Ivo Guimaraes is a professional photographer and he is sharing a great lighting tip over at Digital Photography School.
All you need is a smartphone or a tablet, a camera on a tripod and a long exposure to create an image with perfect lighting. Just use the phone or the tablet to light paint the subject(s) in your image.
This is a typical light painting image, where the camera is placed on a tripod and the image shot at ISO 50, f/5.6 and a 4 second exposure time. I used the lowest ISO so I could make the image as clean and noiseless as possible. F/5.6 was the chosen aperture to give me the depth of field effect I was looking for, and after a couple of test shots, I realized that 4 seconds was the correct exposure time for the light I was using. It also gave me a comfortable time-frame in which I could move the light around and create the desired effect. An infrared remote shutter release was used to avoid touching the camera and keep it as steady as possible.
Light painting is a technique I use a lot in my work. I used to do it mostly with small lanterns and led light panels, but I realized that a tablet screen light and a smartphone camera led light are fantastic tools to use as light sources in this type of images. There are a few apps that can help to control what shows up on the screen, and transform your device into a mean light-painting machine, but you don’t really need them.
The light from these screens is usually soft; you can make it even softer with some tracing paper in front of the screen and you can control the spread of light with some black card flags. This particular image could have been done with only one exposure, but I decided to take it a bit further with a composition of three images with three different types of light.
Source: Digital Photography School