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REAL ESTATE
AND
NEW YORK, DECEMBER 16, 1916
DAYLIGHT ILLUMINATION OF BUILDINGS
Statement by Herbert S. Swan and George W. Tuttie
of the Staff of the Committee on the City Plan
DAYLIGHT ILLU.VIINATIO.N ON STREET
of the United States all the way from 36
to 84 per cent. In New York it is 58
per cent.
.^ssinning that Prof. Basquin's data
for Chicago is true for New York after
allowing for the difference in per cent
of annual sunshine, the mean annual
l>rightness of the zenith sky in New
\ ork is 560 candles per square foot and
the corresponding working 'minimum
280 candles per square foot. The il¬
lumination received at an unobstruced
point on the surface of the- earth is
880 foot candles. The illuinination on
the side of a building at the ground
level where half of the sky is obstructed
is 440 foot candles.
Increasing tlie height of buildings re¬
duces the amount of daylight on both
the street level and tlie street facade.
.\ street improved with buildings once
the street width in height receives only
71 per cent, as niuch illumination from
I N the lighting of buildings, daylight
* is more important than sunshine.
Daylight is the sunlight diffused and re¬
flected by the sky and clouds as dis¬
tinct from that received directly from
the sun. Sunshine is obtained from but
one point, viz., the sun. Daylight, on
the other hand, is obtained from the
whole visible sphere of the sky.
Professor O. H. Basquin of the
.\merican Luxfer Prism Company in
1907 made a series of observations with
a flicker photometer recording the in¬
tensity of the zenith skylight at Chicago
under varying conditions. These meas¬
urements, which were taken daily at 9
a. m., 12:30 and 4:30 p. nj., covering a
two-year period, showed the mean an¬
nual brightness of the zenith sky at
Chicago to be SOO foot candles per
square foot. The intensity taken as a
working minimum was one-half of the
mean annual brightness, or 250 candles
per square foot. This amount was gen¬
erally available as a daily average e-xcept
on one or two days a month—on days
with either a clear blue sky or a stormy
sky, both of these conditions giving a
minimum illumination. The sky at
various altitudes gave substantially the
same illumination as at the zenith.
Tlie curve showing the brightness of
the sky follows quite closely the mean
daily and yearly sunshine curve. The
per cent, of annual sunshine to the max¬
imum possible varies in different cities
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SURFACE AND 0.\ BUILDl.N'G FACADE.
the sky at the curb level as one im¬
proved with buildings half the street
width in height. For a street improved
with buildings twice the street width in
height the direct illumination from
the sky is onlv 42 per cent.; thrice the
street width 29 per cent.; four times 22
per cent.; five times 18 per cent.; and
si.x times 15 per cent.
Illumination on Street Facade.
Tlie illumination on the street facade
diminishes even more rapidly with an
increased height of buildings than that
at the street surface. This fact is
lirought out with considerable emphasis
liy the curves in Figure 1. A point on
the facade once the street width below
the top of the opposite buildings re¬
ceives only 53 per cent, as much direct
daylight illumination as one situated
half that distance down from the top.
-At a point twice the street width below
the top of the, opposite buildings across
the street, the illumination is only 19.2
per cent, of that one-half tiines the
street width down from the top; three
times the street width 9.4 per cent.; four
times 5.5 per cent.; five times 3.5 per
cent., and six times 2.5 per cent.
The illumination in foot candles in
the street and on the street facade due
to direct sky light is shown in the fol¬
lowing table for different points below
the top of the buildings, assuming the
street in eacli case to be uniformly im¬
proved on both' sides with buildings of
different heights.
Height
of build----------Foot Candles--------\
ings times On street On street
street width. level. facade.
0.......... 880. 440.
%.......... 486.8 243.3
1.......... 345.2 128.9
l'/4........ 296.4 96.5
iy2........ 258.1 73.9
2.......... 204.0 46.6
2^........ 167.5 31.6
3.......... 141.6 22.9
4.......... 107.7 13.2
5.......... 86.6 8.6
6.......... 72.5 6.2
Illumination in Inner Courts.
The minimum size prescribed by the
Tenement House Law for an inner court
on the lot line in a tenement 60 feet
high is 12 feet by 24 feet. The law con¬
templates that this court should be com¬
plemented by a court of similar size on
the adjacent lot, but it does not enforce
this provision. Let us assume that the
adjoining building when erected does
not have its courts co-ordinated with