You will need to have some kind of mask on a layer before you see any effect of the Luma Range feature. You can actually create a Luma Range on an Empty Layer, but you will not see any effect before you apply some sort of mask.
- Go to the Layers tool. Long-press the New Layer icon and select New Filled Layer. Alternatively, long-press the Mask icon and select Draw Mask (B), Draw Linear Gradient Mask (L) or Draw Radial Gradient Mask (T) and create a mask.
- Click on the Luma Range… button in the Layers tool (or select it in the menu Layer -> Luma Range…).
- Enable Display Mask to see what parts of the image the Luma Range mask will affect.
- Set your desired Luma Range by dragging the black and white Range points at the top of the graph. Tweak the Falloff points to ensure that the transition from the included areas to those that are excluded are not too harsh. A difference of 20 between the Range and Falloff for each is a good start.
- You can invert the selected Luma Range with the Invert Range option. This basically flips the selected luminosity range, so if you have included only the highlights and then enable Invert Range, you will in effect exclude the highlights and include everything else. Note that this invert command only affects the masked area, not the whole image.
- It is possible to tweak the edge coverage of the Luma Mask further with the Radius and Sensitivity sliders.
- Once you are happy with the Luma Range adjustments, click Apply to close the dialog box.