  Solar irradiance variations over solar rotational time-scales are largely
determined by the passage of magnetic structures across the visible solar disk.
Variations on solar cycle time scales are thought to be similarly due to
changes in surface magnetism with activity. Understanding the contribution of
magnetic structures to total solar irradiance and solar spectral irradiance
requires assessing their contributions as a function of disk position. Since
only relative photometry is possible from the ground, the contrasts of image
pixels are measured with respect to a center-to-limb intensity profile. Using
nine years of full-disk red and blue continuum images from the Precision Solar
Photometric Telescope at the Mauna Loa Solar Observatory (PSPT/MLSO), we
examine the sensitivity of continuum contrast measurements to the
center-to-limb profile definition. Profiles which differ only by the amount of
magnetic activity allowed in the pixels used to determine them yield oppositely
signed solar cycle length continuum contrast trends; either agreeing with the
result of Preminger et al. (2011) showing negative correlation with solar cycle
or disagreeing and showing positive correlation with solar cycle. Changes in
the center-to-limb profile shape over the solar cycle are responsible for the
contradictory contrast results, and we demonstrate that the lowest contrast
structures, internetwork and network, are most sensitive to these. Thus the
strengths of the full-disk, internetwork, and network photometric trends depend
critically on the magnetic flux density used in the quiet-sun definition. We
conclude that the contributions of low contrast magnetic structures to
variations in the solar continuum output, particularly to long-term variations,
are difficult, if not impossible, to determine without the use of radiometric
imaging.
