User guide

LBA-USB User Guide Document No: 11294 Page 93
Chapter 4. Computations
4.1 Computational Accuracy
Having mastered the skills of acquiring a laser beam's profile with LBA-USB, you may
become curious about the accuracy of the quantitative results.
The degree of accuracy of the computed results is based primarily on two factors. The
first, and most significant, is the correct nulling of the background energy. The second
deals with optimizing the presentation of the beam display.
The background energy nulling operation establishes the zero reference from which all
computed results are based. Failure to correctly null and periodically monitor the
background energy will yield inconsistent results. Excessive background energy levels
will yield oversized beam diameters and reduced magnitudes when energy relationships
are compared.
The opposite effects will result if the background levels are excessively suppressed.
LBA-USB is equipped with a patented auto calibration feature called Ultracal!. Ultracal!
performs a nulling operation that is significantly more accurate than that which can be
achieved manually. The Ultracal! algorithm also compensates for background noise and
camera shading.
Important: Allow both the camera to warm up and reach thermal equilibrium before
performing calibrations. One hour is usually sufficient as a warm-up period. If
the ambient air temperature is changing, then periodically recheck the
background energy levels to make sure they haven't been significantly
altered.
4.2 Numerical Formats
LBA-USB receives either 8 or 12-bit video from the Scorpion camera and 8 or 10-bit
video from the Ophir FX cameras. Each video frame is then processed and placed into a
block of memory called a frame buffer. During processing, LBA-USB converts all pixel
intensities to a signed 16-bit fixed-point value. Processing can consist of: Ultracal!
baseline correction, Reference Subtraction, Gain Correction, Gamma Correction, Frame
Summing, Frame Averaging and Convolution. The 16-bit format is used for all
processing except frame averaging and frame summing. Frame averaging and summing
uses a 32-bit format during the summing operation, but returns a 16-bit result after the
division.
This 16-bit format is the basis upon which all computational results are performed, and
all data files are created.
4.3 Beam Presentation Affects Results
Effective beam presentation is essentially an attempt to improve accuracy by increasing
the signal-to-noise property of the digitized data. Since the camera and the digitizing