5. Operating the Instrument
2016-04
IFU for Infinite 200 PRO No. 30052730 Rev. No. 1.6
107
5.9
Optimizing Luminescence Measurements
CAUTION
SWITCH ON THE INSTRUMENT AT LEAST 15 MINUTES BEFORE
STARTING A LUMINESCENCE MEASUREMENT. SOME COMPONENTS
NEED TO WARM UP TO GUARANTEE STABLE CONDITIONS FOR THE
MEASUREMENT.
5.9.1
Integration Time
At very low light levels, a PMT does not yield a continuous output current, which
is necessary for a reliable analog to digital conversion. Instead, it produces a
sequence of pulses the average rate of which can be measured using a counter.
The advantage of the photon counting technique at such low light levels is that
pulse height selection criteria allow electronic noise to be discriminated.
At very low light levels the measured counts per second are proportional to the
light intensity. Increase of measurement time per well yields more accurate
values because of the irregular photon impact (photon statistics). The photonic
noise (shot noise) cannot be reduced technically.
Note
The signal to noise (S/N) ratio can be optimized by increasing the
integration time. Increasing the integration time by a factor of 10
results in an improvement of the S/N ratio by app. a factor of 3.
5.9.2
Light Level Attenuation
When using photon counting detection, optical attenuation of higher
luminescence light levels (>10,000,000 counts per second) is necessary. In such
a case, too many photons enter the luminescence detector at one time and
cannot be distinguished as distinct exit pulses. Count rates would even fall behind
values at lower light levels.
Therefore, values >10,000,000 counts per second (without attenuation) are
displayed as “INVALID” in the results sheet.
The optical system of the
Infinite 200 PRO
can attenuate light levels by a fixed
factor of either 1 (none) or 100 (2 OD). Correspondingly, the usable
measurement range will be shifted to higher light levels (<1,000,000,000 counts
per second).
STOP
