What photoreceptors are necessary to permit infrared vision?
The photoreceptors of the mantis shrimp (Fig. 1) are quite different from that of the human retina (Fig. 2).

Importantly, when light enters the units in the shrimp's eye, it must first pass through a crystalline cone, which lies over the receptors. These crystalline cones contain frequency-specific light-blocking substances called MAAs (mycosporine-like amino acids). These filters help to filter the spectrum and this means that less different opsins are necessary tod etect different wavelengths. In other words, a mantis shrimp does not feature 16 different opsins. Instead it utilizes the same opsins for various wavelengths by putting a filter on top of the cone (source: National Geographic).  


Fig. 1. Mantis shrimp photoreceptors. source: Physiologizing


Fig. 2. Human cone cell. source: Wikipedia