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234 PI-MAX
®
4 System Manual Issue 4
15.3.3 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 pre-trigger. 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.
15.4 Inhibiting the Pulser During Readout
In Gate mode operation, if gating pulses are applied to the camera 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-MAX4, the timing generator is inhibited internally.
15.5 Lens Performance
Imaging applications require that a lens be mounted to the camera. 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, each of which is described the following sections.
15.5.1 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 (i.e., the smallest f/ stop.)
15.5.2 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 of
sharpest focus. For example, with a 50 mm lens, at f/4 the depth of field will extend from
8 ft to infinity. By focusing at full aperture, the depth of field will be as shallow as possible.
As a result, the effects of even very small focusing adjustments will be readily observed,
allowing the focus to be set with precision. Once the optimum focus setting has been
achieved, the aperture can be reduced to the point of maximum sharpness. In some
experiments, you may wish to adjust the aperture for optimum signal level. However, the
experiment setup parameters established with the applications software can also be used to
adjust the signal level, allowing the lens aperture and focus to be optimized.