Our present understanding of electromagnetic radiation grew from Herschel’s simple measurements of temperature in sunlight to the unification of the spectrum mathematically by James Clerk Maxwell in 1861 and, ultimately, to Max Planck’s formulation of the quantum theory in 1900. Herschel could not conclusively prove that light and radiant heat are the same quantity, but his experiments provided very strong evidence and were the first piece of the puzzle that others later built on.

Herschel was missing a concept, unknown in 1800, but fundamental to radiometry today: the effect of sensor response. The reading obtained with any instrument or sensor, including the human eye, results from the product of two curves: the distribution of the received power and the curve of instrument response, which includes the spectral transmission of all optical elements.

He measured the effects of scattering, and found, correctly, that light scatters more than infrared. Herschel attributed the difference to light and heat having different natures, rather than as evidence of similar behavior in a different interaction with matter (as scattering is dependent on wavelength).

In a paper read before the Royal Society on March 27, 1800, Herschel called this warmth felt at a distance “radiant heat.” This description is still a good working term for infrared radiation. The term “infrared” did not enter scientific vocabulary until the 1880s. Infra is Latin for “below,” but researchers have been unable to trace the source of who initially coined the name.

A computer atmospheric model was used to calculate the distribution of solar irradiance that illuminated Herschel’s prism, and the curve was normalized for comparison. Herschel was meticulous in recording his temperatures, but although he owned a number of prisms of both crown and flint glass with a variety of angles, he did not record what was used in his first experiments. The curve shown assumes his prism was of crown, which was common in 1800, and had a 60-degree angle.

The presentation of data can strongly influence their interpretation. Even today, it is difficult to look at Herschel’s graph without the impression that light and radiant heat are two different types of rays. Herschel’s curves are both accurate, but they are of different, almost unrelated quantities and should not be graphed together. His error was not in his basic data but in his assumption that the curves were comparable.

Today, the nonuniform response of a spectrometer can be corrected. But such calibration requires knowledge of wavelength and a source of known radiant power, neither of which were concepts in Herschel’s time. The displacement of his heat curve is not erroneous in itself; it was actually serendipitous because it created the temperature trend that led him into the infrared.

Human color vision is more complicated than shown, but the International Commission on Illumination curve for the light-adapted (or photopic) eye illustrates the concept (see figure 10). The sensation of sight is so rich in information that we don’t often think about how narrow is the slice of the electromagnetic spectrum that we see. Because of its narrowness, the shape of the response curve and that of its product with the solar spectrum are nearly identical.

It wasn’t the concept of invisible rays that so interested Herschel. What captivated him were the properties of these rays. It was clear to him that radiant heat had the same optical properties of “refrangibility” (“refraction” in modern usage) and dispersion as light.

Figure 8. Herschel’s final paper in 1800 contained the first graph showing the spectral distributions of visible light and radiant heat (what he called the “spectrum of illumination” and the “spectrum of heat”). Its two curves are of different, almost unrelated quantities and their misleading appearance on the same graph ultimately led Herschel to wrongly conclude that rays of light and radiant heat are of a different nature after all.

The title of this paper has an intriguing echo of Newton. In Newton’s Opticks (1730), his Proposition II, Theorem II is titled “The Light of the Sun consists of Rays differently Refrangible.”

Figure 1. An image of infrared radiation from a hot mug is colored with red, orange and yellow shades to show heat, but hot objects on their own often do not show any visible signs of their emissions. Although it is now common knowledge that both infrared and visible light are part of the electromagnetic spectrum, connecting the two seemingly disparate energy sources took some intricate experiments—and unfettered curiosity—by the famous 18th-century British astronomer Sir William Herschel.

Herschel embarked on an extensive instrument-building program to examine and measure each property. To the original prism and mercury thermometers he added a variety of lenses and mirrors in a dozen different configurations and an extensive array of transparent materials to compare transmission.

Herschel did not have a scientist’s insight into the causes of these phenomena, and he had limited ability to formulate mathematical descriptions of his findings. His strengths were his practical knowledge of optics combined with craftsmanship in the fabrication of instruments and his powers of observation.

Figure 10. The output of any sensor is the product of two curves: the spectral distribution of received radiation and the spectral response of the sensor. The narrowness of the eye’s response (black dashed line) produces a product curve (red line) that is nearly the same shape as the response curve. The result is more a map of how the human eye responds than of how light is distributed. Herschel’s error was assuming that this curve should resemble his temperature measurement.

He took readings with his thermometer, following the upward trend to a maximum and beyond, until the heating began to diminish. The tone of Herschel’s second paper is one of excitement and confidence in his findings. Throughout his experiments, he ventures few opinions that are not firmly supported by data, but he concludes this paper with an argument based on philosophy:

The paper’s final proposition asks whether radiant heat, if sufficiently strong, is able to stimulate vision. This question is critical because the complete answer explains why light and infrared are usually together but infrared can be present without light. With his 18th experiment, Herschel determined beyond doubt that increasing its power cannot make infrared visible.

To find the maximum of illumination, he directed colors onto a variety of small objects that he viewed through a 27-power microscope. From the brightness and clarity of what he saw, he judged the relative illumination.

Following his appointment, which came with an annual salary of £200, Herschel was able to devote all his time to astronomy. He and his sister, Caroline, settled in the town of Slough, near Windsor Castle. A condition of his appointment was that he be available to King George III and the royal family any time they wished to view the stars.

Figure 7. Herschel had to modify his instrument to follow the heating trend into the invisible, as this drawing from his second paper shows. Lacking the concept of wavelength, his only reference was in relation to where the last visible light fell. He marked off five parallel lines and positioned the board so the edge of red fell at the first line. His temperature readings increased to a maximum at approximately half an inch beyond red and diminished beyond. The experiment demonstrated that radiant heat has the same optical properties as light.

Usually the glazing material is not the object of attention in a room. Special glazing materials, however, such as thin slabs of stone, can be emphasized by the way they transmit light. Under the barrel vault of the trustee’s board room, overlooking the library at the Museum of Contemporary Art (Arata Isozaki, 1981–1986) in Los Angeles, California, onyx has been used to glaze a semicircular opening and four windows below it. The onyx fits tight to the ceiling, so that the glow of the entering daylight is carried along the black concrete ceiling surface. Attention is called to the onyx as it is the brightest surface in the room. The thickness of the material saves the window from being a source of glare. Light reveals and celebrates the onyx, making it the identifying feature of the room.

Herschel’s graph was the product of imagination, insight and months of painstaking work, but it was fatally misleading. Its appearance was probably the deciding factor in his conclusion that light and radiant heat are fundamentally different after all. As he wrote (with the area “ASQA” encompassing the spectrum of heat and “GRQG” the spectrum of illumination):

By November 1800, Herschel was approaching the end of what was possible to achieve given the knowledge and technology of his time. He was also almost certainly feeling pressure to return to astronomy.

Spectrometers today have much higher resolution, greater sensitivity and faster response, but the basic functional elements are the same as Herschel’s. Prisms are still used, but better resolution is usually obtained through wave interference —wavelengths are separated by constructive and destructive interference, where their waves either add together or cancel out. The detector today would be a cryogenically cooled semiconductor—much smaller, faster and more sensitive than a mercury thermometer. But Herschel’s instrument, in the hands of the careful experimenter that he was, gave surprisingly accurate measurements.

Herschel began his second experiment by slightly modifying his spectrometer to take temperature readings into the dark area on his board, beyond red. His only reference was in relation to where the colors fell on the table. He marked off five parallel lines spaced half an inch apart on a sheet of white paper, with the first line at the edge of the band of red light. Thus anchored to the edge of the visible, Herschel ventured into the darkness beyond.

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The heating was greatest in red, but the curve did not appear to reach a maximum in the visible spectrum. Instead, the readings seemed to point somewhere in the dark region beyond red. He felt compelled to follow this trend.

For the spectrum of illumination, it must have seemed logical to Herschel to plot this on the same graph because his original objective was to find a filter that would maximize light while minimizing heat. His curve was accurate, but it did not show what he thought.

Most encyclopedias and physics books credit the great British astronomer Sir William Herschel with the discovery of infrared radiation in 1800. It’s a good story, but it is not strictly correct—it trivializes the true significance of what Herschel found.

Something about the temperature readings clearly bothered him. He had expected, as he found, that the readings would be different for the various colors. But the measurements also showed something he did not expect: a trend, rather than a peak.

He placed one thermometer in the light and kept the other two in darkness to measure the room’s ambient temperature. Herschel understood there were, as he expressed it, “causes acting in different ways” (in other words, conduction and convection) that affected the stabilization temperature of the thermometers, and he wanted to quantify the heating caused by the light alone.

In more than 200 experiments, he recorded page after page of readings using every available illuminating source viewed through different combinations of mirrors, prisms and lenses. He confirmed again and again, in every way he could test, that light and radiant heat have the same optical properties.

His first instrument consisted of three components: a prism set in a south- facing window to catch the sunlight and direct and disperse the colors down onto a table; a small panel of cardboard with a slit wide enough for only a single color to pass through; and three mercury- in-glass thermometers (of which he used two) with their bulbs blackened to better absorb light. Thermometers were not common household items in 1800, but Herschel had one of his own and borrowed two more from a colleague.

Light and materials are mutually dependent on each other. Materials are key to understanding light in architecture because they directly affect the quantity and the quality of the light. Two qualities of materials – their finish and their color – are most important in this regard. Specular materials, such as glossy finishes, reflect light as a mirror does, which can result in reflected images of the light source being visible ‘on’ the surface. Matte surfaces, such as natural stone, wood, and plaster, reflect light diffusely equally in all directions. Of the three aspects of color – hue, value, and intensity – value is the one that determines how much light is absorbed and how much is reflected. A white wall reflects approximately 82 percent of incident light, a light yellow wall 78 percent, and a dark green or blue wall 7 percent. 1 Colored surfaces lend some of their hue to light that is reflected.

If an instrument responds nonuniformly, as most do, then the response shape is impressed on the received radiation in a way that cannot be extricated without independent knowledge of how the radiation is distributed and how the instrument responds. Lacking this concept, Herschel assumed the spectra he measured accurately represented how the radiation was distributed.

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Comparing his spectrum of heat with the solar spectrum shows that Herschel made remarkably good measurements considering the limitations of his instrument and that he had no idea how a solar spectrum should look. His curve is displaced toward longer wavelengths because of the dispersion curve of glass.

If the maximum lay outside the visible spectrum, then the heating was not from light but from something else. Herschel used the expression “invisible light,” cautiously phrasing it in a way that indicated he knew it to be an oxymoron. (If rays are invisible, then they aren’t light.) As he expressed it:

Herschel’s reputation as an astronomer probably helped ensure that his papers were favorably received by most scientists, but not by all. His third paper opens on a decidedly defensive note. He appears to have been attacked by a person he refers to as a “celebrated author,” who took offense at the phrase “radiant heat.” This detractor may have been John Leslie, who was considered an authority on heat and clearly resented the intrusion of an amateur into his domain. In a letter published in A Journal of Natural Philosophy, Chemistry, and the Arts by William Nicholson, Leslie wrote:

In the central six-story light well in the Casa Batllo (Antonio Gaudi, 1904–06) in Barcelona, Spain, Gaudi designed the ceramic tiles that cover its surfaces to manipulate light. By modulating the hue, value, and texture of the tiles, he modified the qualities and quantities of light experienced in the light well itself as well as in the adjacent apartments. The tiles range in color from a deep blue through lighter shades of blue to an off-white. The deep blue tiles are placed in their largest concentration at the top of the light well, on the surfaces directly under the skylight glazing, interspersed with lighter tiles. The effect here is cooling, almost as if one were seeing the light underwater. At the bottom of the light well are placed the lightest tiles, interspersed with a few darker ones. In between, the colors gradually shift from dark to light. This distribution of the colored tiles evens out the perceived light gradient in the light well, establishing a balanced light. Thicker patterned tiles, which reflect the light from their corners, are scattered among the smooth ones along the entire height, adding a glint of sparkle. In addition to the use of materials to manipulate the light, the shape of the light well – wider at the top – and the sizing of the windows – larger at the bottom – serve to balance access to light for all residents. An additional geometric manipulation of the section of the light well is the insertion of balconies, with glass panels serving as flooring for the balcony and as a skylight for the room below The light that enters the apartments through the windows in the light well is therefore more equal than in the usual situation where the rooms at the top garner all the light and the rooms at the bottom are in shadow. Ventilation apertures are separate from the glazed windows, thereby adding more light to the interior when they are open.In the Casa Batllo, light was used in a thoughtful way with consideration for the well-being of the inhabitants, their need for light and air, and artful ways to provide them. Light was apparently considered at each step in the design process: concept, development of plan and section, window size and placement, surface shape and composition, and details. The tile work is not only beautiful, but also serves the purpose of modifying the daylight as it enters the building and is distributed to the apartments. Control and delight are both provided. The materials of electric lighting fixtures are as important as those of building surfaces which are acting as daylighting fixtures. In the Resurrection Chapel (Erik Bryggmann, 1939–41, renovated 1984) in the Turku Cemetery in Finland, the brass lighting fixtures reflect daylight with a cool yellow that warms to an amber glow when the incandescent lamps are turned on. The material and details of the fixture respond to the incandescent Iight. The vertical blades that baffle views of the lamp glow with the light reflected between them. The ‘crown’ of brass loops circling the top catch the light, as do similar ‘crowns’ higher up. The pools of gold light in the cool interior lend a warmth and points of attraction similar to the glow of a fire. The light furnished by electric lighting fixtures is contrasted with the daylight in both color and distribution. While the daylight washes the surfaces of the interior with fairly even light, the electric lighting fixtures act as points of focus.

This observation is remarkable: Yellow-green is near the wavelength where the Sun’s radiant power is a maximum and is exactly where the eye’s sensitivity is greatest.

We all discover infrared at a young age when we feel warmth at a distance from a hot object, and we know that these rays are invisible—warmth can be felt in total darkness. What Herschel discovered was subtler than the existence of invisible radiation. He found the first solid evidence that light and infrared are the same quantity that we know today to be electromagnetic radiation. Through a series of simple experiments, Herschel found the first piece in one of the great puzzles of physics that took another century to solve.

As a result, Herschel’s spectrum of illumination is more a map of how the human eye responds to colors than how light is actually distributed. He saw the shape of his curve as evidence that light is not equally distributed across the spectrum. His hypothesis was correct, but his data did not show it. He would have gotten an almost identical curve even if the distribution were uniform.

In spite of its displacement, the shape of Herschel’s curve confirms his first hypothesis that the heating power of sunlight is not equally distributed across the spectrum. The fact that his curve is continuous in its transition into the infrared supports his second hypothesis that light and radiant heat are the same quantity.

Herschel had observed features on the Sun’s surface for a number of years, presenting a paper on the Sun and fixed stars to the Royal Society in 1794. Being able to observe sunspots with a large telescope without damaging his eyes had long been a challenge. Through experiments with different combinations of colored and darkened glass, Herschel observed, as he noted in this paper:

Herschel accepted without question Newton’s beliefs on the “corpuscular” nature of light. A strong case for light being a wave had been made by Newton’s contemporary, Christiaan Huygens, but the theory of light as streams of minute particles dominated science at the time, especially in Britain. This viewpoint changed within the next 15 years, but at that moment, Herschel thought of light as particles that had more or less “efficacy” in their effect on matter.

To evaluate Herschel’s spectra, we need to see how the Sun appeared from the village of Slough at the time of his measurements. Herschel did not record the date and time, but analysis of the curve shape indicates the data likely came from his first experiments, and thus were probably taken sometime in late February or early March. Slough lies at latitude 51.5 degrees north, which would make the solar zenith around 61 degrees (29 degrees above the horizon) at local noon.

Figure 3. The electromagnetic spectrum stretches from gamma rays to radio waves, but human beings can directly sense only two small segments. Our eyes see light, a narrow band of wavelengths centered approximately where the Sun’s radiant power is at its maximum. Our skin feels warmth across the spectrum, but mainly from infrared, which spans the range of wavelengths between light and microwaves. Everyday experience alone would not lead us to believe they are the same quantity.

In the end, the misleading appearance of his graph and the different sensations of seeing light and feeling heat won out over all of his carefully collected evidence, leading him to conclude that the two energies are not the same after all. Actually, he never reached a firm conclusion. The closest Herschel came was to again invoke philosophy, this time to argue against his original thought:

The best information about Herschel’s experiments is found in his original papers. His recorded data and many of his comments appear to have been taken directly from his lab notes, and their freshness and authenticity come through even today. The biggest challenge reading his work is to follow his line of reasoning through many digressions and pages of raw temperature data.

Figure 5. Herschel directed the dispersed colors from a prism onto a piece of cardboard with a slit that allowed only a single color to pass. To measure relative heating, he kept one thermometer in the light and the other in shadow.

It was a question that dominated Herschel’s thoughts and effort for much of the rest of the year. He must have worked rapidly, because just 9 days after writing his first paper and 10 days before he formally presented it, he wrote a second, shorter paper to the Royal Society titled “Experiments on the Refrangibility of the Invisible Rays of the Sun.”

These differences may be why the connection was not made for so many centuries in spite of much experimentation. Perhaps this counterintuitive association is why the connection was found almost by accident by a person with no formal scientific training.

His final paper, presented November 6, 1800, contained the first graph made showing the spectral distributions of visible light and infrared radiation. He called these curves the “spectrum of illumination” and the “spectrum of heat.” On the vertical axis, Herschel plotted measured temperature and perceived brightness. He set their maxima equal (a format called peak-normalization) to compare the relative extent and shape of the distributions and the location of their maxima. For the horizontal axis, not having the concept of wavelength, he used distance in relation to where the visible colors fell. The horizontal axis is reversed from today’s convention of increasing wavelength from left to right.

Refraction is the change in direction of a ray as it enters or exits a transparent medium that causes a change in velocity, such as between air and glass. Dispersion is the effect of refraction on multiple wavelengths, causing different rays to refract at differing angles. We see the effects of the dispersion of light most commonly from rainbows and prisms. Herschel didn’t think of light in terms of wavelength, but as a lens-maker, he was very familiar with the effects of dispersion and how to correct for it to produce lenses that minimize what is known today as chromatic aberration, where different colors converge to a focus at varying distances from the lens. As he wrote:

If Herschel’s measurements had been made in summer, when the Sun is higher in the sky, the maximum he found would have been closer to the center of the visible spectrum. But in winter, with a longer atmospheric path, the maximum of the spectral irradiance (the incident power density as a function of wavelength) is pushed toward red due to atmospheric scattering of the shorter wavelengths.

By the following year, he was grinding his own mirrors to build larger and better quality telescopes, and spending his nights studying the heavens. Herschel’s craftsmanship rapidly took his hobby to public recognition as the foremost telescope manufacturer of his time. His fame peaked after he discovered the planet Uranus in 1781, which led to his appointment as the king’s astronomer.

Light and materials are inseparably connected, indeed they actually determine each other: neither is visible to the human eye until the two come together. For this reason, great architects have always also allowed themselves to be directed by the light in the choice of their building materials. They use light to draw out contrasts between different materials and they use materials that allow them to create a very specific distribution of light in a room.

This argument would not be credible today and probably sounded weak even in 1800, but it was a way of bringing his quest to a close. Herschel was surely disappointed, but he had achieved more than he or any contemporary realized.

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In Henry’s Church (Pitkänen, Laiho and Raunio, 1980) in Turku, Finland, the material of the lighting fixtures appears entirely different under daylight and under electric light. With daylight streaming in from large windows, the white screen material is almost transparent, and the brick wall shows clearly through it. When illuminated from below, the white material becomes a reflector, casting the light back down to the congregation. As it does so, it obscures the view of the wall directly behind it and casts a warm glow over the nearby wall surface. The material switches between revealing what is behind it and reflecting what is in front of it, as does a scrim curtain in theater productions. Through it, one becomes more aware of the difference between the nature of daylight and electric light.

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The vast electromagnetic spectrum stretches from gamma rays (whose wavelengths can be smaller than the width of an atom) to radio waves (whose wavelengths can reach thousands of kilometers). Of this range, humans are only able to directly sense radiation in two small bands. Our eyes see light, which occupies a narrow sliver of wavelengths from 0.4 to 0.7 micrometers, centered approximately where the Sun’s radiant power is at its maximum. Our skin feels warmth mainly from infrared, which spans the range of wavelengths between light and microwaves, up to about 1,000 micrometers.

Feeling he had proven that both radiant heat and light are not equally distributed across the colors, and with measurement results showing their differences, Herschel should have been ready to move on to applying these results to his problem of viewing the Sun. But he didn’t do so immediately. Instead, he returned to the temperature data.

Any spectrum created from what the eye sees will be zero outside the eye’s limits. Inside its limits, the power received at any wavelength will be weighted by the eye’s sensitivity to that color. The greatest sensitivity, as Herschel correctly determined, is yellow-green at a wavelength of 0.555 micrometers.

Figure 6. With his first temperature readings, Herschel believed he had proven the heating power of light was not equally distributed across the visible spectrum, as he had found the greatest heating from red. But his readings also showed a trend that appeared to point toward a maximum somewhere in the dark region beyond red. It was a trend that he felt compelled to follow.

Herschel’s foray from astronomy to infrared was a fortuitous tangent from his effort to find the best color for a filter that would allow him to safely view the Sun. His speculations and conclusions were often contradictory: Many were wrong, but some were extraordinarily prescient. The story of Herschel’s experiments is that of the role of human perception in scientific discovery, as well as the conflict between conventional beliefs and concepts never before encountered.

This observation led to the thought that different colors might, in Herschel’s words, “have the power of heating bodies very unequally distributed among them.” Herschel further reasoned that if the heating power were unequally distributed, the illuminating power might be as well. There might be a single best color for seeing, and it might be different than the one for maximum heating. Knowing these qualities would help him find the best filter to view the Sun.

Light emphasizes the materials in Patkau Architects’ Newton Library at Surrey, at the same time as the materials emphasize light and foster its distribution. As the architects have stated, ‘because the light of the Vancouver area can be very soft, even weak, under the frequently overcast skies of winter, the robust light-absorbing character of heavy timber and concrete, in themselves, are not appropriate to distribute natural light into a relatively deep floor plate.’ The ceiling surfaces of the library have therefore been treated where needed with material that distributes light to the interior. Near the tall sloped north glazing, where the large area of glass provides abundant light for reading, the ceiling surface is the exposed underside of the wood decking. In such a situation, care must be taken so that the contrast between materials at the perimeter glazing and the sky is not too great, causing discomfort. Here the wash of daylight over the wood beams and onto the underside of the ceiling helps to mitigate the contrast at the edge as well as even out the brightness of the ceiling from the window wall to the center of the room. About midway between the glazing and the low center beam, sheet rock surfaces painted white were applied as the ceiling surface, better reflecting the daylight down to the area below. Each material is used honestly to do what is needed and no more, forming an economical building shell. The layering of materials discloses the role that each plays in the total realized construction.

He discovered that radiant heat has the same optical properties as light. He confirmed his hypothesis that heating power is not equally distributed across the spectrum. He performed the first radiometric measurement of spectral radiant power across the visible into the infrared and found it to be a smooth, continuous curve.

Light emphasizing materials Emphasis on materials is grounded in the interaction between light and material. Highlights arise from glossy materials reflecting discrete points of light. Definition of surface texture comes from grazing light. Revelation of the inner qualities of materials results from light passing through them. Dark shadows result from light being deflected from the surface, and from material absorbing light.

Newton clearly had great influence on Herschel. The latter’s approach was nearly identical to Newton’s experiments using a prism placed in a window to project colors onto a wall. Herschel took Newton’s basic qualitative method of viewing the spectrum and turned it into a quantitative instrument. He may have felt—justifiably—that he was continuing the work Newton had begun with the colors of light by extending the concept of “different refrangibility” to the rays of the Sun that lay beyond the visible colors.

The ‘lume materiale’ (literally: ‘material light’) of Venice 4 seems to glow in Boston, Massachusetts, at the Isabella Stewart Gardner Museum (Willard T. Sears, 1899–1901). The wall was prepared in a similar way to the traditional Venetian stucco, and consists of plaster impregnated with color introduced by using a wash of pink paint. The light of Boston is not the light of Venice, but the appearance can fool the eye on certain days. The surfaces seem to glow in and of themselves so that the light is more real than the material. Likewise, in the exhibition of glass balls, Niijima Floats, by Dale Chihuly (1992 at the Seattle Art Museum), the light that is cast by the glass seems to be the point of the piece. It is not the balls themselves that are so important, but the patterns of light that they cast on the surface below. The material (of the balls) transforms the light which then transforms material (the resting surface).

The travertine, unfilled and unpolished, has certain characteristic ways of reacting with light that complements the reaction of concrete to light. As the light changes – outside and inside – the surfaces of the two materials shift subtly in relationship to each other. First one seems warmer, then the other does. First one appears to be lighter, then the other. One seems to have a glossy surface, and the other a matte finish, and then they switch. One looks more mottled, then the other one does. The surfaces respond to the changing light. Light is the real material here.

The criticism he received did not slow his experiments, but these attacks may have had an impact. By the second part of his final paper, his emphasis changed from finding evidence supporting the similarity between light and radiant heat to that supporting their difference.

Louis Kahn was very aware of the nature of a material’s response to light. Kahn’s selection of concrete and travertine as materials for the Kimbell Art Museum (1966–1972) in Fort Worth, Texas, was related to how their surface characteristics shaped that response: “Travertine and concrete belong beautifully together because concrete must be taken for whatever irregularities in the pouring’are revealed. […] Time, he believed, would unify all materials eventually, but the architect could achieve unity by carefully choosing certain materials-wood, travertine, concrete – ‘which are so subtle that each material never ruins the other […] And that’s why the choice’.”

illumination Definitions from The American Heritage® Dictionary of the English Language, 5th Edition. from The Century Dictionary.

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Herschel positioned the measuring thermometer in the band of colored light for each reading. At each color position, he allowed the thermometer to stabilize for 10 minutes before taking a reading. He took a series of measurements, starting with red, which gave an average reading of 67/8 degrees above ambient. Green gave 31/4, and violet light gave a 2-degree average increase.

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Light muting materials Materials can also be chosen to mute the effects of light, to make dissimilar materials appear similar, or to make the light seem unchanging. The shoji screens in traditional Japanese houses diffuse all the daylight that enters, whether the skies outside are sunny or overcast. The light is first shaded by the large overhanging roofs acting as a parasol, so that the interior muted effect is constant. The interior surfaces are carefully crafted to interact with the light. Junichiro Tanizaki explores this connection between light, materials and culture in his book ‘In Praise of Shadows’, in which he explains the traditional preference of Japanese people for shadows and soft, broken light: “We do our walls in neutral colors so that the sad, fragile, dying rays can sink into absolute repose. The storehouse, kitchen, hallways, and such may have a glossy finish, but the walls of the sitting room will almost always be of clay textured with fine sand. A luster here would destroy the soft fragile beauty of the feeble light. We delight in the mere sight of the delicate glow of fading rays clinging to the surface of a dusky wall, there to live out what little life remains to them.”

By describing the rays in terms of momentum, Herschel was not anticipating the discoveries of quantum physics, still a century in the future. Photons at infrared wavelengths do have less energy than those in the visible band. As a result, they do “have such a momentum as to be unfit for vision.” He was not looking a century ahead but a century back—to Isaac Newton’s experiments with light in the late 1600s.

The illumination experiment also went well. He did 10 separate experiments with objects viewed in different colors. He attempted to distinguish between the color having the maximum of illumination and that having the sharpest resolution or “distinctness.” He was unable to reach a conclusion about resolution, but for illumination, he was able to state:

Drawing on his experience making telescopes, Herschel built an instrument to test his hypothesis. He made what we would call a spectrometer, or more precisely, a spectroradiometer: an instrument to measure the magnitude of radiant power at different wavelengths.

The boundary between light and infrared is determined by the long-wavelength limit of the human eye’s response. Everyday experience would not lead us to believe light and infrared are the same kind of energy. Indeed, two compelling pieces of evidence suggest, logically, that they are not related.

Light was also a contentious issue. Herschel’s brash assertions about radiant heat and light had stepped on the toes of conventional belief. With light, he again adopted a cautious stance, but this time he countered with a challenge calculated to silence his critics:

Materials are important emotionally in relation to light. The sparkle of glass. the glitter of gold mosaics, the depths of dark polished wood, and the shadows on white walls all hold emotional messages. some of them connected with cultural settings, some of them connected to individual recollection. Some regions have building traditions and materials that respond to particular local conditions, such as the stucco alla veneziana favored by Carlo Scarpa. Requiring a labor-intensive process of application with very particular materials, the stucco ‘over time takes on a softer, more moist look, a quality of fantasy and beauty.’

The telescope designer in Herschel grasped the significance immediately: If light and radiant heat have the same optical properties, if they exhibit the same behavior in their interactions with matter, might that indicate they are the same quantity? He notes: “

From these data, Herschel felt that he had proven his hypothesis about heating being unequally distributed and could move on to the illumination experiment. As he stated in his initial conclusion:

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Relating the index of refraction to wavelength, the dispersion curve causes refraction to spread the wavelengths across the target board nonuniformly. At each position, moving toward longer wavelengths, the spectral width or band of wavelengths that the thermometer receives becomes progressively wider. A wider spectral width contains more power, which increases the reading. The result gives readings at longer wavelengths that are weighted more heavily than those at shorter wavelengths.

Herschel was not originally a scientist. He rose from obscurity as a German immigrant in a military band to become an accomplished musician and composer. In 1773, at age 34, six years after moving to Bath to take a position teaching music and playing in concerts, he did something that changed his life and fortunes. He bought a small telescope and a book on astronomy.

If Leslie’s differential thermometer (he called it a “photometer”) found no heating beyond the visible as he claimed, then it was badly in error. This point was later proven dramatically by independent experiments conducted by the Royal Society. To deflect criticism, Herschel switched from the term “radiant heat” to “the rays that occasion heat.”

The frantic pace of his later experiments may have caused him to miss or misinterpret connections, especially after he began to look for differences instead of similarities. He did a detailed experiment showing that the focal length of a refractive converging lens was longer for heat than for light, without realizing that the difference in focal length is caused by the same dispersion as a prism.

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With his device set up on a sunny day, Herschel methodically took temperature measurements, first comparing the thermometers’ readings at ambient conditions to ensure baseline agreement. It was cold in the room. His starting temperatures averaged 43.5 degrees Fahrenheit. After some experimentation, he settled on using his own thermometer with its half-inch-diameter bulb exposed to the light and used the larger of the borrowed ones as the ambient reference.

Figure 9. Comparing Herschel’s “spectrum of heat” with that of sunlight shows his measurement was surprisingly accurate. His curve is displaced toward longer wavelengths due to the dispersion curve of glass. Without the concept of wavelength and a known radiation source, this effect could not have been corrected in Herschel’s time. Herschel’s curve confirms his first hypothesis that the heating power of sunlight is not equally distributed across the spectrum. And, the fact that his curve is continuous in its transition across the visible and into the infrared strongly supports his second hypothesis that light and radiant heat are the same quantity.

A change in materials can alter the feeling of a room and the level of illumination as well. The cheapest wad to increase the amount of light in a dark room is to paint the room surfaces white. A dark room, on the other hand, can be created either by using little light in a white room or through dark surfaces. With dark surfaces, a room will look dark during both daytime and at night. With light or white surfaces, however, the effect changes depending upon the light sources used. This effect can be exploited. For example, the interior surfaces of the chapel of Notre Dame du Haut at Ronchamp are white, but due to the small quantity of daylight admitted, perceptually the surfaces grade horn light gray to dark gray.

Figure 4. Herschel’s mental leap to connect light and radiant heat had to overcome everyday experience with each energy: If light and heat are the same, we’d expect to find them together, and sometimes radiant heat can be found alone (as with hot coffee or the human body, at middle and bottom insets). A century after Herschel, quantum theory explained that radiant power had a wave-shaped distribution whose position on the spectrum shifts upward and toward shorter wavelengths with increasing temperature (blue arrow). Around 700 degrees Celsius, the shortwavelength edge of the curve pushes sufficiently into the visible range for the human eye to see a red glow, such as from an electric grill (top inset). Infrared images were taken in the band of wavelengths indicated by the gray box.

Herschel’s third paper proposed seven comparisons between light and radiant heat. The first concerns two human senses. The next five are interactions with matter that were known in 1800: reflection, refraction, “different refrangibility” (dispersion), transmission through “diaphanous bodies” (transparent media) and scattering from rough surfaces.

First, we experience light and infrared differently with different senses. We see light, perceiving different wavelengths as different colors, but we feel infrared only as warmth. Second, light and infrared aren’t always found together. Most sources of light also emit infrared, but infrared is often found by itself. A familiar example is an electric grill not hot enough to glow: If the room is completely dark, we can still feel the warmth from the grill, but we can’t see it.

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