In an example implementation, a sintering system includes optical fiber installed into a sintering furnace. A support structure inside the furnace is to support a token green object in a predetermined position and to hold a distal end of the fiber adjacent to the predetermined position. A light source is operably engaged at a proximal end of the fiber to transmit light through the fiber into the furnace. A light detector is operably engaged at the proximal end of the fiber to receive reflected light through the fiber that scatters off a surface of the token green object.