June 2016 — Comfort in the visual environment is determined by the location of lighting fixtures, the distribution of light, and the character of the light-reflecting surfaces in a space. Providing the required level of light that matches the work performed ensures that building occupant productivity is maximized; ergonomic issues and costly retrofits are minimized; and visual comfort is achieved. Visual comfort, much like temperature comfort, is an important component of a property or facilities manager’s building requirements. Conditions that can cause visual discomfort include:
- Direct glare
- Veiling reflections or reflected glare
- Eye adaptation and high contrast ratios
- Window wall brightness (daylight)
- Room surface reflectance
Direct glare comes to the eye straight from a light source that is within the eye’s normal field of view. In addition to viewing an object directly, the eye sees a great deal of its surroundings—including light from lighting fixtures, ceilings, and windows—due to peripheral vision and the eye’s normal movement. This happens when you look at a bare bulb or out a south-facing window on a bright but cloudy day.
A direct glare source may affect productivity and performance in the workplace because of the distraction it causes. Direct glare is often expressed as a complaint of too much light; however, the problem may be one of poor lighting quality, not excessive quantity. Often, direct glare is misdirected light coming from lighting fixtures that are excessively bright or windows that are not properly shaded. This light directly shines into the eyes as well as onto the task. The resulting discomfort and distraction can take its toll on productivity.
Eyes can generally be protected from direct glare by using a lighting fixture that shields the light source. The typical viewing angle when looking straight ahead is 90 degrees—45 degrees down and 45 degrees up. Any fixtures that are within the viewing angle have a potential for direct glare on the computer screen or other work surface.
When the light from a source is blocked or cut off by an interposed opaque object, a shadow or partial darkness results. This is a common problem with poorly placed HID task lights. In addition, a shadow can be viewed or a lower light level created when the light fixtures are not spaced properly to illuminate evenly or to provide direct illumination on the task. Light sources located behind the worker or directly above overhead cabinets may also result in distracting shadows. This problem can often be addressed by adding supplemental task lights mounted under shelves to provide direct light on the work surfaces.
Veiling Reflections or Reflected Glare
Veiling reflections result when the light source itself is reflected as a blurred image on a visual task. The effect is obvious on glossy materials, but it can also be present on matte-finish paper covered with printing, typing, or handwriting, especially pencil. The veiling reflections cast on a task in this manner are sometimes so subtle that they are not readily visible, but they can cause substantial losses in contrast and visibility.
To eliminate veiling reflections, first determine which light source is causing the problem. Sometimes the source is obvious. When it is difficult to pinpoint the source, it is helpful to place a small mirror on the work surface at the point of reflection. When looking into the mirror from the normal work position, the source should be seen clearly. Once identified, blocking the source, changing the angle of the work surface in relation to the source, changing the reflective characteristics of the surface, or adding a fine-mesh screen can eliminate the problem source.
The computer screen reflects the ceiling light fixtures. If ceiling lights are of a high brightness level, such as a typical white lens fixture with exposed lamps, the viewing screen will appear to display the lamps and not allow the user to view the data. To alleviate this problem, put a screen on the monitor, raise the worker’s eye position, or tilt the monitor face downward. The monitor can also be relocated, so that a wall or high partition behind the worker shields the brightness of the offending light fixture from the screen.
Eye Adaptation and High Contrast Ratios
The muscles of the eye continually adjust to the brightness of surfaces in the environment. When seeing areas of low brightness, the eye adjusts to increase the amount of light entering the eye. When seeing areas of high brightness, the eye adjusts to reduce the amount of light entering the eye. These adaptations to brightness differences are automatic.
Like other muscles in the human body, however, eye muscles can become fatigued. Extreme brightness differences in an office such as deep shadow next to intense brightness will cause the eye to continually make major adaptations, producing eye fatigue, eyestrain, and headaches. To avoid these conditions, the contrast ratio of light levels, finishes, and furniture orientation must be considered during the design process. A contrast ratio is the ratio of the lighting of a space to the lighting of an adjacent space. It is important the contrast ratio does not exceed 3 to 1 on a workstation in open offices. In areas where the tasks are not as difficult, such as in a reception lobby, a 6 to 1 ratio is more acceptable.
Window Wall Brightness (Daylight)
Daylight is distinguished as a light source by its unique changing spectra and distribution. It can increase occupant satisfaction and conserve energy if its glare can be controlled, and daylight can save energy if it is integrated into building lighting and energy systems.
Many new buildings have incorporated the use of daylight into their overall energy management systems through use of exterior louvers or screens, good E-rated (energy-efficient) glass, and automated building control systems that adjust automatically for the amount of daylight entering a space. This is accomplished by adjusting for the direction the building faces in relationship to the sun, the time of day, and the occupancy of the space. It is helpful if the space can be designed to ensure that the desired level and quality of illumination is achieved. Then those fixtures that fall within the daylight range (the distance into the building where daylight penetrates) can be tied to a photocell. The photocell signals the master control system when artificial lights need to be decreased or increased.
Room Surface Reflectance
The brightness relationship between the work surface and the surfaces of the ceiling, walls, and floor in a room is important.
Reflectance is the ratio of light reflected from a surface (for example, a ceiling) to light hitting the surface. Staying within recommended reflectance ranges will produce a more comfortable visual environment and minimize the need for occupants’ eyes to adapt frequently to overly bright or dark room surfaces. The following is a list of recommended ranges for the percentage of light reflected from various surfaces in the office environment:
- Ceiling—70 to 90 percent
- Walls—40 to 60 percent
- Furniture tops—25 to 45 percent
- Office machines and equipment—25 to 45 percent
- Floors—20 to 40 percent
Building engineers are usually not part of the design process when the finishes for a space are selected. It helps to have an awareness of the impact of those finishes on the reflection of light in anticipating the final results of the lighting system.
This article is adapted from BOMI International’s course The Design, Operation, and Maintenance of Building Systems, Part II, part of the RPA and FMA designation programs. More information regarding this course or the new High-Performance certificate courses is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.