1. A method for extending a dynamic range of an imaging system, the method comprising: acquiring a plurality of images of a sample using a detector with a linear response, each sample image being acquired with a different exposure time, each sample image having a plurality of pixels; obtaining a dark calibration line fit from memory, the dark calibration line fit based on values of corresponding pixels from multiple dark calibration images, at least one dark calibration image having a longer exposure time than another of the dark calibration images, wherein the dark calibration line fit represents a dark image value for each corresponding pixel as a function of exposure time; determining a system-level dark value for each pixel in the sample images, based on the exposure time of the sample image and the dark calibration line fit, to create a system-level dark calibration image; subtracting from each sample image the system-level dark calibration image corresponding to the exposure time for each sample image to produce a plurality of dark-corrected images; and fitting a linear regression to values of pixels that correspond between the dark-corrected images to determine a merged pixel value.