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Nicolet Evolution 500 User Manual
(1) The amount of energy, or signal, reaching the detector at any wavelength
depends on the source energy and the sample absorption at that wavelength.
(2) Noise is proportional to the square root of the signal, therefore the amount of
noise increases as a proportion of the signal as the level of signal falls;
consequently the amount of noise varies across the scan.
(3) Where high absorbance coincides with low source energy the noise due to the
signal is markedly greater than where high absorbance coincides with high
source energy.
(4) Noise can be reduced by increasing the time for which a data point is measured
(integration time).
(5) Noise can be made constant across the scan by varying the integration time at
each data point.
9.5.4.1 Standard
The Standard scan speed option allows the setting of fixed scan speeds. The maximum
rate at which the spectrophotometer can scan is 1200 data points per minute. This
equates to 2400 nm/min at 2nm data interval (or 120nm/min at 0.1nm data interval) with
an integration time of 15 milliseconds. By slowing the scan speed and/or increasing the
data interval the integration time can be increased and consequently noise decreased.
9.5.4.2 Intelliscan
Intelliscan is a means of optimising the noise on a spectrum by varying the integration
time at each data point and therefore the scan speed over the spectrum.
How Intelliscan Works
As the energy hits the detector, an analogue signal is produced which is converted to a
digital signal by the analogue to digital converter (ADC). For the ADC to work effectively,
the signal reaching it has to be within a preset range. If the signal is outside this range
then it is amplified. The level of amplification is called the gain. So if a given signal is
below the range specified for the ADC, another detector reading is taken and the gain
increased. This process continues with the gain increasing until the signal level is within
the acceptable range. A measurement is then taken and the spectrophotometer moves
on to the next wavelength. The level of gain has no effect on the strength of the signal; it
simply multiplies up a weak signal to fit the appropriate ADC range.
The level of gain is, however, a direct indicator of the amount of energy hitting the
detector which in turn is dependent on the combination of source energy less absorbance
of the sample. Therefore the level of gain set at any wavelength can be used to set the
integration time required at that wavelength. High energy at the detector requires no gain
and therefore a short integration time; low energy requires high gain and therefore a
longer integration time.
This is what Intelliscan does by intelligently varying the integration time at each data point
of the scan depending on the amount of light hitting the detector as measured by the gain
required to fill the ADC.
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Issue 1 (August 2003)