Princeton Instruments PI-MAX 3 System, Manual, Page: 87 / 150

Chapter 9 Tips and Tricks 87
trigger the Timing Generator, it may not be necessary to use mirrors or fiber-optic cable
to delay the laser pulse. If there is no
Pre-Trigger , then taking steps to delay the arrival
of the laser pulse at the sample would likely be necessary. The easiest solution would be
to insert electrical delay between the external trigger source and the laser.
Jitter
Jitter, uncertainty in the timing of the laser output, is a critical laser performance
parameter in gated experiments. If the jitter is significant relative to the duration of the
signal pulse, the gate width will have to be wide enough to accommodate it, and the
temporal discrimination against unwanted signal will be reduced. Some types of high
power laser pulse have considerable jitter, even using a pretrigger. Where this is the case,
there is no choice but to trigger from the actual laser pulse. One way of doing that is to
use a pellicle mirror and PIN diode as previously described and then to delay the light
(usually by multiple reflections between mirrors or in an optical fiber) until the gate
“opens.”
Inhibiting the Pulser during Readout
In Gate mode operation, if gating pulses are applied to the detector during a readout, it
will cause undesirable artifacts in the data. In experiments where the time between cycles
is longer than the readout time, there is no possibility of this happening and it is not a
matter of concern. If the experiment is such that it is possible for a new gate to be applied
before the readout of the previously gathered data set is complete, preventive action will
be required. With a PI-MAX3, the timing generator is inhibited internally.
Lens Performance
Imaging applications require that a lens be mounted to the detector. Because the lens
characteristics affect system performance, it may be helpful to review some basic lens
concepts. Basically, light from an object enters the front of the lens and is focused to a
sharp image on the photocathode of the intensifier. The ability of the lens to do this well
depends on a number of factors, as follows.
Throughput
The throughput of a lens is determined by its aperture, which can ordinarily be set to a
number of different values or f/ stops. The higher the number after the slash, the smaller
the aperture and the lower the throughput. Depth of field considerations make the focus
adjustment most sensitive at maximum aperture (smallest f/).
Depth of Field
Depth of field is a measure of how the sharpness of a lens varies with respect to the
distance of an object from the lens. For any given aperture, there is a depth of field,
usually marked on the barrel of the lens. Objects within the zone will be sharply imaged.
Objects closer or further than the depth of field will not be as sharp. The further an object
is from the point of sharpest focus, the less sharp its image on the CCD will be.
The point
of maximum sharpness is located 1/3 of the way into the depth of field zone. For example,
if the indicated depth of field for the selected aperture extended from 3 ft to 6 ft, the point
of maximum sharpness will be at 4 ft.
For good focusing sensitivity, the depth of field should be small (large aperture). If the
aperture is small, the depth of field will be deep, making it difficult to establish the point