A peak hold circuit includes an input transistor, which is provided with an input signal, and a first hold capacitor, which holds a maximum or minimum value of the input signal. The or each nozzle extends circumferentially around the tool so as to provide a continuous stationary jet of fluid. In one embodiment, the particulate foamed elastomer may comprise polyurethane foam. The method further includes encoding each of the STF sequences with a pseudo-random spreading code. The output face of the light guide is contained between two closed lines which define a continuous internal contour and a continuous external contour, and the input face is contained between two further closed lines, also delimiting continuous internal and external contours.