Description

Visual comfort is intrinsically related to light, to light’s interaction with architectural space, the properties of materials employed in forming the spatial membrane, and the elements placed inside for human inhabitation. The complex physical properties of light and light distribution are to be balanced with the physiological sensation of light perception and interpretation, the psychological phenomena connecting light to ambience, and the functional aspects of providing the relevant level of illuminance for tasks to be performed within the space. Architectural lighting design is further complicated by the interaction of artificial light sources with the dynamic illluminance parameters of daylight.[1] The perception of objects in a space, and the ability to perform a visual task, diminishes with increased exposure to glare caused by unbalanced intensity of illluminance, a light source positioned in the field of vision and by reflection.

In general, the various tasks executed in libraries need values in the range of 200–2,000 lux.[2] Minimum illuminance levels averaging between 300 lux and 500 lux are recommended for reading rooms, 300 lux for active stacks areas and 50 lux for inactive stacks. Other activities as book binding and embossing require higher light levels, 500 lux and 1,000 lux respectively.

Reading, the primary seeing task in a library, is directly related to the reading material itself – font size (assuming a standard viewing distance of approx. 25 cm between eye and reading material), the chosen type face, contrast between text and display medium (commonly black ink on white paper), color contrast of illustrations (satura­ted colors achieve the highest level of contrast), graphic design concepts, and the sheen and texture of the paper, as well as the quality and method of printing. Visibility science distinguishes between seeing/reading tasks that demand a high level of mental activity, like the study of mathematical symbols or an unfamiliar language, which employ central or foveal vision (focusing on the task within the 2 cone of foveal vision), and conventional reading tasks, assuming familiarity with the material to be comprehended and, hence, allowing a far wider cone of vision suitable for scanning a printed page visually.

The luminance of the reading environment, the contrast between the reader’s immediate surrounding and adjacent spaces, and the color contrast between the object to be viewed and the surface on which it rests, are the primary factors controlling lighting conditions in the reading and studying areas of a library. Furthermore, the reader’s general eyesight, determined by medical conditions and age and levels of fatigue, is, to some degree, linked to the illumination of the reading environment and requires design consideration and special accommodation. Individual visual comfort is often achieved by adjustable, user-operable supplemental task lighting at workstations and carrels. The concept, allowing a lower overhead lighting level, supports energy conservation, thus complying with energy standards regulating lighting power density and the control of lighting.

Luminance of a surface type task should be equal or marginally higher than the luminance of the background, although ratios of 3 to1 are acceptable in most circumstances. Consequently, the reflectance (defined as the ratio between incident and reflected light) of a worktable surface for reading material printed on matte white paper with a reflectance rate of 0.77 (i.e. 77 % of the light incident on the paper surface is reflected), yielding a luminance of 120 cd/m² under illuminance of 500 lux (as recommended for the performance of visual tasks of medium contrast or small size), should not exceed 0.25. Newspaper, less refined than white paper, has a reflectance rate of 0.68, while the reflectance of glossy white magazine paper is above 0.80. Reflectance is directly material-dependent, and although the human eye is not able to distinguish between a light-emitting source and a mere reflecting source, the distinction of the two is important for a successful lighting concept. Critical in reading tasks is the viewing angle in relationship to the incident light angle, the angle at which the illuminance is directed onto the paper. Light pointing straight down onto the reading material avoids reflected glare or veiling reflection, offering a well-balanced reading environment.

Luminous ceilings, achieved by suspended fixtures with solely upward-pointing light sources or by light sources installed behind a frosted glass or acrylic membrane, avoid the occurrence of glare while rendering the space in uniform light.

Browsing, locating and retrieving books from stacks, as well as the management of the collection, involve the visual task of reading book titles and identifying labels. Correct lighting levels across the vertical surface of the shelving unit, the spines of the books, is achievable by two conceptual methods; canopy lighting, i.e. linear lights parallel to the range cantilevered out from the shelf standards at the height of the canopy, and linear light fixtures, ceiling-integrated or suspended, centered on the aisle or perpendicular to the ranges. Canopy lighting offers the greatest flexibility with regard to changes in the shelving layout; while ceiling-connected lighting aligned with the aisles is considered the least flexible solution.[3] The task-specific distribution characteristic of canopy luminaires, directing light with high efficiency onto the vertical shelving plane, permits low-level luminance for the surrounding areas, ultimately conserving energy. In active stack areas, the recommended luminance level measured vertically at the floor is 80 lux.

Sophisticated 3D rendering programs based on radiosity (representing radiant exitance or radiative flux of both reflected and re-radiated light) and ray-tracing techniques, offering great realism by taking into account absorption, reflection, refraction and fluorescence, support the virtual analysis and manipulation of lighting concepts and lighting effects prior to realization. Computer-based photometric calculation programs rely on the luminaire’s candle power distribution curve, the geometrical relationship between luminaire and light-receiving surface, and distance between source and surface, while the lumen method provides information on illuminance at the light-receiving surface measured in lux and energy code relevant electrical power density measured in watts per square meter. Luminaire-specific photometric data and the integration of daylight, hours of operation, and the degree of sophistication of automated lighting control systems for both day and artificial light are additional parameters towards a balanced system approach.

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Originally published in: Nolan Lushington, Wolfgang Rudorf, Liliane Wong, Libraries: A Design Manual, Birkhäuser, 2016.