Zeiss LSM 880 Operating Manual page 684
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- Notes on device safety and installation requirements (50 pages) ,
- User manual (25 pages) ,
- Basic user's manual (25 pages)
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Sources of Noise
Sources of noise effective in the LSM exist everywhere in the
signal chain – from the laser unit right up to A/D conver-
sion. Essentially, four sources of noise can be distinguished:
Lasernoiseq
Laser noise is caused by random fluctuations in the filling
of excited states in the laser medium. Laser noise is propor-
tional to the signal amplitude N and therefore significant
where a great number of photons (N<10000) are detected.
Shotnoise(Poissonnoise)
Shot noise is caused by the quantum nature of light. Pho-
tons with the energy h·υ hit the sensor at randomly dis-
tributed time intervals. The effective random distribution is
known as Poisson distribution. Hence,
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SNR
where N = number of photons detected per pixel time
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(= photoelectrons = electrons released from the PMT cathode by
SNR
incident photons). With low photoelectron numbers
(N <1000), the number N of photons incident on the sensor can only
be determined with a certainty of ± N .
N =
N can be computed as
N =
where QE (λ) = quantum yield of the sensor at wavelength λ;
1 photon = h·c/λ; c = light velocity; h = Planck's constant
N = se
Secondaryemissionnoise
Secondary emission noise is caused by the random varia-
N = se
tion of photoelectron multiplication at the dynodes of a
PMT. The amplitude of secondary emission noise is a factor
between 1.1 and 1.25, depending on the dynode system
SNR =
SNR =
and the high voltage applied (gain).
Generally, the higher the PMT voltage, the lower the sec-
ondary emission noise; a higher voltage across the dynodes
improves the collecting efficiency and reduces the statistical
SNR =
behavior of multiplication.
V
1.22
.
1.22
1AU =
1AU =
NA
NA
.
1.22
1AU =
NA
.
n
.
n
1RU =
1RU =
NA
2
NA
2
.
n
1RU =
NA
2
SNR
N
= N
N
= N
Poisson
Poisson
N
= N
Poisson
photons
photons
N =
.
QE
pixel time
.
QE
pixel time
( )
( )
photons
.
QE
pixel time
( )
.
N = se
(N+N
) (1+q
2
)
.
(N+N
) (1+q
2
)
d
d
.
2
(N+N
) (1+q
)
d
2
N
N
2
2
2
se
(N+N
) (1+q
)
2
2
se
(N+N
) (1+q
)
d
d
N
2
2
2
se
(N+N
) (1+q
)
d
Darknoise
Dark noise is due to the generation of thermal dark elec-
trons N
, irrespective of whether the sensor is irradiated. Nd
d
statistically fluctuates about
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a PMT voltage of 1000 V; with lower voltages it progres-
SNR
sively loses significance.
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Dark noise can be reduced by cooling the sensor. However,
SNR
the reduction is significant only if N≤N
areas of a fluorescence specimen. In addition, the dark noise
must be the dominating noise source in order that cool-
ing of the detector effects a signal improvement; in most
N =
applications, this will not be the case.
Additional sources of noise to be considered are amplifier
N =
noise in sensor diodes and readout noise in CCD sensors.
In the present context, these are left out of consideration.
The mean square deviation ∆N from the average (N+N
the photoelectrons and dark electrons registered, is
N = se
N = se
so that the total signal-to-noise ratio can be given as
SNR =
SNR =
where
N = number of photoelectrons per pixel time
(sampling time)
se = multiplication noise factor of secondary emission
q
= peak-to-peak noise factor of the laser
N
= number of dark electrons in the pixel or sampling time
d
Example:
For N =1000, N
=100, se =1.2, and q = 0.05
d
1000
SNR =
SNR =
1.2
2
(1000+100) (1+0.05
1.2
2
(1000+100) (1+0.05
1000
2
SNR =
2
1.2
(1000+100) (1+0.05
N
. Dark noise is specified for
d
N
= N
Poisson
N
= N
, e.g. in object-free
Poisson
d
photons
.
pixel time
QE
( )
photons
.
QE
pixel time
( )
d
.
(N+N
) (1+q
2
)
d
.
(N+N
) (1+q
2
)
d
N
2
2
2
se
(N+N
) (1+q
)
d
2
N
se
2
(N+N
) (1+q
2
)
d
2
1000
2
= 25.1
= 25.1
2
)
2
)
= 25.1
2
)
) of
SNR =
1.2
2
(10
SNR =
2
1.2
(10
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