Measured specular layers are better fresnel specular layers. Compared to the latter, measured specular layers are configured by special files containing reflection and refraction information depending on wavelength. You can find these files here or here. Owlet also reads .ior-files from other 3D visualisation applications.
Here’s what you see when you switch specular layer IOR parameter to “Measured Data”:
The next IOR parameter is set to Measured Data to tell Owlet that you have a special “IOR file” for that material.
The preview is black as you need to configure the layer by loading the file with material parameters.
That’s the main field of the layer, here you can load IOR-files that contain wavelength-dependent information about the refraction of material (N and K), laboratory measured. Owlet comes with some sample files in its materials library.
Here are two balls:
The left one is made by fresnel “metallic” layer as explained here, while the right one uses the measured ior-file for silver that comes with Owlet. Both balls look silver, but they are different. Sometimes (mostly in jewellery), the difference is essential. It takes more time to render materials with measured data layers, but sometimes it is worth waiting.
Note that you can’t define reflection color(s), transmission, N or K and so on. Everything is defined by the IOR-file with measurements, so you don’t need to worry about that. If you still need modifications, consider switching to scalar/complex fresnel layer type and configuring all the effects manually.
These fields work exactly the same as in fresnel specular layers, so you can read about them here.