Innovative airglow instrument based on constant photon count control

We propose an airglow instrument design that can expand the dynamic range of light intensity and have the effect of glare protection. This innovative airglow instrument employs the photon count detected by a photomultiplier tube (PMT) as feedback to control the high voltage of the PMT to compensate for any change of light intensity and to maintain the photon count fixed. When the photon count is controlled to remain fixed, the high voltage is inversely proportional to the light intensity so that the light intensity can be estimated from the high voltage. A mathematical model can be surface fitted to the experimental data, and by using this model, the light intensity can be obtained from the photon count and the high voltage. Through the light intensity and the preset photon count, the high voltage for feedforward compensation can be calculated, and then the high voltage adjusted. In this way, the feedforward PMT characteristic compensation control is completed. Additionally, the feedback includes a proportional-integral linear control to eliminate the influence of this model error and to make the measured photon count closer to the preset photon count. We conducted laboratory testing to confirm that the innovative instrument can achieve the design goals. The new instrument expands the linear dynamic range while maintaining high sensitivity.