PSF Measurements on Back-Illuminated CCDs

The spatial resolution of an optical device is generally characterized by either the Point Spread Function (PSF) or the Modulation Transfer Function (MTF). To directly obtain the PSF one needs to measure the response of an optical system to a point light source. We present data that show the response of a back-illuminated CCD to light emitted from a sub-micron diameter glass ber tip. The potential well in back-illuminated CCD's does not reach all the way to the back surface. Hence, light that is absorbed in the eld-free region generates electrons that can di use into other pixels. We analyzed the di usion of electrons into neighboring pixels for di erent wavelengths of light ranging from blue to near infrared. To nd out how the charge spreading into other pixels depends on the location of the light spot, the ber tip could be moved with a piezo-electric translation stage. The experimental data are compared to Monte Carlo simulations and an analytical model of electron di usion in the eld-free region.