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Principles of the 996 PDA Detector Optics
4
Figure 4-3 Photodiodes Discharged by Light
The 996 detector measures the amount of current required to recharge each photodiode.
The current is proportional to the amount of light transmitted through the flow cell over the
interval specified by the diode exposure time.
Exposure Time
The 996 detector recharges each diode and reads the recharging current one diode at a
time. The interval between two readings of an individual diode is the exposure time. The
996 detector requires 11 msec to sequentially read all of the diodes in the array. The
minimum exposure time is 11 msec. You can set exposure time from 11 to 500 msec.
For example, if an exposure time is set to 50 milliseconds, the Waters 996 detector:
1. Recharges diode 1 and reads the current required to recharge diode 1.
2. Recharges diode 2 and reads the current required to recharge diode 2.
3. Sequentially recharges and reads the current required to recharge all the
remaining 510 photodiodes.
Grating
Deuterium Lamp
Light from grating
dispersed into 1.2 nm
wavelength beams
continuously
discharges diodes.
100%
Charge
0 %
Sample in flow cell
absorbs at specific
wavelengths
Flow
Cell
Mirror