In this example we use an image of 1px wide, with the color for grazing angles at the bottom, and the color for normal angles at the top.
- UV transform:
- R.Z = 180°.
- (S.X, S.Y) = (0.5, 0.5)
- (T.X, T.Y) = (0, -0.25)
- Use spherical projection and set coordinate space to "Normal space"
- Set border mode to "Clamp value"
Now the image is mapped in the same way as the ramp of the falloff texture node.
You can change the rotation according to the orientation of the image. If your image contains the ramp along the X axis, use 90° or -90°.