The following excerpts are from the 1911 and 1925 editions of A Text-Book of Physics by Louis Bevier Spinney, Professor of Physics and Illuminating Engineering at Iowa State College (now Iowa State University) in Ames, Iowa.
From the 1911 edition…
ILLUMINATION
516. The intensity of illumination of any surface is defined as the ratio of the light received by the surface to the area of the surface upon which the light falls. A unit of intensity which is oftentimes employed is known as the foot candle, and is defined as the intensity of illumination which would be present upon a screen placed at a distance of one foot from a standard candle. The meter candle is a unit of intensity which is employed to some extent.
The table below gives a number of values of illumination such as are commonly observed, the intensity of illumination being expressed in foot candles.
Suitable for drafting table . . . . . 5 to 10
Suitable for library table . . . . . . 3 to 4
Suitable for reading table . . . . . . 1 to 2
Required for street lighting . . . . . 0.05 to 0.60
Moonlight (full moon) . . . . . . . 0.025 to 0.03
And from the 1925 edition…
ILLUMINATION
532. The eye has a remarkable power of adaptation. In strong light the pupil contracts and in weak light expands, so that we are able to use our eyes throughout a range of illumination which is really quite astonishing. However, the continued use of the eyes under conditions of unfavorable illumination causes discomfort, fatigue, and even permanent injury. Experiment and experience show that eye comfort, efficiency, and health considerations demand for each kind of eye work a certain minimum illumination. Some of these illumination values taken from tables recently compiled are given below.
FOOT-CANDLES
Streets . . . . . . . . . . . . . . 1/20 to 1/4
Living rooms; Halls and passageways . . 1 to 2
Auditoriums; Stairways and exits;
Machine shops, rough work . . . . . . 2 to 5
Classrooms; Laboratories; Offices;
Libraries; Machine shops, close work . . 5 to 10
Engraving; Fine repairing work; Drafting;
Sewing and weaving, dark goods . . . . 10 to 20
By comparing the 1911 and 1925 data with the illumination levels recommended today by IESNA, we can see that recommended light levels for streetlighting have increased anywhere from 40% to 380% since 1925. A cynic might say that we need more light than our ancestors did to see well at night. As you may have noticed, light levels have been steadily creeping upward, everywhere, over the last few decades.
Recommended Illumination Levels for Streetlighting
Year Minimum Average Maximum
1911 0.05 ??? 0.60
1925 0.05 0.25 ???
1996 0.07 1.20 ???
Have you ever noticed how well you can see at night when the full moon is lighting the ground? The full moon provides surprisingly adequate non-glaring and uniform illumination at just 0.03 footcandles! For inspiration, take a look at the following text from an Ames, Iowa city ordinance, dated July 8, 1895:
“The said grantees shall keep said lamps in good condition and repair, and have the same lighted every night in the year from dark until midnight, and from 5:00 a.m. until daylight, except such moonlight nights or fractions of the same as are not obscured by clouds, and as afford sufficient natural light to light the streets of said city.”
This was originally published as IDA Information Sheet 114 in November 1996, and authored by David Oesper.
