A protected structure test fixture comprising: a protected structure including a powder based chemical fire suppressant system configured to supply a dry powder agent to a protected volume; and a measurement system for measurement of the dry powder agent, the measurement system comprising a sensor system including at least one sensor head at least partially within said protected volume, and a control system in communication with said at least one sensor, wherein each sensor head is configured for measurement of light transmission through the dry powder agent, and comprises: a housing defined along an axis along which light is communicated, said housing defining a multiple of apertures transverse to said axis, said multiple of apertures being in communication with a measurement volume along said axis; a light source which communicates light into said measurement volume along said axis through a fiber optic cable, said light source being in communication with said control system; a mirror within said housing to reflect said light through said measurement volume; a sensor in communication with said fiber optic cable, operable to receive reflected light within said measurement volume from said mirror; and a window located opposite said mirror, said window being in communication with said fiber optic cable. a protected structure including a powder based chemical fire suppressant system configured to supply a dry powder agent to a protected volume; and a measurement system for measurement of the dry powder agent, the measurement system comprising a sensor system including at least one sensor head at least partially within said protected volume, and a control system in communication with said at least one sensor, wherein each sensor head is configured for measurement of light transmission through the dry powder agent, and comprises: a housing defined along an axis along which light is communicated, said housing defining a multiple of apertures transverse to said axis, said multiple of apertures being in communication with a measurement volume along said axis; a light source which communicates light into said measurement volume along said axis through a fiber optic cable, said light source being in communication with said control system; a mirror within said housing to reflect said light through said measurement volume; a sensor in communication with said fiber optic cable, operable to receive reflected light within said measurement volume from said mirror; and a window located opposite said mirror, said window being in communication with said fiber optic cable. a housing defined along an axis along which light is communicated, said housing defining a multiple of apertures transverse to said axis, said multiple of apertures being in communication with a measurement volume along said axis; a light source which communicates light into said measurement volume along said axis through a fiber optic cable, said light source being in communication with said control system; a mirror within said housing to reflect said light through said measurement volume; a sensor in communication with said fiber optic cable, operable to receive reflected light within said measurement volume from said mirror; and a window located opposite said mirror, said window being in communication with said fiber optic cable.