Start Photoshop, whichever version you want, 7.0, 8.0, 9.0 will do...
After creating a new image, press "D" on the keyboard to restore the foreground and background colors to the default black + white.
Then select the gradient tool (shortcut key "G") and drag it on the image. Don't drag it too long, just drag it a short distance.
As shown in the picture below, the gradient gray part in the middle is the range of lightning.
Select "Filter" - "Rendering" - "Layered Clouds", and the result will be as shown below. (Press Ctrl+I to invert the colors to look more comfortable, but it can also be omitted)
Press Ctrl+L to open the Levels panel and adjust the second value of the RGB channel to 0.10. The values on both sides are arbitrary. The effect is as shown below.
