It is known that time-to-contact toward objects can be estimated just from changes in the object size in camera images, and we do not need any additional information, such as distance toward objects, camera speed and camera parameters. However, the existing methods cannot compute time-to-contact, if there are no geometric features in the images. In this paper, we propose a new method for computing time-to-contact by using photometric information. When a light source moves in the scene, an observed intensity changes according to the motion of the light source. In this paper, we analyze the change in intensity in camera images, and show that the time-to-contact can be estimated just from the change in intensity in images. Our method does not need any additional information, such as radiance of light source, reflectance of object and orientation of object surface. The proposed method can be used in various applications, such as vehicle driver assistance.