Look at the following example (Fig.19). This is the diffuse texture.
And here is the specular map (Fig.20).
Notice that the ones on the left are respecting the true nature of the materials, with the wood (dielectric) using the inverse color of the diffuse, and the metal (conductor) with same colors of the diffuse. It has some scratches and a little more contrast, but it's still very similar to the diffuse in terms of hue.
The ones on the right are not good, they are just wrong. Wood specular matching the wood diffuse, metal specular with inverse color.
Now, if you put them on layers like in Fig.21, with the diffuse on top of all the layers of the specular file, and set to Linear Dodge, then the result you get is this (Fig.22).
This result will never look very good because it's like having the specular reflection on top of everything, as if there were several different light sources illuminating the surface from all possible angles. But you can already feel that the reflections on the left look more natural than the ones on the right.
Now, let's make this a little more interesting. What about having a preview like this one here, in Photoshop (Fig.23)?
To do that is actually quite simple. Below the diffuse layer, in Photoshop, create a group and make it Multiply. In this group add two new layers. The bottom one you will fill with black. On the top one you will create some white spots, faking the shape of the specular reflection that you would expect to see on the surface (Fig.24).
Now, you just need to drag the spots around to see how the specular reflection works for this material (see Fig.23).
Pretty cool, right? Of course, I don't keep these layers visible all the time while working on the specular texture. Sometimes I just don't use them at all. But every time I want to be sure I'm heading in the right direction I'll toggle them on and off, and move the small white spot around, to see if I'm still on track.
You may be curious about skin reflection. After all, a good skin reflection will be hued as a dielectric material (inverse of the diffuse color), but our skin does conduct electricity. Yeah, this is true. But our skin is a far more complex material, with several different layers and elements - epidermis, fat, oils, muscle, tinny hairs, sweat, salt, etc. Maybe the mix of these different elements results in a dielectric look, or maybe because oils are not conductive of electricity, and they are on top of all layers of the skin, the final appearance is dielectric... I don't know really (hey, I said this is not a science study!) But it's true, if you are trying to make a specular map for skin, you should start with the inverse hue of the diffuse.
That's it - I hope you found this tutorial useful! There are some other things you could do in order to make this preview of the specular reflection even more interesting, but I'll leave it for a next tutorial.