The reduction in light across the tunnel exhaust caused by
smoke generated during the test is measured by a photometer
system consisting of a lamp and photo cell. The attenuation
of light by the passing smoke and particulate is used to compute
a smoke developed index. The smoke developed index for
a test specimen is determined by comparing the reduction in
light caused by the smoke from red oak flooring to the reduction
of light by smoke from a test specimen. Red oak flooring is
arbitrarily assigned a value of 100 and the cementitious board
material is assigned a smoke developed index of 0. Therefore,
a product achieving a 450 smoke developed index produces a
curve of light attenuation versus time which has 4.5 times the
area of the curve produced by red oak flooring (in comparison
with the results for the cementitious board). In simplistic terms,
one may think of a product having a 450 smoke developed index as one that produces 4.5 times the amount of smoke that red oak flooring generates during a 10 minute tunnel test.