In this tutorial you will learn how to make the scanline effect you might have seen on some images. This is only one way of doing it, in fact you could vary a few details in this tutorial till you arrive at your desired result.
Step 1: Create a new document with the following settings:
Step 2: You'll probably notice that the document you just created is a too small to work with, to rectify this, zoom in, I went to 800%.
Using the PENCIL tool with size 1px fill the top half of the image with black (#000000), use white for darker backgrounds.
Step 3: Go to EDIT->DEFINE PATTERN
You will see a window appear where you have to give your pattern a name. I called it scanlines, this makes it easier to find in photoshops library where it is saved.
Step 4: Click OK and open the image you want to add the scan lines to, I choose this Bugatti EB110 image. Or just make a selection if you don't want to apply the scan lines to the entire image. Create a new blank layer above the image and make sure that it remains selected (the layer should have a dark blue background).
Step 5: Now go to EDIT->FILL and select your pattern you just created, if you wish you can set the opacity here, as you can see I set it to 25%. I did it here as I already know how it looks at 100%. Click OK and there you go. If you didn't set the opacity in the FILL dialogue then you might find the scanlines too prominent so reduce the opacity and/or fill (in the layers palette) until your are satisfied with the result (go under 50 % for best results). The scanlines should be visible, but not overwhelming.
Here's how mine turned out.