As shown in the figure above, this is a single-layer filter glass with a coating on only one side. Let’s assume that the top surface (green in the diagram) is the coated side. Now, observe the red line in the diagram from the perspective shown above. If you cannot see the red line, then the top surface is the coated side. If you can see the red line, you can confirm that the top surface is not the coated side. In that case, flip the bottom surface to the top and repeat the observation method to identify the coated side.
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Distinguishing the coated side helps us position the filter correctly during use. Different orientations can produce varying results in specific environments.
For certain types of lenses, such as mirrors, the coated side facing outward results in external reflection, while the coated side facing inward results in internal reflection. In these cases, distinguishing the coated side is particularly important.