<unk> is the difference in visual properties that makes an object (or its representation in an image) distinguishable from other objects and the background. In visual perception of the real world,<unk> is determined by the difference in the color and brightness of the object and other objects within the same field of view. Because the human visual system is more sensitive to<unk> than absolute luminance, we can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place.
One measure of<unk> is Michelson<unk>  calculated using the formula
where 
L
m
a
x
{\displaystyle L_{max}}
 is the maximum luminance and 
L
m
i
n
{\displaystyle L_{min}}
 is the minimum luminance value.
The human<unk> sensitivity function shows a typical band-pass shape peaking at around 4 cycles per degree with sensitivity dropping off either side of the peak. This tells us that the human visual system is able to detect gratings of 4 cycles per degree at a lower<unk> than at any other spatial frequency.
The high-frequency cut-off represents the optical limitations of the visual system's ability to resolve detail and is typically about 60 cycles per degree. The high-frequency cut-off is related to the packing density of the retinal photoreceptor cells: a finer matrix can resolve finer gratings.
The low frequency drop-off is due to lateral inhibition within the retinal ganglion cells. A typical retinal ganglion cell presents a centre region with either excitation or inhibition and a surround region with the opposite sign. By using coarse gratings, the bright bands fall on the inhibitory as well as the excitatory region of the ganglion cell resulting in lateral inhibition and account for the low-frequency drop-off of the human<unk> sensitivity function.
For example, in the case of graphical computer displays,<unk> depends on the properties of the picture source or file and the properties of the computer display, including its variable settings. For some screens the angle between the screen surface and the observer's line of sight is also important.<unk> is also the difference between the color or shading of the printed material on a document and the background on which it is printed, for example in optical character recognition.
Contrast 
Radiocontrast
Fluid
Colloid
Steroid
Imaging agent
Exudate
Radionuclide