When light hits the surface of the CCD, some of it bounces off because there is a transition between air (low index of refraction) and the silicon (high index of refraction). For bare silicon, this loss can be up to 60% depending on the wavelength. This reflection can be minimized using “anti-reflection” (or AR) coatings. These coatings are carefully designed to allow the most light to pass through, making the CCD more sensitive. AR coated CCDs can approach 100% absorption at selected wavelengths! :: More information on AR coatings ::

When light enters the CCD from the side that contains the electrodes, the CCD is called “front illuminated”. These devices are easier to make, but have several shortcomings. Most advanced CCDs are “back illuminated”, meaning that the side without the electrodes is thinned and then used to absorb the light. The thinning makes the absorption surface nearer to the potential wells set up by the electrodes so that spatial resolution is not lost. :: More information on thinning ::

Top image courtesy of University of Arizona Imaging Technology Laboratory