A laser scanner is provided with a transmission optic whose laser pencil beam is oriented coaxially with the axis of rotation of a deflecting mirror and coaxially with the optical axis of the associated detector optic. The instrument has a source of penetrating radiation for irradiating an irradiated region of the test material, a detector for detecting fluorescence emission by the test material and for generating a detector signal, and a controller for converting the detector signal into a spectrum characterizing the composition of the test material. A field-emission array device has a gate, an emitter array, a first terminal electrically connected to the gate, and a second terminal electrically connected to the emitter array. The water retentive layer may also include a hydrophilic additive to facilitate movement of water from the adhesive to the super absorbent water retentive particles. The bracket includes a first set of resilient fingers for engaging the button assembly and a second set of resilient fingers for engaging the doorbell cover.