Table 1 Description of main innovations in NRS methods

Chemo-metric

One or a group of chemical components [19]

Electronic nose [20], optrode [13], IR spectroscope

Quantity of O₂ [18, 21], H₂O₂ [21], VOCs [20], Protein [22]

Chemical bond: C-H, C-O, O–H [23]

NIR spectroscope, hyperspectral imaging

“Heat map” whose quantity in each point can be summed up

Calori-metric [14]

Heat

IR spectroscope

Imaging

X-ray [24, 25], infrared light, visible light

Photographing

Shape [26], size, color intensity,” heat map”