Photons enter the silicon by passing through the transparent overlying layers of the integrated-circuit elements from the front of the device. The advantages are low cost and good selection. Advantages are low cost, though not so low as standard front-illuminated, and improved quantum efficiency. A disadvantage is that selection is limited. The disadvantages are higher cost and poorer selection.
Some manufacturers offer special coatings to enhance spectral response of CCDs.
For example, see the curves for the SITe CCDs in Figure 2. They do this by taking radiation at a wavelength that the CCD is not sensitive to and converting it to a longer wavelength that the CCD is sensitive to. The response is extended into the shorter wavelengths, but the improvement amounts only to about 10-15 percent. For example, see the curves for the Kodak CCDs in Figure 2. Some CCDs are made with features that reduce blooming.