The collision between a single-sphere and a double-sphere is measured in this study by a high speed camera to verify the fundamental issue of tangential and normal restitution coefficients for a particular shape of double-sphere. In this experiment, a single-sphere moves and collides on a stationary double-sphere, and a high speed camera is used to capture the collision of them. Numerous tests were performed by varying the pre-collisional inclinations and pre-collisional velocities of the single-sphere. Two different definitions of the kinematic coefficient of restitutions, both normal and tangential, have been scrutinized by the experimental data to see which one is realistically true or reasonable. After comparison, the contact-point (surface) based definition is the right one which has almost case-independent value for the restitution coefficient, either in the normal or tangential direction. This verifies a key point which would be very useful for modeling the rigid body collisions, either spherical or non-spherical. (c) 2020 Elsevier B.V. All rights reserved.