Daylight Illumination on Horizontal, Vertical and Sloping Surfaces

The report summarizes sky-brightness and daylight-illumination measurements made during the year ending April 6, 1922. For ten months the measurements were made in a suburb of Washington that is comparatively free from city smoke. During the other two months, one in summer and one in winter, the measurements were made in the smoky atmosphere of the city of Chicago.

The measurements were made as nearly as possible with the sun at altitudes above the horizon of 0 deg, 20 deg, 40 deg, 60 deg, and 70 deg. From the sky-brightness measurements the resulting illumination on vertical surfaces differently oriented with respect to the sun, and on surfaces sloping at different angles and in different directions, has been computed. These computed values have been utilized, in connection with daylight-illumination measurements, to construct charts showing for latitude 42 degrees north, illumination intensities for each hour of each day of the year as follows:
(1) On a vertical and on a horizontal surface, from a cloudy day.
(2) On a horizontal surface and on vertical surfaces facing the eight principal points of the compass, from a clear sky.
(3) On a horizontal surface and on vertical surfaces facing the eight principal points of the compass, from the sun and clear sky combined.

The illumination on sloping surfaces from skylight and from solar and skylight combined has been summarized in tables.