A COMPARISON OF INTENSITIES REQUIRED UNDER 
DIRECT AND UNDER INDIRECT LIGHT- 
ING SYSTEMS 
WM. KUNERTH 
Although artificial indirect lighting dates back twenty-five years 
or more, it was not very extensively patronized until after 1906 
when the advent of the more efficient Mazda B lamp made its 
commercial use possible. At once comparisons were made between 
the direct and the indirect systems on various points. 
It was soon recognized that when we consider the efficiency of 
installation only, the direct system is by far the better, efficiency 
of installation meaning the number of lumens obtained on the 
working plane for every watt of power input. In modern resi- 
dences the installation efficiency is in the neighborhood of six 
lumens per watt, when the direct system is used ; whereas when 
the indirect system is used the efficiency of installation is only 
about three lumens per watt. In a very general way the ratio of 
efficiencies between the two systems has been found to be about 
two to one. This low efficiency for the indirect system is due to 
the increased absorption by walls, ceiling, and glassware. The 
ratio has been determined repeatedly and is given here merely to 
recall to mind that from this point of view the direct system is to 
be preferred. 
If we next consider the question of eye fatigue under the two 
systems we can do no better than to refer to the extensive series 
of experiments by Drs. Ferree and Rand 1 of Bryn Mawr College. 
They found that under conditions of natural illumination the eye 
is fatigued practically not at all, as is shown by the curve here- 
with. Under indirect lighting, however, they found that the eye 
is appreciably but not greatly fatigued after working for a period 
of three hours, and that under direct lighting the eye is excessive- 
ly fatigued after working for the same period. These curves show 
conclusively that when eye fatigue only is considered the indirect 
system is to be preferred. The greater fatigue caused by the 
direct system is no doubt due to the great difference in intensity 
in different parts of the field of vision. This causes great ac- 
tivity in the ocular muscles and hence fatigue. 
Trans. I. E. S., Vols. 8-15. 
