Film is usually sensitive to visible light too, so an infrared-passing filter is used; this lets infrared (IR) light pass through to the camera, but blocks all or most of the visible light spectrum (the filter thus looks black or deep red). (“Infrared filter”, confusingly, may refer either to such a filter, or to one that blocks infrared but passes other wavelengths.)

 

 

 

 

 

 

 

 

 

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