6. Operating the infinite® 200
2008-07
Instructions for Use for
infinite® 200
No. 30017581 Rev. No. 1.4
113
6.7 Optimize Luminescence Measurements
STOP
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.
6.7.1
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 relevant signal to (shot) noise ratio can be improved by a factor
when measurement time is multiplied with the square of the desired
factor.
6.7.2
Light Level Attenuation
When using photon counting detection, optical attenuation of higher
luminescence light levels (> 10,000,000 RLU) is necessary. In such a case, too
many photons entering the PMT at a time cannot be distinguished as distinct exit
pulses. Count rates would even fall behind values at lower light levels.
Therefore, values >10,000,000 RLU (without attenuation) are marked as
“INVALID” on the result sheet.
The
infinite
®
200
hardware can attenuate light levels by a fixed factor of either
1 (none) or 10 (1 OD). Correspondingly, the usable measurement range will be
shifted to higher light levels (< 100,000,000 RLU).
