Jet engine power loss due to ice particle accumulation is a recognized aviation hazard. High-altitude cirrus clouds can have ice particle concentrations high enough to be dangerous; therefore, pilots need to be informed when aircraft enter such environments. One approach to determining ice particle concentration is an onboard lidar system. A lidar does not directly measure ice particle concentrations but uses the measured backscatter coefficient. Concurrent lidar measurements are compared to backscatter coefficients derived from the particle size distribution. Particle measurements are obtained from wing-mounted, in-situ probes during four sixty-second flight segments at different temperatures (+7°C, +4°C, -33°C, -46°C). The backscatter coefficients derived from external cloud probes (ECP) are correlated (R2 of 0.9) with measurements by an airborne lidar system known as the Optical Ice Detector (OID). Differences between the backscatter coefficients range from less than one, to more than three standard deviations of the combined OID and ECP uncertainties. The OID and ECP backscatter coefficients are primarily in agreement for three of the four cases, with disagreement for the -33°C case. The ECP derived backscatter coefficients are lower than the OID for three of the cases, with +7°C being the exception. Measurements over four research flights indicate that the total water content is correlated (R2 of 0.74) with the OID backscatter coefficient. The strong correlation indicates that the OID is a useful instrument for determining ice particle concentrations over a broad range of environments, including at ice water contents as low as 0.02 g per m3.