  Future food security is a major concern of the 21st century with the growing
global population and climate changes. In addressing these challenges,
protected cropping ensures food production year-round and increases crop
production per land area by controlling environment conditions. Maintaining the
growth and health of crops in these facilities is essential to ensure optimum
food production. However, this is a laborious work and is currently done
manually. Image-based non-destructive plant phenotyping is an emerging research
area that reduces the skilled labour cost while enhancing the monitoring of
crop growth, health, and identifying phenotype-genotype relations for plant
breeding. With the proliferations of protected infrastructures and targeted
plants, different technologies and sensor setups are needed for image-based
crop monitoring. Conveyor-type plant-to-sensor systems, bench-top or
gantry-based systems are commonly found in research facilities focussing on
phenotyping of small, relatively short, or movable model plants. This review
examines the literature on crop monitoring and phenotyping platforms in both
field and protected facilities and explains different camera technologies and
their ability to extract different plant traits. The review highlights the
future research directions of image-based monitoring of commercial scale
protected crops where crops can be relatively tall or vertically supported
under semi controlled environments, which presents new challenges and is rarely
covered in the literature.
