  In this article, we discuss an observation phenomenon where the total amount
of photons in the full passband of the Birefringent lter is a constant number
that is considered by removing the spectrum of the light source irrespective of
the instrument transmittance. This conclusion is only noticed and considered to
be correct in Huairou Solar Observing Station since 1980s. This article will
give a further discussion to the question that had been proposed by the
previous researchers. The article structure is organized as history (Sec. 1),
experiment (Sec. 2), math (Sec. 3), and discussion (Sec. 4). This issue should
be the Paseval-Theorem manifesting itself in astronomical measurement, even
though we rigorously demonstrate that this photons conservation has its
mathematical generality in Sec. 3.
