Under Various Daylight Conditions

In an artificially illuminated interior, e.g., during evenings, the overall luminance distribution is almost stable and constant in time. So, the glare situation can be unchanged, with probably only interference with temporary changes caused by the TV monitor excitation. However, during daytime, owing to outdoor daylight condition changes, i.e., gradual or abrupt increasing or moving cloudiness patterns and the sun position movements, the window luminance and consequently also glare situations can vary. The window luminance pattern is influenced by sky luminance in its solid angle and the window orientation. So, in a side-lit room the GI can reach different values indicating glare-free situations under overcast skies or extreme discomfort or disability conditions when the sun appears in the window view. Such varieties can be documented in an example of a room with one window earlier used in the appendix in Chap. 8 applying the MAM modeller computer possibilities (Roy et al. 2007) to check window luminance under typical sky standards.

The glare source luminance, the maximum luminance in the window solid angle ${L_{{{\rm{W}}\max }}}$ seen from the three elements of the working plane as task luminance L _t that can cause to rise the ${\hbox{GI}}$ in the simple formula (12.5) to rise will be evaluated:

$$ {\hbox{GI}} = 0.8\frac{{{L_{\rm{Wmax}}}}}{{{L_{\rm{t}}}}}. $$

(A12.1)

The momentary maximum luminance within the window solid angle was estimated from the upper corner of the window image in MAM while the task luminance of common reading print with reflectance ${\rho_{\rm{t}}} = 0.7$, illuminated by window luminance with horizontal sky illuminance ${E_{\rm{vS}}}$, was increased by interreflection $r$. Thus, the task luminance for the printed text on a horizontal table desk is

$$ {L_{{{\rm{t,text}}}}} = \frac{{{E_{\rm{vS}}}\,{\rho_{\rm{t}}}\,r}}{\pi }\,({\hbox{cd}}/{{\hbox{m}}^{{2}}}), $$

(A12.2)

where approximate $r$ values rise from the elements close to the window to the element deep inside.

To enable the comparison with the example in a previous article (Kittler et al. 2010), the same interior and window size as well as the elements on the working plane were chosen:

• An administrative room 3 m wide and 4.5 m deep with one unobstructed window 1.5 m in width and 1.8 m high was chosen as representative for office work.

• In this room three reference points on the working plane 0.85 m above the floor were placed along the window axis noted as A at 1-m distance from the glazing, B at 2-m distance, and C at 3-m distance, respectively.

• Sky luminance seen through the glazing is reduced by normal glass transmittance of 80%.

• Paper work and text reading are still very common visual tasks. The reflectance of printed paper of 70% was considered.

• The room surface reflectance for the ceiling (80%), for walls (50%) and for the floor (20%) represent most common cases, and exterior terrain reflectance was taken as 20%.

• As MAMmodeller calculates only the horizontal sky illuminance $\,{E_{\rm{vS}}}$ in elements A, B, and C these, were multiplied by factor r, i.e.,

  • At A: $r = 1.15,$; thus, ${L_{{{\rm{t,text}}}}} = {E_{\rm{vS}}}\,{\rho_{\rm{t}}}\,r/\pi = 0.256\,{E_{\rm{vS}}}$ _.

  • At B: $r = 1.35$, i.e., ${L_{{{\rm{t,text}}}}} = 0.3\,{E_{\rm{vS}}}$ _.

  • At C: $r = 1.75$ and ${L_{{{\rm{t,text}}}}} = 0.39\,{E_{\rm{vS}}}$.

Regular office, school, and other cerebral work requires a mixture of both paper and video display reading tasks to be accomplished without inducing undue visual comfort. In addition to satisfactory task visibility requirements, a range of task-related GIs are employed to ensure freedom from excessive visual discomfort. Therefore, daylighting levels must be determined in partnership with the GI levels they induce. There are two obvious ways to reduce an excessive GI: reduce the luminance of the source or increase the luminance of the task. The first should be a last resort if daylighting is prioritized and building energy use parametric simulations, for instance, by eQUEST (http://doe2.com/equest/index.html), are used to provide an agreeable energy cost. The second choice, also with energy cost implications, is to increase the task luminance by either supplementary task lighting, for paper tasks, or by increasing video monitor display luminance. The following tables provide a snapshot of the visual alternatives to be contemplated against incentives such as building energy use, maintenance, visual quality, amenity, and workplace productivity. Bear in mind, with the latter, the very high financial cots of people in the workplace, in relation to the cost of materializing and operating a large-scale work environment. The following tables only sketch the possibilities for intervention to correct a wayward design, but hopefully illustrate a virtually intuitive approach to dealing with the potential pitfalls of daylighting design of cerebral work environments.

The screen luminances have been selected after sampling literature on readily available computer video display luminance and contrast capabilities of readily available video monitors. Also, bear in mind that LED backlit monitors are proliferating, are energy-efficient, and will produce luminances far beyond the luminances selected for this demonstration: 100, 200, and 800 cd/m² were adopted as ${L_{\rm{tC}}}$ in candelas per square meter.

Owing space restrictions in this appendix only one window orientation could be considered in order to demonstrate that any room orientation can be chosen, in this example a random option is the window azimuth ${A_{\rm{W}}} = 140^\circ$ from north. Furthermore, to demonstrate the dependence of ${\hbox{GI}}$ on the geographical latitude of the locality, the 45° latitude was chosen, along which lie North American cities such as Minneapolis/St. Paul, Ottawa, and Montreal and European centers such as Bordeaux, Turin, and Belgrade. As in MAMmodeller, a certain location has to be logged. In the following example was assumed a locality east of Bordeaux with geographical latitude 45° and longitude 0°. Of course, because of different climate and weather influences besides the relevant sun-path similarities, the glare problem can be also different in the momentary, daily, or seasonal occurrence of sky patterns, cloudiness, and sunshine duration expectancies. Experienced designers know that the roughly southeastern window has to have some protection against excessive heat gains and glare during sunny conditions. In Design, appropriate external shading and building configuration alternatives should have been explored and deployed. If the window is causing discomfort glare use venetian blinds, reduced transmittance glass, retrofit film or other shading to alleviate the malady while maintaining the external view (Köster 2004). If the GI rating is too high, try raising the video luminance and interior lighting will also reduce the window and luminaire effective impact on the rating on paper tasks. Do not reduce the window transmittance below about 25% or you will depreciate the view (MacGowan 1984). A favourable energy balance between the efficiency and the utility of windows usually requires a computer simulation, then the most favourable luminance for a work environment can be achieved. The tables provided in this appendix are only simplistic outlines to be played with in order to engender understanding by providing an intuitive feel for the intervention strategies.

For this example only, several outdoor conditions and ISO/CIE sky types have been chosen to demonstrate and test glare conditions calculating ${\hbox{GI}}$ values either for paperwork (${\hbox{G}}{{\hbox{I}}_{\rm{P}}}$) or for visual work on a computer screen (${\hbox{G}}{{\hbox{I}}_{\rm{C}}}$):

Under the overcast sky ISO type 1 (ISO 2004), if ${D_{\rm{v}}}/{E_{\rm{v}}} = 0.1$, ${\tau_{\rm{g}}} = 0.8$, on December 15 in Table A12.1.

Table A12.1 Daytime changes of glare situations on a winter day under sky type 1

In this first test the lowest winter daylight conditions were taken into account as well as the lowest computer screen luminance ${L_{\rm{tC}}} = 100\;{\hbox{cd}}/{{\hbox{m}}^{{2}}}$. In this example, interior illuminance levels are so low that certainly artificial illumination will be used, which will better the overall satisfaction. Therefore, also better daylighting under overcast sky type 4 was tested assuming the same day in December but a higher illuminance level in the following example:

Under the overcast sky ISO type 4 (ISO 2004), if ${D_{\rm{v}}}/{E_{\rm{v}}} = 0.22$, ${\tau_{\rm{g}}} = 0.8$, on December 15 in Table A12.2.

Table A12.2 Daytime changes of glare situations on a winter day under sky type 4

From Tables A12.1 and A12.2 it is evident that point C, with low interior illuminance, causes the low task luminance and in comparison wit window luminance also intolerable glare (indicated by “a”). Some brighter sky patches under sky type 4 and ${D_{\rm{v}}}/{E_{\rm{v}}} = 0.22$ are in the window solid angle at 10:00–12:00, the computer screen luminance becomes too low, and ${\hbox{GI}} \geqslant 28$.

Under the clear sky ISO type 13 (ISO 2004), if ${T_{\rm{v}}} = 5.5$, ${\tau_{\rm{g}}} = 0.8$, on March 1. The window orientation to 140° exposes it to direct sunshine, which is evident in Table A12.3.

Table A12.3 Daytime changes of glare situations on a spring day under sky type 13

In spite of the gradual decreasing effect of sunbeams on the window façade in afternoon hours, the unacceptable glare conditions are only slightly lowered. Under sunshine and clear sky conditions, the glare problem can be solved by several alternative possibilities, e.g., applying window transparent shading (${\tau_{\rm{g}}} = 0.35$) while still retaining an acceptable view (MacGowan et al. 1984). At the same time, if computer screens are manually or automatically controlled to respond to higher window luminances, by display luminance adjustment, e.g., to 800 cd/m², then the reduction from ${\hbox{G}}{{\hbox{I}}_{\rm{P}}}$ to ${\hbox{G}}{{\hbox{I}}_{\rm{C}}}$ would be significant as shown in Table A12.4.

Table A12.4 The glare situation on March 15 at 8 a.m. under sky type 13 when the window is shaded and the display luminance is 800 cd/m²

Shading alone will not result in the solution of disability glare because the interior illuminance also drops because of the window’s shading. Thus, additional artificial illuminance of the visual task is one simple solution. For instance, only turning on night lighting with an additional 200-lx level to complement the shaded window’s contribution would result in a quite high temporarily satisfactory ${\hbox{GI}} = 24$. Hence, the example above shows how higher interior artificial illuminance and video display luminance could be used to lower the GI under sunny conditions and shading while preserving a key function of the window view.

If a video display screen has a “controlled” working luminance of up to 800 cd/m², then an insolated window’s interior luminance could reach 12,000 cd/m², ${\hbox{GI}} = 12$, and be unlikely to cause any perceptible discomfort for most people who work in such a “daylit” luminous environment.

As soon as the sun path goes over the window head in summer, the glare situation is less severe owing to the absence of the solar disc in the window solid angle. For critical point A, the window head on the centerline is $\varepsilon = {\hbox{arc}}\;{\hbox{cotan}}\;1/1.8 = 60.97^\circ$, whereas in the relevant upper window corner it is $\varepsilon = {\hbox{arc}}\;{\hbox{cotan}}\left( {\sqrt {{1 + {{0.9}^2}}} } \right)/1.8 = 53.2^\circ$.

Such a situation can be documented, e.g., by the following example.

Under the clear sky ISO type 12 (ISO 2004), if ${T_{\rm{v}}} = 4$, ${\tau_{\rm{g}}} = 0.8$, on June15. The window orientation to 140° exposes it to direct sunshine, but under relatively high solar altitudes. The glare situation is evident from inspecting Table A12.5.

Table A12.5 The glare situation on June 15 under sky type 12 when the window at orientation 140° is unshaded

When the sun position is not passing the window solid angle, the glare situation better resembles the conditions under cloudy sky type 4 in December. However, owing to relatively higher illuminance levels outdoors, the maximum window luminance is also high and window shading with additional artificial lighting is recommended especially in position C.

Every window orientation is exposed to a variety of daylight conditions and sun-path changes, as documented by interior glare situations in the example above.