Architectural lighting design
Architectural lighting design is a field within architecture, interior design and electrical engineering that is concerned with the design of lighting systems, including natural light, electric light, or both, to serve human needs.
The design process takes account of:
- The kind of human activity for which lighting is to be provided
- The amount of light required
- The color of the light as it may affect the views of particular objects and the environment as a whole
- The distribution of light within the space to be lighted, whether indoor or outdoor
- The effect of the lightened system itself on the user
The objective of lighting design is the human response, to see clearly and without discomfort. The objective of architectural lighting design is to further the design of architecture or the experience of buildings and other physical structures.
Gas lighting was economical enough to light streets in major cities starting in the early 1800s, and was also used in some commercial buildings and in the homes of wealthy people. The gas mantle boosted the luminosity of utility lighting and of kerosene lamps. The next major drop in price came about with the incandescent light bulb powered by electricity.
Architectural lighting design focuses on three fundamental aspects of the illumination of buildings or spaces. The first is the aesthetic appeal of a building, an aspect particularly important in the illumination of retail environments. Secondly, the ergonomic aspect: the measure of how much of a function the lighting plays. Thirdly is the energy efficiency issue to ensure that light is not wasted by over illumination, either by illuminating vacant spaces unnecessarily or by providing more light than needed for the aesthetics or the task. Cultural factors also need to be considered; for example, bright lights was a mark of wealth through much of Chinese history.
As the Sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing color of the Sun over the course of the day is mainly a result of scattering of light and is not due to changes in black-body radiation. The blue color of the sky is caused by Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red light.
For colors based on black-body theory, blue occurs at higher temperatures, while red occurs at lower, cooler, temperatures. This is the opposite of the cultural associations attributed to colors, in which red represents hot, and blue cold.
Lighting fixtures come in a wide variety of styles for various functions. The most important functions are as a holder for the light source, to provide directed light and to avoid visual glare. Some are very plain and functional, while some are pieces of art in themselves. Nearly any material can be used, so long as it can tolerate the excess heat and is in keeping with safety codes.
An important property of light fixtures is the luminous efficacy or wall-plug efficiency, meaning the amount of usable light emanating from the fixture per used energy, usually measured in lumen per watt. A fixture using replaceable light sources can also have its efficiency quoted as the percentage of light passed from the "bulb" to the surroundings. The more transparent the lighting fixture is, the higher efficacy. Shading the light will normally decrease efficiency but increase the directionality and the visual comfort probability.
The PH-lamps are a series of light fixtures designed by Danish designer and writer Poul Henningsen from 1926 onwards. The lamp is designed with multiple concentric shades to eliminate visual glare, only emitting reflected light, obscuring the light source.
Photometric studies (also sometimes referred to as "layouts" or "point by points") are often used to simulate lighting designs for projects before they are built or renovated. This enables architects, lighting designers, and engineers to determine whether a proposed lighting setup will deliver the amount of light intended. They will also be able to determine the contrast ratio between light and dark areas. In many cases these studies are referenced against IESNA or CIBSE recommended lighting practices for the type of application. Depending on the type of area, different design aspects may be emphasized for safety or practicality (i.e. such as maintaining uniform light levels, avoiding glare or highlighting certain areas). Specialized software is often used to create these, which typically combine the use of two-dimensional digital CAD drawings and lighting simulation software.
Color temperature for white light sources also affects their use for certain applications. The color temperature of a white light source is the temperature in kelvins of a theoretical black body emitter that most closely matches the spectral characteristics of the lamp. An incandescent bulb has a color temperature around 2800 to 3000 kelvins; daylight is around 6400 kelvins. Lower color temperature lamps have relatively more energy in the yellow and red part of the visible spectrum, while high color temperatures correspond to lamps with more of a blue-white appearance. For critical inspection or color matching tasks, or for retail displays of food and clothing, the color temperature of the lamps will be selected for the best overall lighting effect. Color may also be used for functional reasons. For example, blue light makes it difficult to see veins and thus may be used to discourage drug use.
Correlated color temperature
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields. In practice, color temperature is only meaningful for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e., those on a line from reddish/orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of, e.g., a green or a purple light. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K.
For lighting building interiors, it is often important to take into account the color temperature of illumination. For example, a warmer (i.e., lower color temperature) light is often used in public areas to promote relaxation, while a cooler (higher color temperature) light is used to enhance concentration in offices.
CCT dimming for LED technology is regarded as a difficult task, since binning, age and temperature drift effects of LEDs change the actual color value output. Here feedback loop systems are used for example with color sensors, to actively monitor and control the color output of multiple color mixing LEDs.
The color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in kelvins, or alternatively in mireds (micro-reciprocal kelvin). This permits the definition of a standard by which light sources are compared.
Categorizing different lighting
|1,700 K||Match flame, low-pressure sodium lamps (LPS/SOX)|
|1,850 K||Candle flame, sunrise, sunset|
|2,700–3,300 K||Incandescent lamps, soft-white fluorescent lamps|
|3,000 K||Warm-white fluorescent lamps|
|4,100–4,150 K||Moonlight, cool-white fluorescent lamps|
|5,000 K||Horizon daylight|
|5,500–6,000 K||Vertical daylight, electronic flash|
|6,200 K||Xenon short-arc lamp|
|6,500 K||Daylight, overcast, daylight fluorescent lamps|
|6,500–10,500 K||LCD or CRT screen|
|15,000–27,000 K||Clear blue poleward sky|
|These temperatures are merely characteristic;|
considerable variation may be present.
To the extent that a hot surface emits thermal radiation but is not an ideal black-body radiator, the color temperature of the light is not the actual temperature of the surface. An incandescent lamp's light is thermal radiation, and the bulb approximates an ideal black-body radiator, so its color temperature is essentially the temperature of the filament.
Many other light sources, such as fluorescent lamps, or LEDs (light emitting diodes) emit light primarily by processes other than thermal radiation. This means that the emitted radiation does not follow the form of a black-body spectrum. These sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black-body radiator which to human color perception most closely matches the light from the lamp. Because such an approximation is not required for incandescent light, the CCT for an incandescent light is simply its unadjusted temperature, derived from the comparison to a black-body radiator.
For simple installations, hand-calculations based on tabular data can be used to provide an acceptable lighting design. More critical or optimized designs now routinely use mathematical modeling on a computer.
Based on the positions and mounting heights of the fixtures, and their photometric characteristics, the proposed lighting layout can be checked for uniformity and quantity of illumination. For larger projects or those with irregular floor plans, lighting design software can be used. Each fixture has its location entered, and the reflectance of walls, ceiling, and floors can be entered. The computer program will then produce a set of contour charts overlaid on the project floor plan, showing the light level to be expected at the working height. More advanced programs can include the effect of light from windows or skylights, allowing further optimization of the operating cost of the lighting installation. The amount of daylight received in an internal space can typically be analyzed by undertaking a daylight factor calculation.
The Zonal Cavity Method is used as a basis for both hand, tabulated, and computer calculations. This method uses the reflectance coefficients of room surfaces to model the contribution to useful illumination at the working level of the room due to light reflected from the walls and the ceiling. Simplified photometric values are usually given by fixture manufacturers for use in this method.
Computer modeling of outdoor flood lighting usually proceeds directly from photometric data. The total lighting power of a lamp is divided into small solid angular regions. Each region is extended to the surface which is to be lit and the area calculated, giving the light power per unit of area. Where multiple lamps are used to illuminate the same area, each one's contribution is summed. Again the tabulated light levels (in lux or foot-candles) can be presented as contour lines of constant lighting value, overlaid on the project plan drawing. Hand calculations might only be required at a few points, but computer calculations allow a better estimate of the uniformity and lighting level.
International professional organizations
The Illuminating Engineering Society of Australia and New Zealand was established in 1930 during the Great Depression.
The International Association of Lighting Designers (IALD) was founded in 1969, and its current mission is "to serve the IALD worldwide membership by promoting the visible success of its members in practicing lighting design." The organization created a new attitude towards the profession and raised the profile of architectural lighting design, one its principal goals.
The Professional Lighting Designers Association (PLDA) was formed in 1993 as the European Lighting Designers' Association (ELDA, later ELDA+). Until it was dissolved in 2014, it was with the IALD one of the main authorities regarding lighting design in architecture.
The Illuminating Engineering Society of North America (IESNA) seeks to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public.
The National Council on Qualifications for the Lighting Professions (NCQLP) is a non-profit organization founded in 1991 to serve and protect the well-being of the public through effective and efficient lighting practice. Through a peer-review process, the NCQLP establishes the education, experience and examination requirements for baseline certification across the lighting professions. The NCQLP has established a certification process by which practitioners in lighting and related fields, through testing, demonstrate their knowledge and experience across the lighting professions. Those who successfully complete the NCQLP Lighting Certification Examination are entitled to use the appellation LC (Lighting Certified) after their name for professional purposes.
The International Commission on Illumination (CIE) is an organization "devoted to international cooperation and exchange of information among its member countries on all matters relating to the science and art of lighting." CIE works globally to develop and publish lighting design standardization and best-practice documents.
The Professional Lighting & Sound Association (PLASA) represents the interests of many lighting designers and manufacturers, several of which are involved in the Architectural lighting market. PLASA is UK orientated, but does represent companies on a European and International level.
There are many more nationally-based organizations such as the Schweizerische Licht Gesellschaft (SLG) in Switzerland, the Association des Concepteurs Lumière et Éclairagistes (ACE) in France, the Hellenic Illumination Committee (HIC) in Greece and the Associazione Professionisti dell'Illuminazione (APIL) in Italy.
- Saw-tooth roof design, circa 1827, British engineer and architect William Fairbairn, credited with the first designs for what he termed the shed principle
- Austrian Postal Savings Bank Vienna 1904-1912, architect Otto Wagner
- AEG turbine factory 1909, Berlin district of Moabit, architect Peter Behrens. It is an influential and well-known example of industrial architecture. Its revolutionary design features 100m long and 15m tall glass and steel walls on either side.
- Fagus Factory, Germany 1913, by architect Walter Gropius
- Bauhaus, Desau Germany 1919, by architect Walter Gropius
- Villa Savoye, France (1929–31) by architect Le Corbusier. The second floor includes long strips of ribbon windows that allow unencumbered views of the large surrounding garden, and which constitute the fourth point of his five-point system for architecture.
- Glass House, Connecticut 1949, architect Philip Johnson, Mies van der Rohe concept, lighting design by Richard Kelly
- General Motors Technical Center 1949, architect Eero Saarinen, lighting design by Richard Kelly.
- MIT Chapel Massachusetts Institute of Technology 1955, architect Eero Saarinen, a non-denominational chapel
- Seagram Building, New York City 1958, architect Mies van der Rohe with Philip Johnson, lighting design by Richard Kelly
- Kimbell Art Museum 1972, architect Louis Kahn, lghting Design by Richard Kelly
- Jatiyo Sangshad Bhaban (National Assembly Building) in Dhaka, Bangladesh, 1962 to 1974, designed by Louis Kahn
- Musée d’Orsay Paris 1984, lighting Design by Gae Aulenti
- Louvre Pyramid (Pyramide du Louvre) designed by the architect I. M. Pei, completed 1999
- Institut de Monde Arabe 1987, architect Jean Nouvel and Architecture-Studio
- Neues Museum Berlin reopened 2009, architect David Chipperfield, lighting designer Kardorff Ingenieure
- Arena do Morro, Brazil 2014, architect Herzog & de Meuron
- Auditorium of the Vyborg Library, 1930s, architect Alvar Aalto
- 30 St Mary Axe (The Gherkin) 2004, London architect Foster and Partners, lighting designer Speirs and Major Associates
- Gae Aulenti was an Italian architect, lighting designer, interior designer and industrial designer for buildings such as the Musée d’Orsay.
- Ray Grenald, founding member of the IALD
- George Izenour, theatrical lighting designer. His patents form the modern lighting control consoles.
- Rogier van der Heide, theatre and architectural lighting designer, recipient of the IALD Radiance Award and awarded Lighting Designer of the Year. Chief Design Officer of Philips Lighting
- Richard Kelly, lighting designer for significant modernist buildings
- Leslie Wheel, founding member of the IALD
Decorative luminare designers
- Charlotte Perriand 1950 table lamp for Philips
- Marianne Brandt Kandem Bedside Table Lamp (1928) and pull lamp (1926)
- Poul Henningsen lighting designer for Louis Poulsen
- Christian Dell Head of silversmithing at the Bauhaus, As an early industrial designer and pioneer of plastic design, Dell used bakelite and aminoplastics as materials for his works for Molitor-Zweckleuchten in 1929-30. Well known are the lights for the lamp factory Gebr. Kaiser & Co. in Neheim Hüsten beginning in 1933-34, which were produced in large quantities.
- Wilhelm Wagenfeld
- Eileen Gray Her architecture demonstrates a profound knowledge for space, the use of light, and ingenious planning.
Publications on architectural lighting design
- In Praise of Shadows by Jun'ichirō Tanizaki is an essay on the Japanese aesthetic in contrast with change. Comparisons of light with darkness are used to contrast Western and Asian cultures.
- The Structure of Light by Richard Kelly
- The Illumination of Modern Architecture by Dietrich Neumann
- Made Of Light | Speirs + Major | Designers working with light
- A Method of Lighting the Stage by Stanley McCandless
- Architectural Lighting: Designing with Light and Space by Hervé Descottes with Cecilia Ramos (Author)
- Lighting Design Basics (US empirical system) by Mark Karlen (Author), James R. Benya (Author),
- The Architecture Of Light: A textbook of procedures and practices for the Architect, Interior Designer and Lighting Designer. by Sage Russell
- Lighting Retrofit and Relighting: A Guide to Energy Efficient Lighting by James R. Benya (Author), Donna J. Leban
- Fundamentals of Lighting by Susan M. Winchip
- Designing With Light: The Art, Science and Practice of Architectural Lighting Design by Jason Livingston.
- Lighting : basic concepts / Warren G. Julian, editor ; written by members of the Architectural Science Dept, University of Sydney
- Architectures de lumières (2003) by Louis Clair (bilingual publication, in French and English)
Architectural design media
With the increase in global focus on green design and energy codes, lighting design and its role in sustainability have become more well known, resulting in a number of lighting-specific trade publications and an increase in coverage in architectural publications.
- Recessed light
- The protective housing is concealed behind a ceiling or wall, leaving only the fixture itself exposed. The ceiling-mounted version is often called a downlight.
- "Cans" with a variety of lamps
- Jargon for inexpensive downlighting products that are recessed into the ceiling, or sometimes for uplights placed on the floor. The name comes from the shape of the housing. The term "pot lights" is often used in Canada and parts of the US.
- Cove light
- Recessed into the ceiling in a long box against a wall.
- Floor lamp
- Recessed fluorescent light fixtures, usually rectangular in shape to fit into a drop ceiling grid.
- Surface-mounted light
- The finished housing is exposed, not flush mount with surface
- Pendant light
- Suspended from the ceiling with a chain or pipe
- Provide up or down lights; can be used to illuminate artwork, architectural details; commonly used in hallways or as an alternative to overhead lighting.
- Track lighting fixture
- Individual fixtures ("track heads") can be positioned anywhere along the track, which provides electric power.
- Under-cabinet light
- Mounted below kitchen wall cabinets
- Emergency lighting or exit sign
- Connected to a battery backup or to an electric circuit that has emergency power if the mains power fails
- High- and low-bay lighting
- Typically used for general lighting for industrial buildings and often big-box stores
- Strip lights or Industrial lighting
- Often long lines of fluorescent lamps used in a warehouse or factory
- Outdoor lighting and landscape lighting
- Used to illuminate walkways, parking lots, roadways, building exteriors and architectural details, gardens, and parks.
- A type of architectural outdoor lighting that is a short, upright ground-mounted unit typically used to provide cutoff type illumination for egress lighting, to light walkways, steps, or other pathways.
- Street light
- Flood lighting
- Usually pole- or stanchion-mounted — for landscape, roadways, and parking lots
Types of electric lighting include:
- Incandescent light bulbs
- Arc lamps
- Gas-discharge lamps (e.g. fluorescent and compact fluorescent lamps, neon lamps, metal halide lamps, modern photographic flashes)
- Light-emitting diodes (LEDs), including OLEDs
- Sulfur lamps
|Name||Optical spectrum||Nominal efficiency |
|Incandescent light bulb||Continuous||4-17||2-20000||2400-3400||Warm white (yellowish)||100|
|Halogen lamp||Continuous||16-23||3000-6000||3200||Warm white (yellowish)||100|
|Fluorescent lamp||Mercury line + Phosphor||52-100 (white)||8000-20000||2700-5000*||White (various color temperatures), as well as saturated colors available||15-85|
|Metal halide lamp||Quasi-continuous||50-115||6000-20000||3000-4500||Cold white||65-93|
|Sulfur lamp||Continuous||80-110||15000-20000||6000||Pale green||79|
|High pressure sodium||Broadband||55-140||10000-40000||1800-2200*||Pinkish orange||0-70|
|Low pressure sodium||Narrow line||100-200||18000-20000||1800*||Yellow, no color rendering||0|
|Light-emitting diode||Line plus phosphor||10-110 (white)||50,000-100,000||Various white from 2700 to 6000*||Various color temperatures, as well as saturated colors||70-85 (white)|
|Induction Lamp (External Coil)||Mercury line + Phosphor||70-90 (white)||80,000-100,000||Various white from 2700 to 6000*||Various color temperatures, as well as saturated colors||70-85 (white)|
*Color temperature is defined as the temperature of a black body emitting a similar spectrum; these spectra are quite different from those of black bodies.
The most efficient source of electric light is the low-pressure sodium lamp. It produces, for all practical purposes, a monochromatic orange/yellow light, which gives a similarly monochromatic perceprtion of any illuminated scene. For this reason, it is generally reserved for outdoor public lighting usages. Low-pressure sodium lights are favoured for public lighting by astronomers, since the light pollution that they generate can be easily filtered, contrary to broadband or continuous spectra.
Incandescent light bulb
The modern incandescent light bulb, with a coiled filament of tungsten, was commercialized in the 1920s developed from the carbon filament lamp introduced in about 1880. As well as bulbs for normal illumination, there is a very wide range, including low voltage, low-power types often used as components in equipment, but now largely displaced by LEDs
There is currently interest in banning some types of filament lamp in some countries, such as Australia planning to ban standard incandescent light bulbs by 2010, because they are inefficient at converting electricity to light. Sri Lanka has already banned importing filament bulbs because of high use of electricity and less light. Less than 3% of the input energy is converted into usable light. Nearly all of the input energy ends up as heat that, in warm climates, must then be removed from the building by ventilation or air conditioning, often resulting in more energy consumption. In colder climates where heating and lighting is required during the cold and dark winter months, the heat byproduct has at least some value.
Halogen lamps are usually much smaller than standard incandescents, because for successful operation a bulb temperature over 200 °C is generally necessary. For this reason, most have a bulb of fused silica (quartz), but sometimes aluminosilicate glass. This is often sealed inside an additional layer of glass. The outer glass is a safety precaution, reducing UV emission and because halogen bulbs can occasionally explode during operation. One reason is if the quartz bulb has oily residue from fingerprints. The risk of burns or fire is also greater with bare bulbs, leading to their prohibition in some places unless enclosed by the luminaire.
Fluorescent lamps consist of a glass tube that contains mercury vapour or argon under low pressure. Electricity flowing through the tube causes the gases to give off ultraviolet energy. The inside of the tubes are coated with phosphors that give off visible light when struck by ultraviolet energy. have much higher efficiency than Incandescent lamps. For the same amount of light generated, they typically use around one-quarter to one-third the power of an incandescent.
Solid state light-emitting diodes (LEDs) have been popular as indicator lights since the 1970s. In recent years, efficacy and output have risen to the point where LEDs are now being used in niche lighting applications.
Indicator LEDs are known for their extremely long life, up to 100,000 hours, but lighting LEDs are operated much less conservatively (due to high LED cost per watt), and consequently have much shorter lives.
Due to the relatively high cost per watt, LED lighting is most useful at very low powers, typically for lamp assemblies of under 10 W. LEDs are currently most useful and cost-effective in low power applications, such as nightlights and flashlights. Colored LEDs can also be used for accent lighting, such as for glass objects, and even in fake ice cubes for drinks at parties. They are also being increasingly used as holiday lighting.
LED efficiencies vary over a very wide range. Some have lower efficiency than filament lamps, and some significantly higher. LED performance in this respect is prone to being misinterpreted, as the inherent directionality of LEDs gives them a much higher light intensity in one direction per given total light output.
Single color LEDs are well developed technology, but white LEDs at time of writing still have some unresolved issues:
- CRI is not particularly good, resulting in less than accurate color rendition.
- The light distribution from the phosphor does not fully match the distribution of light from the LED die, so color temperature varies at differing angles.
- Phosphor performance degrades over time, resulting in change of color temperature and falling output. With some LEDs degradation can be quite fast.
- Limited heat tolerance means that the amount of power packable into a lamp assembly is a fraction of the power usable in a similarly sized incandescent lamp.
LED technology is useful for lighting designers because of its low power consumption, low heat generation, instantaneous on-and-off control, and in the case of single color LEDs, continuity of color throughout the life of the diode and relatively low cost of manufacture.
In the last few years, software has been developed to merge lighting and video by enabling lighting designers to stream video content to their LED fixtures, creating low resolution video walls.
- Active daylighting
- Architectural glass
- Architectural light shelf
- Architecture of the night
- Daylight factor
- Daylight harvesting
- Deck prism
- Light art
- Light + Building
- Lighting control system
- Lighting for the elderly
- List of lighting design applications
- Lumen method
- Passive daylighting
- Passive solar building design
- Seasonal affective disorder (SAD)
- Smart glass
- Sun path
- Transom (architectural)
- Vivid Sydney
- Lighting : basic concepts / Warren G. Julian, editor ; written by members of the Architectural Science Dept, University of Sydney
- "What Is Architectural Lighting". AlconLighting.com. 2017-10-23. Retrieved 2018-03-15.
- "Episode 534: The History of Light". NPR.org (Podcast). NPR. 2014-04-25. Retrieved 2017-11-11.
- Campanella, Thomas J. (24 October 2017). "Mapping the Edison Bulbs of Brooklyn". Citylab. The Atlantic. Retrieved 25 October 2017.
- Chris George (2008). Mastering Digital Flash Photography: The Complete Reference Guide. Sterling Publishing Company. p. 11. ISBN 978-1-60059-209-6.
- "The PH lamp", Design, Visit Denmark, archived from the original on 2012-02-15.
- "Poul Henningsen". Louis Poulsen Lighting. Archived from the original on 2013-11-18.
- Watts, Amanda (7 December 2017). "Gas station installs blue lights to combat drug use". CNN. Retrieved 16 January 2018.
- Rüdiger Paschotta (2008). Encyclopedia of Laser Physics and Technology. Wiley-VCH. p. 219. ISBN 978-3-527-40828-3.
- Thomas Nimz, Fredrik Hailer and Kevin Jensen (2012). Sensors and Feedback Control of Multi-Color LED Systems. LED Professional. pp. 2–5. ISSN 1993-890X.
- Wallace Roberts Stevens (1951). Principles of Lighting. Constable.
- Parrott, Steve. "Moonlighting: Landscape Lighting Design Imitates Nature". Archived from the original on 2012-07-30. Retrieved 2011-09-29.
- "OSRAM SYVLANIA XBO" (PDF). Archived from the original (PDF) on 2016-03-03.
- Watkin, David (1996). The History of Western Architecture (Second ed.). Laurence King Publishing. pp. 508–509. ISBN 1-85669-082-2.
- Belinda, Stening. Curve. Curve http://www.curvelive.com/Magazine/Archives/thirty-one/Lighting-the-way. Missing or empty
- Desktop. Desktop Magazine https://desktopmag.com.au/features/the-occupation-lighting-designer/. Missing or empty
- Elizabeth, Donoff. Architectural Lighting http://www.archlighting.com/news/briefs/philips-appoints-new-chief-design-officers_o. Missing or empty
- Perkowitz, Sidney; Henry, A. Joseph (23 November 1998). Empire of Light:: A History of Discovery in Science and Art. Joseph Henry Press. ISBN 978-0309065566. Retrieved 4 November 2014.