Optical Low-pass Filter
A birefringent layer in front of the sensor that slightly blurs the image to prevent detail finer than the pixel grid from being recorded.
In depth
Detail beyond the sensor's sampling limit does not simply vanish — it reappears as false patterns and colours. The low-pass filter, also called an anti-aliasing filter, splits each ray into slightly offset copies so that such detail is spread across neighbouring photosites. High-resolution sensors increasingly omit it, since their pixel density makes aliasing rare.
Examples in practice
Two versions of the same camera, one with and one without the filter, differ by a small but visible amount of fine detail.