Disclosed is a hand held calibration device for use with a sprayer utilising optical sensors to control the distribution of a substance to plants via one or more controllable spraying outlets. The calibration device includes an optical sensor for sensing optical characteristics of plants in an area of interest and producing optical sensor data, and a processor which utilises the optical sensor data to develop one or more calibration parameters for controlling whether each optical sensor of the sprayer controls each controllable spraying outlet to spray or not spray. A transmitter communicates the one or more calibration parameters to a sprayer, wherein each calibration parameter is an average value obtained from a plurality of optical sensor data samples.
