A fundamental relationship of inherent optical properties (IOP) is tha
t the beam attenuation coefficient is the sum of the volume absorption
and scattering coefficients (c = a + b). A relative calibration of a
set of instruments can be provided using this IOP closure equation. Me
asurement of the true beam attenuation coefficient c is not practical
as all attenuation instrumentation has some finite acceptance angle in
which scattered light is collected. We provide a theoretical framewor
k for measuring the attenuation and scattering coefficients in a consi
stent manner. Using this framework, we provide a practical version of
the IOP closure equation. We apply the practical IOP closure equation
to measurements made at Lake Pend Oreille, Idaho, in the spring of 199
2. Results of this IOP closure indicate that the practical closure equ
ation is a useful approach. Closure was achieved during some measureme
nt sets but not at others. The intermittent lack of closure may be due
to the method of determining the scattering coefficient from the gene
ral angle scattering meter or that the calibration of at least one of
the instruments drifted during the time of the experiment.