Understanding the principles of light and vision helps lighting designers to identify and solve office lighting problems.

Light and Vision
Light is radiant electromagnetic energy (between wavelengths 380 and 770 nm). When it strikes the retina, light creates visual sensations, which, in turn, stimulate a neurological response in the brain. Light is usually emitted from a source and is either absorbed by, reflected from, or transmitted through various objects before it hits a viewer's eye. Light reveals the shape, size, texture, colour, depth, and location of an object.

The illuminance on a surface is the amount of light incident on that surface. It is measurable using a light meter and is recorded in lux (lx) or footcandles (fc).

Luminance is the amount of light emitted from an object in a given direction. The eye detects and interprets luminance patterns to form perceptions of objects.

It is measured in candelas per square meter (cd/m²).

For any object, its illuminance proportionally determines its luminance; however, luminance is further affected by the reflectance and finish of an object. High reflectances and/or glossy finishes increase luminance and redistribute a lot of light into a space, even if the illuminanceremains constant. Luminance contributes to sensations of brightness, which dependon the illuminance of the object, adaptation of the eye, and the luminances of thesurrounding surfaces. It can be difficult to look at surfaces with very high illuminances.

Visibility Influences
How well we see depends on the contrast, size, luminance level of an object, the age of the viewer, and the time available for viewing. Contrast and size are task design issues and can be easily improved without costly changes to the lighting system.

Contrast is the relationship between the luminances of an object and its immediate background ^[1]. We see contrasts of brightness, colour, and pattern.

All our visual perceptions are relative. Visual processing compares items in the visual field to judge colour, size, distance, and texture.

Size: Small details are harder to perceive because they do not contrast greatly with the background. In an office setting, increasing the task size is one of the most effective ways of making a task easier to see.

Luminance Level: The visual system can adapt to a wide luminance range, from lower than 0.001 cd/m² (starry night) to more than 3000 cd/m² (mid-day in summer). In general, increasing luminance levels improves visibility; however, high luminances may cause glare. In offices, it is more effective to increase task size or contrast to improve visibility.

Age of Viewer: As we age, the eye changes in many respects. These changes reduce our detail perception, contrast sensitivity, colour discrimination, and adaptation and visual processing speed. Older people take longer to adjust to changes in light level and are more sensitive to glare. The effects are generally noticeable after age forty.

The age of employees cannot be controlled, and offices generally have a range of ages and visual abilities. Lighting designers should know the average age of the occupants for a given office and design the lighting for the worst-case scenario. Office appropriate lighting and tasks with large size and contrast compensate for age-related visual problems.

Time Viewed: This generally refers to moving objects. Movement blurs contrast and definition. Therefore, if employees have to look at moving objects, the contrast and the luminances should be increased, or the movement should be slowed.

Open-plan Office Lighting Problems
In general, there are two major lighting problems that arise in open-plan offices. Glare is often caused because the partitions (partial-height screens) do not block light coming from distant fixtures. Open-plan offices may also have non-uniform lighting because the system was designed for an empty room; the addition of partitions and modular furniture can block light, create shadows, and create bright spots due to different surface reflectances.

The light fixtures themselves can sometimes cause a third problem if they flicker.

Glare refers to a number of problems that can be created by patches of luminance that are much greater than the surrounding luminances and the luminance to which our eyes are adapted. Glare can be either uncomfortable or disabling.

Discomfort glare is a sensation caused by bright areas in the field of view, other than the spot on which one is focussing. Viewers can experience eyestrain or fatigue if exposed to discomfort glare for a long time.

Discomfort may result from direct glare, from sources such as an exposed light source or sunlight from an unshaded window; overhead glare, from sources such as a light source that is directly overhead, not in the field of view; or reflected glare, from sources such as the reflection off a glossy surface.

Disability glare reduces task contrast and thereby reduces visibility. It exists in two forms:

• Disability glare can occur within the eye when unwanted light is scattered in the eye and prevents contrast detection; one can experience disability glare in the office when sunlight is shining in a window behind an object one is trying to see. This is a type of direct glare;
• Disability glare can also occur when veiling reflections scatter light over a surface, creating a "veil", or sheen, of light that reduces task contrast. These reflections often occur on magazine pages, computer screens, and other glossy surfaces, obscuring the images on the surfaces. A special case of veiling reflections occurs when light fixtures are reflected from a specular surface such as a computer screen (see photo). This is a type of reflected glare.

Non-uniform light distribution on the task surface can reduce visibility and comfort. It can be annoying because the eye must frequently adapt to different light levels. In extreme cases, task surfaces may be in shadow.

Perfect uniformity is not desirable over the whole office space because occupants want luminance variety for visual interest. Uniformly lit scenes appear flat and unrealistic. Object modelling is weak, meaning that the form, texture, and depth of objects is not clearly revealed. Complete uniformity should be limited to task surfaces.

Low-frequency flicker of light sources (below 150 Hz) can interrupt visual processing and lead to discomfort. Electronic ballasts for fluorescent lamps operate at a high frequency (over 20 kHz) and can eliminate this problem, while saving energy.

Types of Lighting in Open-plan Offices
There are three types of lighting that can be used in open-plan offices. Quality office lighting designs generally use a combination of all three.

Ambient lighting illuminates the whole environment and can be direct (down-light) or indirect (up-light).

Direct-downward ambient lighting in offices comes in two main types: prismatic and parabolic fixtures.

• Prismatic fixtures use a plastic lens to distribute the light.
• Parabolic fixtures use louvres to cut off light at certain angles to reduce reflected glare. Parabolic fixtures place more light on horizontal surfaces than on vertical surfaces.

These fixtures are either flush with the ceiling or recessed.

Indirect lighting provides more diffuse light over all interior surfaces. Indirect fixtures often have a direct-downward component as well, which targets some light on horizontal surfaces such as desktops. Research suggests that people tend to prefer a mixture of direct and indirect illumination, with 40% of the light being indirect. These fixtures may be free-standing, suspended from the ceiling, or wall- or partition-mounted.

Task lighting provides bright light on a specific surface, where a task requires higher illuminance. Linear task lights are often used under partition-hung storage elements to counter shadowing. With careful design, ambient light levels can be lowered when task lighting is available, thus, conserving energy.

Accent lighting is used to highlight areas of importance or interest and to decorate a space. It balances ambient light, fills shadows, and can create some viewpoints for eye relaxation.

References:

1: Illuminating Engineering Society of North America (IESNA). (2000). The IESNA Lighting Handbook: Reference and Application. 9th edition. Ed M.S. Rea. New York: IESNA. Pg G-21.