Though the candela is one of the seven SI base units, it receives little attention from physics teachers. This paper will discuss the history of the candela, its measurement techniques (photometry), and its relation to the lumen. The luminous properties of incandescent and fluorescent lamps are compared. Of the SI base units, only the candela is linked to the peculiarities of human perception.
REFERENCES
1.
An exception is the half-page treatment by D.C. Giancoli, Physics for Scientists and Engineers, 3rd ed. (Prentice Hall, 2000), p. 882, though it quotes the pre-1979 definition.
2.
See http://www.sizes.com/units. This website is a compendium of unit definitions, including those of obsolete units.
3.
Other early photometer designs are shown in
T. B.
Greenslade
Jr., “19th century textbook illustrations XII: Two photometers
,” Phys. Teach.
15
, 44
–45
(Jan. 1977
).4.
The history of photometry is thoroughly and engagingly documented in S.F. Johnston, A History of Light and Colour Measurement (Institute of Physics Publishing, Bristol, 2001).
5.
B.G. Aldridge, R.L. Barnett Jr., and G.S. Waldman, The Incandescent Lamp: A Module on Thermodynamics, Current Electricity, and Photometry (AAPT, College Park, MD, 1985).
6.
N.C. Harris, Experiments in Applied Physics, 2nd ed. (McGraw-Hill, New York, 1972).
7.
This curve is formally called the spectral luminous efficiency function for photopic vision. It describes the spectral response of eyes that are adapted to moderate-to-bright light with a two-degree field of view. A slightly different curve, the V′(λ) function, describes dark-adapted eyes (scotopic vision).
8.
Tabulated data of the V(λ) curve may be downloaded from http://cvrl.ioo.ucl.ac.uk/index.htm.
9.
H. E.
Ives
and E. F.
Kingsbury
, “Physical photometry with a thermopile artificial eye
,” Phys. Rev.
6
, 319
(1915
).10.
Typical photometers these days use a silicon photodiode (which does not have a flat spectral responsivity) combined with a correction filter. The filter is specially designed so that the total spectral responsivity (of the photo diode and the filter) is matched to the V(λ) function.
11.
Y. Ohno, “NIST Measurement Services: Photometric Calibrations,” NIST Special Publication 250-37 (U.S. Department of Commerce, 1997).
12.
A.C. Parr, “A National Measurement System for Radiometry, Photometry, and Pyrometry Based Upon Absolute Detectors,” NIST Technical Note 1421 (U.S. Department of Commerce, 1996).
13.
Paraphrased from The Penguin Dictionary of Physics, 2nd ed., edited by V. Ellington (Penguin Books, London, 1991), p. 51.
14.
The CIE distinguishes two meanings of luminous efficacy: lumens per watt of optical power (termed “luminous efficacy of radiation”) and lumens per watt of input electrical power (termed “luminous efficacy of source”). The figure quoted here is the former. The value quoted for a fluorescent lamp is the latter, meaning that the inefficiency of the lamp's electrical to optical power conversion is included.
15.
D.
MacIsaac
, G.
Kanner
, and G.
Anderson
, “Basic physics of the incandescent lamp (lightbulb
),” Phys. Teach.
37
, 520
–525
(Dec. 1999
).16.
B.
Denardo
, “Temperature of a lightbulb filament
,” Phys. Teach.
40
, 101
–105
(Feb. 2002
).17.
H. S.
Leff
, “Illuminating physics with light bulbs
,” Phys. Teach.
28
, 30
–35
(Jan. 1990
).18.
There is a veritable menagerie of photometric variables. Reference 11 is useful in sorting them out. A lucid chapter on photometry is found in F.W. Sears, Optics, 3rd ed. (Addison-Wesley, Reading, MA, 1949). Also useful is A. Ryer, Light Measurement Handbook (International Light, Newburyport, MA, 1997), available online at http://www.intl-light.com/handbook/index.html). Another insightful online resource is C.P. Halsted, “Brightness, luminance, and confusion,” http://www.crompton.com/wa3dsp/light/lumin.html (1993).
19.
Y. Ohno, private communication (2002).
This content is only available via PDF.
© 2003 American Association of Physics Teachers.
2003
American Association of Physics Teachers
AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.