The intensity of light is measured by a photon counting device such as a charge-coupled device (CCD) or photomultiplier tube Errors in measurement using a CCD arise from a number of Noise is a word often used to describe random errors factors. Errors generated by the image itself (due to random fluctuations, say) is called shot noise. Dark noise occurs in the compete absence of light, again due to random thermal fluctuations. Finally, the read noise is caused by errors in the readout device. The overall noise in the CCD is thus CCD noise = p (dark noise)
2 + (shot noise)
2 + (read noise)
2 .

b. What is unique about the last row? (Hint: think about whether or not the CCD noise is principally due to one of the individual noise terms, or whether they all contribute about equally.)