High Key is a photographic technique where you create bright, very low contrast images. Usually, the background is white, though any color can be used.
Originally, High Key lighting techniques were developed for early film and television, which could not handle high contrast scenes. Nowadays, High Key is mostly used to create an upbeat or dreamy mood.
For the image above, I used a light box. I lit the back screen with spotlights from behind. The mask was lit by two flashes from the front, as in the lighting diagram.
|The original photo, before post processing|
To begin with, I cropped the image to remove the space between the chin and the edge of the photo. I then used a Repair brush to remove the eye, nostril, and mouth holes in the white mask.
Then, I reduced contrast, and increased Brightness and Exposure.
Throughout the process, I kept a careful look on a histogram showing me the brightness levels in the picture. My goal was to shift low and mid level parts of the curve to the right, without having the rightmost parts pushed off the right edge. That way, I could create a low contrast image, without overexposing parts of it.