Seekers of Precision Guided Munition, which are visible or infrared light based, and are self-sufficient, or fire-and-forget, have 2 main sub systems. First is detection of the incident light coming from the target area then processing the generated signal to recognize the target and establish a "track". IIR seeker systems use a 2d array of infrared detector elements in a certain size (eg. 640x480) at the front for detection. This is like our eye, with multiple photosensors and a lens in front to focus the incident light. Also called focal plane array or staring array.
After this, there are image processing electronics to make out the target object from the "viewed" image. Since this is an actual image like our eyes see, the electronics has to do relatively more complex signal processing like our brain does to correctly determine the target object, but it's much harder to fool this kind of seeker since the tracking algorithm can clearly "see" the difference between the outline of a Fighter and a point source like a flare.
Older seekers use a single detector and some sort of scanning system to sequentially look at different parts of the view. These could be linear scanning, or circular spinning scanning. Other enhancements like drawing rosette patterns or optics with extra mirrors and prisms can be added to widen the field of view or make the tracking more spoof resistant etc.
The tracking electronics are easier with these, and can use simpler logic switches. They generally determine a point in the fov, which is most likely the target, but the downside is these tracking algorithms are easier to spoof with flares, although the effectiveness of the spoofing varies depending on what enhancements are used.
In the cutting edge of technology for imaging systems, be it fixed LORROS, or portable, handheld and head mounted devices, or airborne pods, the trend seems to be focusing on combining multi-spectral signals into a single augmented visible image, which is presented to the user. Electronics has enabled techniques like compressive sensing which can produce a full multi pixel image from a single detector. Material science plays a big role, sensors made out of exotic substances like strained superlattice gallium arsenide promises to bring down cost, power consumption and cooling needs while increasing sensitivity. The research and engineering activities in this field are very much active and newer more effective techniques and "things" come out very often.
