NEOcam will be observing asteroids in the infrared - a range of wavelengths that cannot be seen by the human eye. A telescope operating at these wavelengths will be sensitive to warm objects, like asteroids whose surfaces have been warmed by the sun.
If you look at the sky on a dark night, your eyes will see tens of thousands of stars. Although beautiful, these bright stars can act as camouflage, hiding the dimmer asteroids and comets we want to spot. However, look at the same night sky with a heat-sensing infrared camera, and many of the bright stars are filtered out, leaving the asteroids and comets clearly visible.
Ground-based surveys that scan the sky for hazardous asteroids in visible wavelengths are most sensitive to more highly reflective asteroids. Like someone wearing a white T-shirt at night, light-colored asteroids are easier for visible light telescopes to spot. Darker asteroids tend to be fainter at visible wavelengths, making them difficult to detect with this type of telescope.
NEOcam's infrared camera is equally sensitive to dark and bright asteroids. Like a black T-shirt on a hot summer's day, darker asteroids absorb more heat than more reflective asteroids. Hotter asteroids shine brighter in infrared wavelengths, making them equally easy to spot with NEOcam.
Not only will NEOCam be sensitive to dark asteroids, but infrared observations can also be used to make accurate measurements of an asteroid's size. Unlike visible light telescopes, which have a tough time distinguishing between asteroids that are small but highly reflective and those that are large and dark, and infrared telescope can usually measure an object's effective diameter to within about 10%.
The infrared camera on NEOcam will need to be kept cold in order to function properly. Previous missions, such as WISE, relied on expensive cryogens for refrigeration. Cryogens limit the life of the mission - once the cryogens are used up, the camera's usefulness decreases dramatically. NEOcam sidesteps this issue by operating at the L1 Lagrange point and employing a sun shield, so that it can remain cool and cryogen-free throughout its four year mission.