Disclosed is a detector for monitoring a UV false positive from genuine X-ray positive of color developing radiation sensitive devices. A layer which can be scratched off and is opaque to undesired radiation but transparent to X-ray is printed on the sensor. If the sensor displays a signal (i.e., develops color), whether it is genuine or false positive can be confirmed by scratching off the opaque layer. If the signal is a genuine positive, the whole sensor will be uniformly exposed/colored including the area under the opaque layer. If it is a false positive, the area under the scratched off layer will be of lighter color than the rest of the sensor.