1. An imaging apparatus including an infrared video camera comprising a sensor array and operable to produce successive video images in the form of pixelated two-dimensional images, the imaging apparatus further comprising a processing arrangement operable to reduce or increase an overall brightness of an enhanced image in dependence upon an overall brightness of a pre-enhanced image and an overall brightness of a preceding enhanced image, so that a change in the overall brightness between the enhanced image and the preceding enhanced image is decreased to be less than a difference in overall brightness between the pre-enhanced image and the preceding enhanced image; wherein the output at time t is calculated using 
 f out ( x,y,t )= f ( x,y,t )+γ· DC residual ( t )  (1) where 
 DC residual ( t )= DC out ( t )− DC f ( t )  (2) DC f (t) being a mean brightness of the pre-enhanced image and 
 DC out ( t )=(1−λ)· DC out ( t− 1)+λ· DC f ( t )  (3) where γ is a parameter controlling a strength compensation and λ is a parameter determining an adaptation rate of the compensation.