A method has been proposed for acquiring an omnidirectional image of a scene using a specular sphere (namely a mirror ball). The method takes the reflected image on the specular sphere located at a desired position in a scene by an ordinary camera. This paper discusses the relation between the reflected image of the scene and the radiance of the scene. Because of the reflection on the sphere, the final image on the camera has geometric distortion. There are imaging systems covering wide angle by a lens system it-self, e.g., a fish-eye lens. The image obtained by such lens systems has similar geometrical distortion. The effect of the distortion on the image brightness is different however. We point out that the image distortion due to the lens system affects the image brightness, while that due to the reflection on the specular sphere does not. Most of the discussion can be applied to the methods using a curved mirror to obtain a wide-angle image.