There are some applications where light needs to be focussed into a point or along a line. The main uses for this technique are to either concentrate the light into a smaller area, thus increasing its intensity such as would be the case for high-speed applications, or to illuminate only a specific part of an object.
A common application for this technique would be a line scan web inspection, where a thin, highly focused beam of light is required across the whole width of the web. The illumination must be able to provide high intensity, even illumination for the line scan camera.
The normal method would involve using an array of high intensity LEDs to provide very uniform illumination along the line. A focussing/rod lens is then used in front of the LEDs to focus the light to a fine line.