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New iStar ICCD

Appendix

NOISE

Noise is a complex topic, a full exploration of which is beyond the scope of this glossary. Noise may, however, be

broken down into two broad categories:

1. Pixel Noise

2. Fixed Pattern Noise

PIXEL NOISE

Let us rst attempt to dene pixel noise. Assume that a light signal is falling on a pixel of the CCD. If the charge on the

pixel is read, and the read process is repeated many times, the noise may be taken as the variation in the values read.

The Root Mean Square (r.m.s.) of these variations is often used to express a value for noise. As a rule of thumb, the

r.m.s. is four to six times smaller than the peak to peak variations in the count values read from the pixel. Pixel noise has

three main constituents:

• Readoutnoise

• Shotnoisefromthedarksignal

• Shotnoisefromthelightsignalitself

Shot noise cannot be removed because it is due a known noise factor derived from these signals. Most simply dened,

shot noise is the square root of the signal (or dark signal) measured in electrons.

READOUT NOISE

Readout noise is due to the amplier and electronics: it is independent of dark signal and signal levels; it is only very

slightly dependent on temperature; and it is present on every read, as a result of which it sets a limit on the best

achievable noise performance.

• Shotnoiseisdependentonthedarksignal.Thedarksignalisitselfdependentontheexposuretimeandisvery

dependent on the temperature.

• Shotnoisefromthesignalisadditionallydependentonthesignallevelitself.Ifeitherthesignalorthedark

signal falls to zero, their respective shot noise also falls to zero.

The total pixel noise is not, however, simply the sum of the three main noise components (readout noise, shot noise from

the dark signal, and shot noise from the signal). Rather, the Root Sum Square (r.s.s.) gives a reasonable approximation -

thus:

total = sqrt (readnoise² + darkshot² + sigshot²)

where:

Fixed Pattern Noise (FPN) consists of the differences in count values read out from individual pixels, even if no light is

falling on the detector. These differences remain constant from read to read. The differences are due in part to a variation

in the dark signal produced by each pixel, and in part to small irregularities that arise during the fabrication of the CCD.

Since xed pattern noise is partly due to dark signal, it will change if the temperature changes, but because it is xed, it

can be completely removed from a measurement by background subtraction.

 total is the pixel noise

 readnoise is the readout noise

 darkshot is the shot noise of the dark signal

 sigshot is the shot noise of the signal

Andor Technology New iStar User Manual

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