text: The development of fluorometers allowed chlorophyll fluorescence analysis to become a common method in plant research. Chlorophyll fluorescence analysis has been revolutionized by the invention of the Pulse-Amplitude-Modulation (PAM) technique  and availability of the first commercial modulated chlorophyll fluorometer PAM-101 (Walz, Germany). By modulating the measuring light beam (microsecond-range pulses) and parallel detection of the excited fluorescence the relative fluorescence yield (Ft) can be determined in the presence of ambient light. Crucially, this means chlorophyll fluorescence can be measured in the field even in full sunlight.
  Today, chlorophyll fluorometers are designed for measuring many different plant mechanisms. The measuring protocols: F/F and OJIP measure the efficiency of Photosystem II samples at a common and known dark adapted state. These protocols are useful in measuring many types of plant stress. Bernard Genty's light adapted measuring protocol ΔF/F’, or Y(II), is an effective and sensitive way to measure plant samples under ambient or artificial lighting conditions. However, since Y(II) values also change with light intensity, one should compare samples at the same light intensity unless light stress is the focus of the measurement. Y(II) can be more sensitive to some types of plant stress than F/F, such as heat stress.
  Other plant mechanism measuring protocols have also been developed. When a chloroplast absorbs light, some of the light energy goes to photochemistry, some goes to regulated heat dissipation, and some goes to unregulated heat dissipation. Various chlorophyll fluorescence measuring parameters exist to measure all of these events. In the lake model, q measures photochemical quenching, Y(NYO) measures plant regulated heat dissipation, and Y(NO) measures unregulated heat dissipation. An older quenching protocol, called the puddle model, uses q for photochemical quenching, q for nonphotochemical quenching of both regulated and unregulated heat dissipation and NPQ for an estimate of nonphotochemical quenching. NPQ has also been resurrected to the lake model mathematically.
  In addition, the parameters q, and pNPQ have been developed to measure the photoprotective xanthophyll cycle. q is a measure of state transitions. q is a measure of chloroplast migration, and q is a measure of plant photoinhibition.
  At lower actinic light levels NPQ = qE+qT+qI 
  At high actinic light levels NPQ = qE+qM=qI 
  Some fluorometers are designed to be portable and operated in one hand.
  Consistent further development into imaging fluorometers facilitate the visualization of spatial heterogeneities in photosynthetic activity of samples. These heterogeneities naturally occur in plant leaves for example during growths, various environmental stresses or  pathogen infection. Thus knowledge about sample heterogeneities is important for correct interpretation of the photosynthetic performance of the plant sample. High performance imaging fluorometer systems provide options to analyze single cell/single chloroplast as well as sample areas covering whole leaves or plants.