This can be found in the Image options and effectively combines two channels into a new Alpha channel.
Once inside the Calculations dialog box there are some familiar settings you should activate. You’ll discover there are two sources. In nearly all cases, set the first Source 1 Channel to Red and the Source 2 Channel to Blue. Inverting the Blue channel will clearly define the hair from the backdrop, which is ultimately what you want.
However, applying either Add or Subtract blending modes complements this effect. In our case, Add worked best. In some cases you’ll discover all the hair value, or most of it, has been selected. But it’s not always perfect, so here tweak your Offset value – we’ve set ours to -200 in this image.
Clicking OK will create your Alpha channel and you can now perfect hair selection by applying an Overlay-set soft white brush to bring out more highlights and an Overlay-set soft black brush to fill in the black background. Of course, further Pen Path selection and brushwork will fill in the rest of your model.