Cloud Droplet Probe (CDP-2)

The CDP is a forward-scattering optical spectrometer. For accurate sizing, the CDP accepts and sizes only particles that pass through a region of the laser beam with uniform power. This region of the laser is called the depth of field. As particles pass through the laser beam, light scatters in all directions. The CDP collects forward-scattered photons within an annular cone that is 4° to 12° from the laser beam. The collected light is then directed onto a 50/50 optical beam splitter and finally to a pair of photo detectors, referred to as the sizer and the qualifier. (See figure below) There is a mask in front of the qualifier detector to define the depth of field. The edge of the depth of field is defined by the points where half of the light scattered from a particle is blocked by the mask. The photo detectors then convert the photon pulses into electrical pulses. The pulse from the qualifier is multiplied by two, and if the resulting signal exceeds the pulse from the sizer, the particle is deemed within the depth of field. The particle is then sized based on the amplitude of the sizer pulse.