  The RGB Bayer filter system consists of a mosaic of R, G, and B filters on
the grid of the photo sensors which typical commercial DSLR (Digital Single
Lens Reflex) cameras and CCD cameras are equipped with. Lot of unique
astronomical data obtained using an RGB Bayer filter system are available,
including transient objects, e.g. supernovae, variable stars, and solar system
bodies. The utilization of such data in scientific research requires that
reliable photometric transformation methods are available between the systems.
In this work, we develop a series of equations to convert the observed
magnitudes in the RGB Bayer filter system ($R_B$, $G_B$, and $B_B$) into the
Johnson-Cousins BVR filter system ($B_J$, $V_J$, and $R_C$). The new
transformation equations derive the calculated magnitudes in the
Johnson-Cousins filters ($B_{Jcal}$, $V_{Jcal}$, and $R_{Ccal}$) as functions
of RGB magnitudes and colors. The mean differences between the transformed
magnitudes and original magnitudes, i.e. the residuals, are
$\Delta(B_J-B_{Jcal})$ = 0.064 mag, $\Delta(V_J-V_{Jcal})$ = 0.041 mag, and
$\Delta(R_C-R_{Ccal})$ = 0.039 mag. The calculated Johnson-Cousins magnitudes
from the transformation equations show a good linear correlation with the
observed Johnson-Cousins magnitudes.
