P: 269 Here's what I'm thinking. The sun is too bright to measure directly with our equipment. If I calibrate without a filter, and capture the reference spectrum with and without the filter, then I can model how the filter changes the spectrum. This way I can capture the sun spectrum with the filter, and transform the data as if I had not used it. What I mean to say is, if $I_{r}(\lambda)=Y(\lambda)$ and $I_{r+f}(\lambda)=G(\lambda)Y(\lambda)$ then is it valid to argue that $I_{s}(\lambda)=\frac{I_{s+f}(\lambda)}{G(\lambda)}$ Or is it that $G(\lambda)$ is dependent on I? where $I_{r}$ is the irradiance of the reference $I_{r+f}$ is the irradiance of the reference measured through a filter $I_{s}$ is the irradiance of the sample $I_{s+f}$ is the irradiance of the sample measured through the same filter