As the efficiency of light sources, and the sensitivity of photographic imaging sensors, increased, the use of a continuous ring light instead of a flash increased. A ring light has the same advantages as a ring flash, but produces continuous light at a lower intensity, so the image can be examined before taking the photograph. In the past a circular fluorescent tube was often used; LEDs are now frequently used. Brightness and colour temperature can be changed on some devices. In addition to lights that are fitted to the filter ring of a camera, inexpensive versions that clip onto a mobile phone or other device equipped with a camera (facing away from or towards the user) are available.