From its beginning, NBS had studied properties of building materials in order to meet the construction industry's recognized need for specific technical information. It was not until 1921, however, when Herbert Hoover became Secretary of Commerce, that these activities were brought together under a Division of Building and Housing and grew into a significant and effective program. The impact of this effort was noted with the publication of the Building and Housing Series, and perhaps the most important document in the series was the very first publication, Recommended Minimum Requirements for Small Dwelling Construction [1]. This publication is often viewed as the first national model building code, and the Building and Housing Series [2] established NBS as an important source of information to serve the public and stimulate economic growth.

The significance of this publication and the events leading to its release are documented most clearly in the book, Measures for Progress [3].

66

... Hoover entered office determined to recover the Nation, singlehandedly if necessary, from its wartime splurge, its consequent depletion of resources, and the general economic demoralization into which it had plunged. Recovery, by raising as rapidly as possible the level of productivity, was the first essential; reconstruction would follow.

"Hoover's plan for recovery, in order to open employment offices again and start the wheels of industry, was to stimulate building and housing, lend direct assistance to both new and established industries, and minister to the new aviation and radio industries. Reconstruction, providing long-range benefits to the economy, aimed at a progressive elevation of the standard of living, principally by a campaign to eliminate economic wastes. "Although the building trades themselves badly needed reconstruction, they offered the most likely means of achieving immediate and massive results in reviving depressed industry and providing maximum employment across the Nation. The housing shortage as a result of the war was estimated at more than a million units. Stimulate homebuilding, and the

brick, lumber, glass, hardware, plumbing,
appliance, textile, and furniture industries
and all that served and supplied them would
revive."

Hoover's new division of building and housing set its sights on revising an infrastructure consisting of poor housing designs, costly materials and labor, outdated building and zoning regulations that were barriers to development, and a shortage of mortgage funds. The circumstances at the time of issuance of Recommended Minimum Requirements for Small Dwelling Construction are summed up in a preliminary report of the 1920 Senate Committee on Reconstruction and Production:

"The building codes of the country have not
been developed upon scientific data but rather
on compromise; they are not uniform in prac-
tice and in many instances involve an addi-
tional cost to construction without assuring
more useful or more durable buildings." [4]

NBS's program was announced with national publicity. Information was to be published on home building, home ownership, technical revisions to construction codes, plumbing codes, zoning ordinances, etc., suitable for adoption within codes or as separate ordinances. Beyond these recommended minimums, NBS was to provide standards for better construction practices that would improve workmanship, seek simplification and standardization of building materials, reduce dimensional variations and deviations, and otherwise serve to lower costs. To meet these objectives, Recommended Minimum Requirements for Small Dwelling Construction included an extended appendix in which information was presented on good construction going beyond the limits of minimum safe standards.

The spring of 1922 saw the start of a major spurt in the construction of housing. Secretary Hoover chaired the National Advisory Council of the Better Homes in America movement. This program mobilized chambers of commerce, women's clubs, and better homes and gardens organizations to promote more and better housing across the country. The Department of Commerce's housing division, working with NBS and in consultation with building officials, architects, engineers, fire chiefs, materials experts, and their related associations, gathered and organized technical

New homes construction in 1922 was reported at over 700,000 units, nearly doubling the total from the prior year. By 1925 housing production had risen to 937,000 units. The 8-year period of 1922-1929 saw an average of 750,000 homes per year completed, far in excess of the 450,000 units per year estimated as being necessary to overcome the postwar shortage.

Recognition of the importance of sound and economical construction, coupled with home ownership as a significant contributor to the welfare of the nation, was manifested in December 1931 when over 3000 civic

leaders from around the country came to Washington to attend the President's Conference on Home Building and Home Ownership. The conference emphasized improvements in the building code situation and the importance of sound regulations.

In the spring of 1932 Recommended Minimum Requirements for Small Dwelling Construction was re-issued as Building and Housing Publication No.18 (BH18) [4] and superseded BH1. The 102 page publication consisted of three parts: Introduction, Minimum Requirements for Safe and Economical Construction of Small Dwellings, and Appendix. The introduction provides background, purpose and intent information for the reader. The minimum requirements are organized by types of construction with separate sections describing different types of horizontal and vertical space dividers and miscellaneous components (e.g., chimneys and fireplaces, heating appliances). The bulk of the document is the appendix, which provides detailed technical information on a wide variety of materials and methods of construction assembled from a number of industry sources.

BH18 was a continuation of the work of the Department of Commerce's Building Code Committee. The committee consisted of seven members (four of whom were members of the original group) and was chaired by William K. Hatt, professor of civil engineering and director, Laboratory for Testing Materials, Purdue University. The committee continued to operate under the Division of Building and Housing of NBS with James S. Taylor, Chief. George N. Thompson of NBS, who served as Secretary of the Committee from its inception, provided direct liaison with NBS and thereby contributed the scientific and engineering knowledge necessary to develop the technical information and answer committee questions throughout the deliberations.

Recommended Minimum Requirements for Small Dwelling Construction was the forerunner for the Federal Housing Administration's Minimum Property Standards, which provided quality requirements for the post World War II housing construction boom, and for the One and Two Family Dwelling Code (OTFDC) [6], published first in 1971 by the three model code organizations. In 1972 the Council of American Building Officials (CABO) was formed, and later in that decade CABO took over the publication of the OTFDC. This code provides the technical requirements for most of the Nation's conventionally constructed housing. In 1994 the CABO constituency formed the International Code Council (ICC), and in 2000 the ICC published the International Residential Code [7], which supersedes the OTFDC.

In 1933, as a result of an economy program in the Federal Government, the American Standards Association (ASA) established a Building Code Correlating Committee to take over the work of the Department of Commerce's Building Code Committee. Technical questions that arose in ASA sectional committees continued to form the basis for many scientific investigations at NBS. George N. Thompson became chairman of ASA's Building Code Correlating Committee, then chairman of its successor, the Construction Standards Board.

NBS provided leadership in many important construction standards activities by directly sponsoring the following American Standards: the National Electric Safety Code (ASA C2); the Safety Code for Elevators (ASA A17); Building Code Requirements for Minimum Design Loads in Buildings and other Structures (ASA A58); and Building Code Requirements for Masonry (ASA A41). In addition, NBS staff members participated actively in the development of other American Standards for building construction including: the National Electrical Code (ASA C1); the Safety Code for Building Construction (ASA A10); Building Code Requirements for Fire Protection and Fire Resistance (ASA A51); and the Safety Code for Mechanical Refrigeration (ASA B9). These standards were recognized in virtually all state, local, and model building codes in the U.S., and by all federal construction agencies. They unified technical requirements for buildings based on consensus of the construction industry, and contributed substantially to the efficiency and economy of construction while improving safety and quality. NBS research results, many of which are documented in the 150 titles of the NBS Building Materials and Structures Reports published between 1938 and 1957, provided the foundation for many of the provisions. Many of their technical requirements survive as the basis for today's construction standards and codes.

When the American Standards Association decided to incorporate in 1947, the U.S. Department of Commerce withdrew as a member body, and NBS

ceased to lead construction standards management activities until its participation in the renamed American National Standards Institute's Construction Standards Management Board was renewed in the 1970s. However, technical contributions to and participation in consensus standards committees has continued unabated to the present time. A number of particularly significant contributions are described elsewhere in this centennial publication.

Prepared by Joel P. Zingeser.

Bibliography

[1] F. P. Cartwright and the Building Code Committee of the Department of Commerce, Recommended Minimum Requirements for Small Dwelling Construction, Building and Housing Publication BHI, National Bureau of Standards, U.S. Government Printing Office, Washington, DC, July 1922.

[2] For a summary, see P. R. Achenbach, Building Research at the National Bureau of Standards, Building Science Series 0, National Bureau of Standards, U.S. Government Printing Office, Washington, DC, October 1970.

[3] R. C. Cochrane (with J. R. Newman), Measures for Progress, A History of the National Bureau of Standards, NBS Miscellaneous Publication 275, National Bureau of Standards, U.S. Department of Commerce (1966).

[4] Department of Commerce, Building Code Committee, Recommended Minimum Requirements for Small Dwelling Construction, Building and Housing Publication No. 18, National Bureau of Standards, U.S. Government Printing Office, Washington, DC (1932).

[5] Additional publications included: BH2-Recommended Minimum Requirements for Plumbing in Dwellings and Similar Buildings; BH3-A Zoning Primer; BH4-How to Own Your Home; BH5A Standard State Zoning Enabling Act; BH6-Recommended Minimum Requirements for Masonry Wall Construction; BH7Minimum Live Loads Allowable for Use in Design of Buildings; BH8-Recommended Practice for Arrangement of Building Codes. [6] International Conference of Building Officials, Inc., Building Officials and Code Administrators International, Inc., and Southern Building Code Congress International, Inc., One and Two Family Dwelling Code, 1971.

[7] International Code Council, Inc., International Residential Code, 2000.

This classic paper [1] from 1923 reports the results of one of the most enduring projects ever undertaken at NBS, research into the physical description of human vision. The principal result of this work was the "visibility curve," a quantified model of how well a typical person can see the different wavelengths (colors) of light. Today this model function, essentially unchanged, underlies all physical measurements of photometric quantities and their interpretation in photometric units of measure.

It has been understood since the time of Isaac Newton that white light is a combination of a spectrum of different wavelengths, each seen as a pure color. Light is a form of radiant energy, with a power that can be measured in watts, but the connection between this physical description (or the "mechanical" description as it was then known) and the visual result in the human eye was not well established. This was the challenge undertaken by K. S. Gibson and E. P. T. Tyndall: to carry out a study of the visibility of radiant energy or, in quantitative terms, the ratio of the luminous (perceived) power to the radiant (physical) power at the different wavelengths in the spectrum.

Gibson and Tyndall were neither the first nor the last to study the visibility of light, but their work is perhaps the most notable for its thoroughness, timeliness, and impact. The first experiments on this subject were undertaken by Fraunhofer in 1817, and the first energy measurements were made by Langley in 1883 [2]. By 1905, Goldhammer had crystallized the idea of a definite relationship between visibility and power at each wavelength, and at the young NBS, Nutting introduced the term "visibility curve" in 1908 [3]. The Bureau's forefront research continued through the subsequent decade, leading to the major study of the sensitivity of the eye across the spectrum by Coblentz and Emerson in 1918 [4].

However, these and other data accumulated around the world were not consistent. Different experimental methods were a chief cause. In the equality-of-brightness matching method, two lights were projected onto a split-screen viewer, while with the flicker method, two lights were alternately projected on a simple viewing screen in rapid succession. In each case, the lights were adjusted in a known way until an observer declared a brightness match. The equality-of-brightness method was the more precise of the two, but only so long as the color quality of both lights was similar. When the colors

were very different, different observers would make different matches. The flicker method did not give as sharp results for similar lights, but the data quality was not much affected by color differences.

Seeing the need to bring closure to the question, Edward P. Hyde (who had left the NBS staff in 1908 to go to the General Electric Nela Research Laboratories), as president of the U. S. National Committee of the International Commission on Illumination (the CIE), requested the Bureau of Standards to make an additional investigation using the so-called step-by-step method. This form of equality-of-brightness matching, where comparisons were made between a series of only slightly different colors, held promise as a means of obtaining more reliable data.

Gibson and Tyndall were neither the first nor the last to study the visibility of light, but their work is perhaps the most notable for its thoroughness, timeliness, and impact.

NBS undertook the challenge under the sponsorship of General Electric. Director Burgess appointed a special committee of experts to oversee the work, which was conducted by Gibson and Tyndall. The University of Nebraska loaned a Brace spectrometer to the Bureau, to be incorporated into an elaborate apparatus that made the best use of the Bureau's primary standard lamps. Special care was taken in all aspects of the experiment; issues that were believed to affect the consistency between previous experiments such as the size and brightness of the viewing fields-received particular attention.

The results included the brightness-matching data from 52 observers, some of them in common with previous studies. As hoped, the new equality-of-brightness data were within the range of data obtained with flicker methods (except in the outer regions of the spectrum).

However, the strength of the paper was not so much in the new experimental results as it was in its extensive analysis and critical review of all existing data. Gibson

and Tyndall carefully compared their own results with those of their predecessors and proposed a mean visibility curve based upon the accumulated data from more than 200 different observers. They were guided in this task by the prevailing theories of the day, which were believed to dictate certain balance in the curve [5].

The result was a smash success, quickly winning wide acclaim. In 1924, the 6th Session of the CIE adopted the Gibson-Tyndall curve as a world standard. In 1933, the Comité International des Poids et Mesures (the supervisory body of the world's metric system) followed suit.

The achievement of Gibson and Tyndall might have remained an academic one were it not for the changing needs in metrology and the advances of technology. As surprising as it might seem today, until 1948 there was no universal standard for the brightness of light. The "standard candle," once made from whale oil, is a part of popular lore, but in reality, different laboratories each had their own favorite "standard." Some used gas lamps, some used liquid-fueled lamps, and following the trend towards electric lighting at the turn of the century, some (including NBS) used electric lamps. It was difficult to compare lighting devices to the standards, and the standards to each other, because different fuels and different lamp constructions would produce lights of different color. The only available instruments that could reliably report how bright a light appeared, or how lights compared to standards, were humans, and as we already know, equality-of-brightness matching was unreliable when the colors were significantly different.

Research in the 1930s, interrupted by World War II, led to international agreement in 1948 to use a platinum-point blackbody as the sole international standard. of the luminous intensity of light. When objects are hot, they give off light. By "blackbody," we mean that the object does not reflect ambient light-all the light we see from it is thermally generated, an intrinsic function of the object's temperature. The trick was to operate a blackbody at a temperature that anyone could reproduce in this case, the temperature of molten platinum as it begins to freeze while cooling. Many felt that this would provide the necessary world-wide stability and uniformity. A unit of measure of luminous intensity was then defined, now known as the candela, to relate the new blackbody standard to a typical standard candle of times past.

This development had an unintended consequence. Unlike the previous lamp and flame standards, the behavior of blackbodies are calculable from first principles, using Planck's radiation law. We had a light source that we understood in detail. The other piece of the puzzle was an understanding of how the eye

responded to this light, and this is where the work of Gibson and Tyndall fit in. Suddenly, it became feasible to design and build electrical devices to measure brightness just as a human would, or at least the ideal human modeled by the Gibson-Tyndall curve 25 years earlier. The definition of the candela in 1948 had the effect of eliminating the need to have someone actually looking through a visual comparator, a process today called "visual photometry." It began an era of "physical photometry" in which luminous intensity could be evaluated through more objective measurements, yielding better precision and accuracy.

As time went on, the platinum-point standard fell into disfavor. The devices were difficult to maintain, their temperature was much lower than that of common electric lamps, and the melting-point temperature itself was too uncertain. This limited how well their emission spectra could be calculated. Finally, in 1974, Bill Blevin from the National Measurement Laboratory in Australia and Bruce Steiner at NBS published the seminal paper that said enough is enough.' They formally proposed that the SI base unit for photometry, the candela, be redefined so as to provide an exact numerical relationship between it and the SI unit of power, the watt [6]. They stated the case so well that, in 1979, the world metrology community effected a redefinition of the candela.

The 1979 redefinition puts even more reliance upon the work of Gibson and Tyndall. It says, "the candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian." That specific frequency corresponds to a wavelength of 555 nm, which is where the peak of the Gibson-Tyndall curve lies. In order to determine the luminous intensity of light at other wavelengths, one uses the Gibson-Tyndall curve (more precisely, its modern, smoothed form, denoted as V(A)) to find the corresponding number of watts. The era of visual photometry is truly over. Today, essentially all photometry is physical photometry, relying upon this definition and the V(A) curve to characterize any real light source or the performance of any tangible light detector.

This success of the Bureau in the early 1920s led to another important success towards the end of the decade. Having solved the problem of modeling brightness, Bureau staff next turned their attention to modeling color. This was a field that remained active well into the 1960s, but in those early days, a young staff member named Deanne Judd made his mark through another compelling analysis of existing data, resulting again in establishing the principles and methods that led to international consensus [7]. In 1931, the CIE adopted the