  In 2016, an exposure meter was installed on the Lijiang Fiber-fed
High-Resolution Spectrograph to monitor the coupling of starlight to the
science fiber during observations. Based on it, we investigated a method to
estimate the exposure flux of the CCD in real time by using the counts of the
photomultiplier tubes (PMT) of the exposure meter, and developed a piece of
software to optimize the control of the exposure time. First, by using
flat-field lamp observations, we determined that there is a linear and
proportional relationship between the total counts of the PMT and the exposure
flux of the CCD. Second, using historical observations of different spectral
types, the corresponding relational conversion factors were determined and
obtained separately. Third, the method was validated using actual observation
data, which showed that all values of the coefficient of determination were
greater than 0.92. Finally, software was developed to display the counts of the
PMT and the estimated exposure flux of the CCD in real-time during the
observation, providing a visual reference for optimizing the exposure time
control.
