When choose the analytical lines of photoelectric spectral analysis, it must meet the following requirements：
1 when do spectrum analysis, it generally using the internal standard method. Due to the internal standard method analysis is often use with many analytical line and a marking composition, commonly the matrix elements used in the samples internal standard element.
The line formed of was required to be symmetry, when the excitation light source has some fluctuation, the two lines on the spectral line intensity has something changed, but the strength ratio and relative intensity can remain unchanged。
For example R： Intensity ratio then R=I1/I0
I1： The intensities of analysis line,
Io： The strength of inner laber line,
It showed that I1 and Io changed at the same time, and the R is not affected. R and content C have the linear relation.
When do the photoelectric direct reading spectrum analysis, there are a lot of channels for analysis, it is difficult to install many internal standard channels, so it needs using an inner laber line. But some people believe that in order to improve the accuracy of photoelectric spectral analysis，it need use the different line, which remains to be solved by the miniaturization of photoelectric conversion elements.
In the Photoelectric method, sometimes it using a line to control the amount of exposure, called automatic exposure, when the sample is in the exposure, the analysis line and internal standard charging to the respective integral capacitor separately, when the line integral charging the capacitor reaches a predetermined voltage, the automatic exposure stopped.
The analysis of line integral charging the capacitor to achieve voltage represents the intensity analysis of line I, and which is represent the intensity of analysis line ratio R (because R=I1/Io, while the Io is maintained constant) the strength I or intensity ratio R was expressed by the photometric numerical readings .
Now generally the use of timed exposure method is more common