1. A method of detecting motion between two images of a scene, the method comprising: obtaining a first image of the scene, the first image having a first plurality of pixels; obtaining a second image of the scene, the second image having a second plurality of pixels, wherein an illuminance of the scene in the second image is different than the an illuminance of the scene in the first image; determining, for each pixel in both the first plurality of pixels and the second plurality of pixels, a spatial gradient of an intensity of the pixel; and determining that motion has occurred in the scene by comparing the spatial gradient of the intensity of at least one pixel in the first plurality of pixels with the spatial gradient of the intensity of at least one pixel in the second plurality of pixels.