Visual Comfort

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Adaptation

Adapting to bright windows reduces the visibility of the bulletin board. (Source: Hayden McKay)
Individuals may chose visual discomfort for the pleasure of sunlight. (Source: Murray Milne)

Visual comfort is one of the key elements of lighting quality, and the criteria changes depending upon the application. Discomfort is most often caused by an excessive contrast in perceived brightness. Our eyes adapt to the most prevalent brightness in our field of view, and when the brightness is fairly uniform we are generally comfortable, whether the levels are high or low. We can adapt to a wide range in brightness, from a moonlit night to a bright sunny day. The discomfort occurs when our eyes try to adapt to two levels at once, like trying to read a bulletin board with bright windows on either side. Discomfort also occurs when the contrast is sudden, like exiting from a darkened theater into a sunlit street. Our tolerance for high contrast also varies depending on the circumstances. In general, people are more tolerant of extremes if the light source is natural, if they have the option to make adjustments to avoid discomfort, and if there is little pressure to perform difficult visual tasks. Occupants in a library may choose a chair in bright sunlight, even though reading is more difficult. But in working environments, the goal is to avoid visual discomfort and provide acceptable levels or contrast. To assist in this goal the Illuminating Engineering Society of North America (IESNA) publishes recommendations for luminance ratios for various space types like offices and classrooms. For example, a maximum 3:1 or I:3 ratio is recommended between the luminance of the task and the immediate surround, and a maximum of 1:40 between the task and a bright window.

However, total uniformity is not the goal. A small amount of contrast creates soft shadows that are desirable to make people and objects look three‐dimensional and natural. A totally uniform, shadowless luminous environment also lacks interest and visual stimulation, like a fully overcast sky.

Section Key Resources
  • Hopkinson, R.G., (1963) Architectural Physics: Lighting, Chapter 1. Her Majesty’s Stationery Office, London. Philips, Derek.
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

Glare

Contrast between the window and face renders the facial expressions unrecognizable, resulting in a form of disability glare. (Source: Hayden McKay)
Luminaires with exposed lamps led to a new “overhead glare zone”. (Source: DOE)

Glare is a condition of excessive contrast in perceived brightness in the field of view and is dependent on our visual sensitivities, the application, and our adaptation level. The two primary classifications of glare are “discomfort” glare where the contrast is unpleasant or distracting and requires extra effort to perform a work task. More serious is “disability” glare, making it difficult or impossible to perform a visual task. When our eyes are adapted to the nighttime environment, on‐coming car headlights can be temporarily blinding, causing disability glare. During the daytime, the same headlights cause neither discomfort nor disability. In shiny television screens or computer monitors, reflections of luminaires or windows create a “veiling reflection” which can be mildly obscuring or so severe that the visual task cannot be adequately viewed. Luminance ratios are a good predictor of our perception of brightness contrasts, but there is not a straightforward relation between objective ratios and the subjective perception of glare. Under the same conditions, one individual may consider a 1:10 ratio to be uncomfortable while another may not. Likewise, in one setting, like an office, a 1:20 ratio of brightness between a task and surrounds might be uncomfortable for most workers, but in an entertainment environment this same ratio could be delightful, considered “sparkle”, not glare.



Section Key Resources
  • Hopkinson, R.G., (1963) Architectural Physics: Lighting, Chapter 1. Her Majesty’s Stationery Office, London. Philips, Derek.
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

Distribution

Uniform distribution of luminances is important for a number of reasons. By definition, it avoids high contrast that can cause glare. It also creates a calm, harmonious and neutral background that does not distract from the visual task at hand. Uneven lighting may create a chaotic luminous environment, distracting the observer and drawing attention to the brightest surfaces, which are not necessarily the most important. Uniformity is most desirable in classrooms and workspaces, where challenging visual tasks must be performed quickly and accurately. Uniformity is most easily achieved when all the surfaces in a space are relatively light‐colored and matte in finish. Task ambient lighting is most successful when the ambient lighting uniformly illuminates the ceilings and walls of a space, allowing the occupants to comfortably adapt to those luminance, and the task light only provides supplemental illumination and focus, without high contrast. The mantra of “putting light only where it is needed” during the first energy crisis produced lighting systems with unacceptable quality because there were bright pockets of light randomly dispersed in a dark surround. Total uniformity is not required, and in fact can be boring and soporific. Even in a workspace, some variation is necessary for proper facial definition, three‐dimensional modeling, and visual interest. In non‐work environments, contrast is desirable for visual stimulation and draw attention to specific areas or objects, like in stores, theaters, restaurants and entertainment spaces. Uniformity is evaluated by luminance ratios, and the Illuminating Engineering Society of North America, among others, publishes recommended or maximum luminance ratios for various space types (offices, classrooms) and various applications (roadways, pedestrian paths, egress corridors, parking lots.)

Section Key Resources
  • No publications specific to this section have been listed.
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

Direction

The directionality of light sources, luminaires and reflective or transmissive surfaces are key elements in lighting design, and contribute to visual comfort or glare. Light sources range from narrowly direct with a tightly focused beam, to widely diffuse, emitting light in all directions. Luminaires can redirect diffuse light into a narrower range, typically through the use of reflectors, prisms or baffles. Likewise, a directional source can be diffused by the addition of spread lenses. Transmitting media such as glass or plastics can minimally affect the originating beamspread, or entirely change it. Finally, reflecting material can maintain a mirror image, or provide a uniform, diffuse appearance. The goal of lighting design is to create a combination of lighting distributions and a visual hierarchy of brightness that is appropriate to the needs of the space and occupants. The extremes (totally direct or totally diffuse) are seldom desirable. A retail store with nothing but spotlights fails to highlight any merchandise in particular. An office luminaire that only directs light to the working plane fails to light the space comfortably. Too much directionality results in excessive shadows, contrast and glare. Total diffuseness results in loss of shadows and a flat, uninteresting visual environment.

Reflectance values are independent of the shininess (specularity) or diffuseness of a material. A shiny surface will reflect the same overall percentage of light as a matte surface, except in a shiny surface, all the light is reflected in one direction (a specular reflection), whereas on a matte surface, the same percentage of light is reflected in all directions (a diffuse reflection).

Section Key Resources
  • Hopkinson, R.G., (1963) Architectural Physics: Lighting, Chapter 1. Her Majesty’s Stationery Office, London. Philips, Derek.
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

Room Surface Reflectances

Dark finishes absorb most of the light, and make it impossible to balance luminances in a workspace. (Source: Hayden McKay)

The use of light‐colored walls, ceilings and furniture has a greater impact on lighting energy efficiency than any other strategy, and costs nothing. Room surface reflectances are essential components of lighting design and how a space is perceived. Reflectance of a material is a function of its light “value”, that is the amount of black or white in the color. All materials absorb some light, but bright white surfaces can reflect 90% or more of the incident illumination, and dark interior finishes may reflect less than 5% of the light. Reflectance values are independent of the shininess (specularity) or diffuseness of a material. A shiny surface will reflect the same overall percentage of light as a matte surface, except in a shiny surface, all the light is reflected in one direction (a specular reflection), whereas on a matte surface, the same percentage of light is reflected in all directions (a diffuse reflection).

Our perception of brightness in a space is most often based on the appearance of the vertical surfaces in a space, rather than the light on a horizontal plane. High illumination on the task may be perceived as insufficient if the walls, ceiling or adjacent partitions are dark. Individuals typically prefer working in an environment with lower task illumination and higher room surface luminance, rather than the reverse condition.

People frequently over‐estimate the reflectance values of materials. In an experience where observers evaluate multiple cards of varying reflectances from black through gray to white, the selection of “middle gray” approximating 50% reflectance was in fact closer to 20% reflectance. This can have significant energy consequences. A designer may believe they are picking colors that are light or medium reflectance, and inform the lighting designer of that assumption, but the materials specified may be considerably more absorptive, reducing the energy efficiency and luminous appearance of the space. Light reflectance values are available from manufacturers of paints and fabrics. An additional column added to furniture and finish schedules listing lighting reflectance of materials is good design and energy practice. In workspaces, (offices, classrooms, factories) light colored materials with a matte finish are the most effective for utilizing daylight and electric light, by minimizing absorption and creating inter‐reflections. If the light is distributed to walls and ceiling, the space will feel bright and relatively uniform, promoting a high adaptation level and visual comfort. Inter‐reflections increase the usable light on the working plane and reduce unwanted shadows.

In workspaces, ceilings should be white, at least 80% reflective if luminaires are recessed, and at least 90% reflective for designs with any indirect lighting. These reflectance values are readily available, even with acoustical tiles. Walls, internal furniture partitions and overhead bins should have a reflectance of 70% or higher, which equates to pastel colors. Desktops should be at least 60% reflective. Saturated colors and darker values are acceptable in limited applications, or when used below the working plane. The reflectance of the floor has less of an impact on the efficiency of a space, but lighter finishes are desirable in open circulation areas. In other applications, darker finishes may be advantageous (in a movie theater), or necessary to achieve a saturated color for a feature wall, or to create desirable contrast for a museum display. But even in these applications, dark surrounds can be achieved through the use of medium reflectance materials accompanied by reduced lighting, to minimize energy wasted through absorption.

Section Key Resources
  • No publications specific to this section have been listed.
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

Flexibility for user preference, tasks, photobiological needs

In a natural setting where we have access to natural light, the body synchronizes its internal clock to the changing quality of daylight through the day and night. The body’s circadian systems and sleep/wake cycle are regulated by this regular oscillation between day and night. Light responsiveness regulating physiological processes highlight the fact that light must be thoughtfully examined in the built environment. Darkness at night is as important as light during the day for maintaining normal circadian rhythms and sleep patterns. Light pollution at night can come from many sources: from outside light sources shining through the window, from adjacent rooms, as well as from lights in the room. This extra light that illuminates our nights can be disruptive to full sleep and the natural circadian rhythm. Night shift workers also have a disruption in the normal cyclical relationship to light and darkness. While light is important for visual task performance, it is also stimulating the circadian system at night. When the cycle of light/wakefulness followed by darkness/sleep is disrupted, our physiological processes can be thrown off balance. Brightly illuminated nights over a long period of time can cause stress and exhaust the body, making it vulnerable to a variety of illnesses. In a recent study of a large population of nurses, it was discovered that there was a correlation between nightshift work and increased risk for cancer (Dimich-Ward, 2007). Productivity can also suffer from light disruptions. In a study in a hospital setting in Alaska, it was found that medication errors appear to be nearly twice as prevalent in the Winter months compared to Summer months (Booker 2002). The optimal balance of light as well as dark is critical for maintaining a healthy cyclical balance – and the built environment can be designed to help support the oscillation between high light exposure during the day and true darkness at night.

Section Key Resources
  • Berson, D.M., Dunn, F.A., Takao, M. (2002). Phototransduction by retinal ganglion cell that set the circadian clock. Science, 295: 1070-1073.
  • Booker, J., Roseman, C. (1995) A Seasonal Pattern of Hospital Medication Errors in Alaska. Psychiatry Research 57(1995) 251-257.
  • Dimich-Ward, H., Lororenzi, M., Teschke, K., et. al. “Mortality and Cancer Incidence in a Cohort of Registered Nurses from British Columbia, Canada.” American Journal of Industrial Medicine 50 (2007): 892-900.
  • Evening Light Exposure, Implications for Sleep and Depression -- JAGS -- Wallace-Guy etal -- 2002.pdf
  • Seasonal Pattern of Hospital Medication Errors in Alaska -- Booker and Roseman -- Psychiatry Research -- 1995.pdf
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

Nighttime / adaptation / scotopic

Adaptation is a process where the visual system changes its sensitivity to get accustomed to more or less light than it was exposed to during an immediately preceding event. The retina, which is the innermost layer of eye, contains two classes of light sensitive cells- rods and cones.

Light adaptation, which involves cones, occurs when moving from a dark to bright environment. This adaptation, which may be momentarily painful, occurs in milliseconds. Dark adaptation, which involves rods, occurs very slowly when moving from a bright to dark environment. The time to adapt to darkness depends on the difference between the luminances of the bright and the dark environments and can take as long as 20 minutes. However, the dark adaptation can take up to one hour to complete.

Schematic representation of visual adaptation. Source: www.visual-3d.com
Source: www.visualexpert.com

Adaptation to the prevailing light condition determines how well the visual system is functioning. The visual system of human beings can process information over an enormous range of luminances (approximately 12 log units), however not all simultaneously.

The process of adaptation takes the visual system through three distinct operating states- photopic, scotopic and mesopic vision. Photopic vision, which is aided by cone cells, occurs at luminances higher than approximately 3 cd/m2. In this state color and fine details are perceived. Scotopic vision, which is aided by rod cells, occurs at luminances less than approximately 0.001 cd/m2. In this state color and fine details are not perceived. Mesopic vision is the intermediate state between photopic and scotopic. This state is aided by both rod and cone cells.

It is important to understand the concept of adaptation when design buildings, as it may have serious consequences. For example, a person entering a dark space from a bright space may experience temporary. Thus, it is important to design buildings, where different spaces help transition from bright spaces to dark spaces.



Section Key Resources
  • IESNA Lighting Handbook, New York: Illuminating Engineering Society of North America, 2000
  • IESNA Lighting Education, ED-100
Links

Lead Author(s): Prasad Vaidya

Naturalness

“Naturalness” is a desirable characteristic of lighting, and has two aspects. First is a connection of lighting to nature and the maintenance of natural bodily rhythms for psychological and physiological health. The second is lighting that provides a natural appearance to people, objects and spaces.

Connection to Nature

Humans evolved under natural light, but most Americans spend over 85% of their lives indoors. Maintaining a connection to the natural environment has many benefits. Windows give us visual connections to landscape and sky, providing information on the time of day, time of year, and the weather. A view outside reduces visual fatigue by allowing us to change our focus from close up to distant views. Natural light can provide us with high levels of illumination and a full spectrum of color at the right times of day to meet our photo‐biological needs and to maintain our circadian rhythms for a normal sleep‐wake cycle. The dynamic variations in the intensity, direction, distribution and color of natural light provide both stimulation and a sense of well‐being. The design of buildings should promote access to nature, and in workspaces, great care should be taken not to distort the natural effects. For example, glass should be spectrally neutral to minimize changing the color of the light transmitted, Vertical glazing should not be diffuse, causing sun light to turn the entire surface into an excessively bright luminous plane. Tinting should not be so dark as to create a permanent impression of twilight, and fritting should not be so dense as to create a permanent impression of an overcast sky, Horizontal window banding, sun control devices and interior blinds are more effective in creating a panoramic view of the external environment than vertical windows, fins or vertical blinds.

Natural Appearance

While it seems obvious, it is worth discussing the importance of lighting designs that make people, objects and spaces appear natural. Attributes of natural appearance include good color rendering, reduction of shadows, high contrast or harsh patterns, balance and direction of direct and reflected light, elimination of flicker, and spatial order. Much of human communication is non‐verbal, so it is essential that faces and bodies are lighted in such a way that individuals can be recognized, and their facial expressions and body language are clearly visible. Natural skin tones and soft lighting improve appearance, and increase the sense of well‐being in all involved. Color rendering may be critical for some tasks and for safety issues, but even in casual settings it is important to be able to distinguish between colors in the environment, inside during the day or outside at night. The directionality of light contributes to the clarity of facial expressions and to flattering conditions. Strong, directional down lighting can create deep shadows beneath the eyes (“raccoon eyes”) as well under shelves and horizontal objects in the environment. Lighting that comes from below the face, or is reflected off of glass or mirrored table tops, can create a ghoulish appearance. Indirect lighting reflected from the walls or ceiling reduces shadows, is flattering to faces and promotes non‐verbal communication. In most cases, lighting is used to enhance the architecture, providing three‐dimensional comprehension of the spaces and clear way‐finding. In these cases, The occupants are comfortable with their understanding of the space and their location within it. It the lighting is not organized in a clear manner, confusion and disorientation may result. While there are certainly occasions when the appropriate lighting is dramatic and even chaotic, this should be part of, and perceived as an intentional effect.

Nighttime lighting is particularly prone to unnatural effects, where spaces that were clearly understandable under daylight conditions may be unrecognizable under nighttime conditions. Continuous paths have spotty pools of light and deep shadows. Peoples’ faces are in silhouette, or difficult to recognize, or are often not lighted at all. Care should be taken to provide acceptable lighting for faces, especially in public environments when personal security is an issue, and good visibility supports the visitors’ sense of safety.

Section Key Resources
  • No publications specific to this section have been listed.
Links
  • No links specific to this section have been listed.

Lead Author(s): Hayden McKay

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