Microlens arrays (also referred to as microlenticular arrays or lenslet arrays) are used to increase the optical fill factor in some CCDs, which suffer from reduced aperture due to metal shielding. These tiny lens systems serve to focus and concentrate light onto the photodiode surface instead of allowing it to fall on non-photosensitive areas of the device.
The tutorial consists of a schematic diagram of a interline CCD pixel pair, one equipped with a microlens to concentrate light into the photodiode, while the other must absorb incident light rays without the benefit of optical assistance from a microlens. Incident photons that strike the microlens are directed into the photodiode by refraction through the glass or polymer comprising the microlens. The photodiode without a microlens collects a significantly lower portion of incoming photons, because those that impact on shielded areas (the exposure gate and neighboring structures) are not useful in charge integration. Two sliders control incoming photon parameters. The Illumination Wavelength slider is used to control the color (wavelength) of photons striking the CCD, and the Photon Intensity slider can increase or decrease the number of photons in the applet.