A method for generating and localizing a signal in a solid phase substrate detection system comprises applying a solution of target material to a substrate; binding the target with a specific affinity molecule having an attached label, the label comprising multiple signal precursor molecules; applying a carrier to the substrate, and treating the label to convert the signal precursor molecules to signal generating molecules. The carrier comprises solvent for the label and thickener for localizing the signal. The carrier may include developer that converts signal precursor molecules to signal generating molecules. Developer is not necessary if the signal precursor molecules are converted to signal generating molecules by e.g. temperature change, pH change, sonication, light irradiation, microwave heating. A test device for detecting target in a fluid sample, and a kit of parts for determining the presence of target in a fluid sample are also disclosed.