Illumination Levels: Then and Now

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.

Science News

Some people are molded by their admirations, others by their hostilities. – Elizabeth Bowen (1899-1973)

I have many admirations, and one of them is for a bi-weekly magazine called Science News.  My first introduction to this amazing publication was in 1973, when a friend of my recently-divorced mother, Frank Gillotti, started giving me his copies after he was finished reading them.  I was a sophomore at Hoover High School in Des Moines then, and by my senior year I was a subscriber for life.

Science News has been around a long time.  It started way back in 1922 as Science News-Letter, and remained that until 1966, when it became Science News.  Today, Science News has an international circulation of about 94,000—alarmingly, down quite a bit (like most magazines) from its peak circulation of nearly 250,000 in the late 1980s.  Unlike most magazines these days, Science News is not saturated with advertising, but is instead chock-full of well-written, accurate, and timely news and feature articles about all areas of science, technology, and mathematics.  Yes, astronomy and space science are covered thoroughly!  And, with each bi-weekly1 issue numbering 32 pages (though, occasionally 40+), it is easy to find the time to read or at least skim it cover-to-cover every two weeks.

In my early years reading Science News, one writer I particularly admired was senior editor / physics editor Dietrick E. Thomsen, whom I was so fortunate to meet at the AAS Meeting in Ames, Iowa in June 1986.  Sadly, he passed away in 1988.  One thing I remember about him besides his always-excellent articles was his passion for passenger trains, and his growing distaste for air travel at the time (and it has only gotten worse).  At that time, I had never ridden on a passenger train, but nowadays I ride Amtrak regularly, and love it!

Another fantastic writer in those days at Science News was space science editor Jonathan Eberhart (1942-2003) whose brilliant and unconventional career was sidelined by multiple sclerosis by 1991.  The AAS Division for Planetary Sciences (DPS) has awarded the Jonathan Eberhart Planetary Sciences Journalism Award annually since 2009.  J. Kelly Beatty (Sky & Telescope) was the first recipient (in 2009), and Emily Lakdawalla (The Planetary Society) won the 2011 award.

Science News maintains an excellent web site.  One feature I really like is they provide a complete list of sources and references for their magazine articles.

And, Society for Science & the Public (SSP), the nonprofit corporation that produces Science News, also produces an excellent website for readers ages 9-14, Science News for Students.

Check out these wonderful resources regularly, and while you’re at it, don’t forget to subscribe!

1Science News published weekly through April 12, 2008.  Science News began publishing bi-weekly on May 10, 2008.