User guide

LBA-USB User Guide Document No: 11294 Page 109
Position the camera in the beam path to acquire a first, or Reference, beam
width. It is assumed that this first sample will be the one nearest the beam
waist, and thus the smaller sample width. Next, move the camera a distance
further from the beam waist. Note the distance the camera has traveled as the
Separation distance.
The Divergence result is computed as follows:
divergence
WW
S
CR
=⋅
−
⋅
⎛
⎝
⎜
⎞
⎠
⎟
−
2
2
1
tan
Where: W
R =
The width of the beam at the Reference (nearer to
the waist) location.
W
C
= The width of the beam at the Current (further from
the waist) location.
S = The separation distance between the two beam
width sample locations.
4.22 Histogram
LBA-USB can produce a fluence Histogram of the currently displayed frame of data.
Each bar in the display represents a fluence Bucket. Each Bucket describes a range of
quantized fluence values. The minimum Bucket Size is based upon a single count of
the digitized output of the A to D converter. The Bucket will be scaled if energy
calibration is in use. With energy calibration in effect, the raw values are simply
multiplied by a scaling factor that converts them to energy densities. For simplicity, all of
the following discussions will assume no energy calibration. Thus, a raw A to D pixel
intensity range of from 0 to 255 or 4095 will be assumed.
Image processing can alter the numerical value of a pixel’s intensity. Ultracal!, Reference
Subtraction, Frame Averaging and Frame Summing are all processes that transform the
simple 8 or 12-bit integer input from the A/D conversion, into a signed 16-bit fixed point
value. For ease of use, we have forced all buckets to be defined in raw pixel integers.
Thus a Bucket size of 4, starting at zero, will contain the intensity values from 0 to
3.992, the next bucket goes from 4 to 7.992, and so on.