Method and apparatus for increasing the accuracy of dimensional measurements made with video cameras

Video cameras, such as charge coupled device cameras, are used for optically making dimensional measurements of laser beams. Such cameras have drawbacks including baseline offset error, shading error, pixel-to-pixel fixed pattern offsets, and poor signal-to-noise ratio. Methods and apparatus to correct for these errors, without losing any desired signal components, wherein the baseline offset error is corrected without the loss of any signal components that may otherwise be obscured due to noise. The baseline of the signal is raised above a digitizer's zero level, an average baseline without an input signal present is determined, and then the average baseline is subtracted from the subsequently obtained signals. The subtraction process is performed such that the subsequent frames in memory retain all negative signal components, as well as positive signal components, and thus there is no loss of the desired signal. Any subsequent quantitative measurements consider both the positive and negative signal components and thus measurements are made as accurately as possible. After the baseline error is corrected, one or more of a number of noise reduction methods are implemented to improve the signal-to-noise ratio.

BACKGROUND 
1. The Field of the Invention 
This invention relates to devices and methods used to make dimensional 
measurements using video cameras. More particularly, the present invention 
relates to making dimensional measurements of a laser beam which are used 
to determine important parameters and characteristics of the laser beam. 
2. The Background Art 
A variety of video cameras accompanied by appropriate signal processing 
equipment are advantageously used to make quantitative dimensional 
measurements in scientific and industrial fields. For example, video 
cameras based upon charge coupled devices (CCD) and vidicon devices are 
commonly used to make dimensional measurements. Cameras based upon CCD and 
vidicon devices are rugged mechanisms with many desirable characteristics 
for use in making quantitative dimensional measurements. 
Laser devices have become common in modern technology. A laser beam 
provides a source of highly concentrated optical energy which has found 
use in many diverse applications in research, medicine, and industry. The 
primary features that make a laser beam useful for such applications 
include, for example: a single wavelength or color, i.e., monochromatic 
light; collimation of the optical energy so that the laser beam travels in 
a narrow beam over a very long distance; the ability to concentrate large 
amounts of energy in a very short period of time; and, the ability to 
focus the laser beam to a very small spot. 
The laser's capability to control the exact spot size and spatial 
distribution of laser beam energy is of critical importance in nearly all 
applications of lasers. In order to control the spot size and spatial 
distribution of laser beam energy it is necessary to measure the spatial 
energy profile, and in particular, the precise dimensions of the laser 
beam. It is only when accurate measurements of the spatial profile and 
dimensions of laser beams are possible that the performance and 
effectiveness of a laser beam can be significantly improved. 
Video cameras using CCD (charge coupled devices), CID (charged injection 
devices), self-scanned arrays, vidicon tubes (video cameras using vidicon 
tubes sometimes being referred to as vidicon cameras) and other camera 
technologies have become widely used as part of a system to measure laser 
beam dimensions as well as to carry out other optical dimensional 
measurements. When using video cameras to measure laser beam parameters, 
the laser beam is directed to impinge upon the video camera's light 
sensitive surface and the video camera is able to record the spatial 
intensity profile of the laser beam over its entire two-dimensional 
matrix. The signal which is output from the video camera is then subjected 
to a video digitizer and appropriate digital signal processing hardware 
and software to provide laser beam parameter measurements. 
The video cameras and associated signal processing equipment provide two 
principal functions when performing laser beam measurements. The first 
function is the ability to record and display the laser beam profile. 
Visualization or displaying of the laser beam profile assists a laser user 
by providing an intuitive feel for the results of quantitative 
measurements. The fast response of such video cameras and signal 
processing equipment can provide a display of laser beam profiles in real 
time and in both 2D and 3D modes. 
The second function is to make quantitative measurements on laser beam 
parameters. Such laser beam parameters include total power, power density, 
and in particular, the width or diameter of the laser beam. These detailed 
quantitative measurements of laser beam characteristics allow a user to 
precisely determine the properties of the laser beam and to make 
incremental adjustments and improvements in its performance. 
Video cameras using CCDs, hereinafter referred to as CCD cameras, in 
particular have become popular as a tool for conducting laser beam 
diagnostic measurements. The ability of the CCD camera to simultaneously 
measure the entire surface area of the laser beam and perform detailed 
spatial measurements makes it well suited for conducting laser beam 
diagnostics. These cameras are often used by scientists and engineers who 
are either designing lasers or who are using lasers in applications where 
the spatial profile of the laser beam is critical. 
The before-mentioned video cameras, in spite of their advantages, possess 
certain characteristics that limit the precision with which laser beam 
diagnostics can be carried out. Still, some of the properties of CCD 
cameras make them well suited for spatial profile measurements of laser 
beams. These video cameras, however, have some characteristics that limit 
their usefulness in laser beam diagnostics and in other industrial 
quantitative measurement applications. 
First, CCD cameras and vidicon cameras typically have a low signal-to-noise 
ratio, even when the signal is approaching saturation, which causes 
problems in obtaining precise measurements under varying camera 
conditions. 
Second, some video cameras possess a measurement error resulting from a 
fixed baseline offset error inherent in the camera. The fixed baseline 
offset error exacerbates the seriousness of the low signal-to-noise ratio. 
It has been shown (Jones, R., Laser Focus World, January 1993) that a 1% 
error baseline offset error can contribute up to 20% error in the measured 
width of a laser beam. Moreover, fixed baseline offset can also come from 
signal processing equipment. 
Third, some video cameras, and particularly CCD cameras and vidicon 
cameras, exhibit what is referred to as "shading error" in the baseline 
which results in the baseline offset changing from one portion of the 
spatial plane in the camera to another. 
These inherent characteristics have limited the ability of some commercial 
grade video cameras to make accurate measurements. Generally, accurate 
measurements using video cameras is possible only when the signal produced 
by the camera is very close to saturation and only when the signal covers 
a relatively large area of the light sensitive surface in the video 
camera. 
The problems encountered with video cameras such as CCD cameras and vidicon 
cameras are accentuated in laser beam diagnostics applications because the 
measured dimensions of the laser beam impinging upon the light sensitive 
surface of the camera is highly dependent upon the proper detection of low 
level beam intensities in the wings, or outer regions, of the laser beam 
where the signal-to-nose ratio of the video camera is inherently very low 
and may even be less than one. In contrast, the integrated total energy in 
the wings can be significant due to the relatively large area of the wings 
found in some laser beams. In cases where the wing portion of the laser 
beam is large or exhibits high energy the noise which is inherent in video 
cameras, especially in the presence of baseline offset error and shading 
errors, can create very large errors in calculated beam dimensions. 
In addition to the above noted difficulties with video cameras, there are a 
number of different definitions for laser beam width which must be 
considered. Some of the definitions for laser beam width are useful for 
some types of laser beams, and other definitions are useful for other 
types of laser beams. Nevertheless, for a great majority of laser beams, 
many of the definitions have an equivalence to one another. Information on 
the equivalence of various definitions of laser beam width is available 
from Seagman, Johnston and Sassnet, "Choice of Clip Levels for Beam Width 
Measurements using Knife-Edge Technologies," IEEE Journal of Quantum 
Electronics (April 1991). The most useful of these definitions of laser 
beam width, and their accompanying methods, are known in the art and will 
be described below. 
One fundamental approach to defining laser beam widths is based upon the 
second moments of the energy distribution in a two-dimensional intensity 
profile. This definition of laser beam width is referred to as the 
D.sub.4.sigma. approach. However, the calculation of the D.sub.4.sigma. 
beam widths is compromised due to the limitations of the recording 
devices, e.g., the videocameras and signal processing equipment. In 
particular, noise and baseline offset have a stronger effect on creating 
errors when using the second moment definition. Thus, other definitions of 
laser beam widths are used because they agree with the d.sub.40 definition 
to within a high degree of accuracy under certain conditions and are less 
vulnerable to the limitations of the recording devices. 
Another definition of beam width is called D.sub.86 and is one in which all 
of the pixels which are measuring a signal in the beam are summed, 
starting at the highest magnitude, until 86% of the total energy impinging 
on the light sensitive surface of the video camera is counted. At this 
point, a diameter is calculated based upon the area containing those 
pixels which make up 86% of the total energy of the laser beam. 
Disadvantageously, the shape of the beam is assumed to be circular and 
only a single diameter is computed. 
A third definition of laser beam width is called knife-edge. The knife-edge 
definition is one in which an equivalent knife-edge is drawn across the 
signal from the video camera until it cuts off 10% of the energy of the 
laser beam. The knife-edge continues to be drawn across the signal from 
the video camera until it cuts off 90% of the laser beam. The laser beam 
width is defined as the distance between 10% and 90% positions multiplied 
by a correction factor relating it to the second moment value. The 
knife-edge and D.sub.86 definitions function to approximate the second 
moment definition widths of a laser beam. 
A fourth definition of laser beam width is referred to as aperture wherein 
an aperture is drawn around the laser beam centroid. The size of the 
aperture is increased until it includes 86% of the energy of the laser 
beam whereupon the size of the aperture is taken as the diameter of the 
laser beam. 
A fifth definition of laser beam width is known as Full Width Half Max 
(FWHM) and is performed by making a measurement on the width of the laser 
beam at the positions that are exactly half the energy of the peak. 
Alternatively, the Full Width Half Max definition can be altered so that 
the percent energy of the peak is defined by the user. A common percent is 
13.6%, which corresponds to the 1/e.sup.2 point, and on a perfect Gaussian 
beam provides results which are equivalent to those obtained using the 
D.sub.86 definition or the corrected knife-edge definition described 
above. 
The commonly accepted beam widths for non-top hat laser beams fall into two 
basic categories, second moment and FWHM. The D86, knife-edge, and 
aperture definitions are all attempts at finding a the second moment beam 
width equivalent. In all of the above mentioned definitions, except FWHM 
and some alteration of FWHM, any baseline offset error or shading present 
in the video camera has a very significant effect on the resulting 
measurement. For example, if a relatively small laser beam covers 
10.times.10 pixels out of a 120.times.120 pixel array, the total 
integrated energy in the beam, if the peak is saturated, would be a 
maximum of 25,600 digital counts using an 8-bit digitizer. If the baseline 
offset were in error by only 1 digitizer count out of 256, the baseline 
would contribute 14,400 counts, or more than half the signal in the laser 
beam. Thus, a beam width measurement that integrated energy in the area of 
the beam, could be in error by more than 100%. Thus, it is critical that 
the baseline of the camera be properly set in order to make valid laser 
beam width measurements. 
In view of the foregoing, it would be an advance in the art to provide a 
method and apparatus which overcomes the noted problems and which provides 
improved dimensional measurements using commercially available video 
cameras. 
BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
In view of the above described state of the art, the present invention 
seeks to realize the following objects and advantages. 
It is a primary object of the present invention to provide a method and 
apparatus for correcting deficiencies in video cameras used for performing 
dimensional measurements. 
It is also an object of the present invention to provide a method and 
apparatus for correcting baseline offset inherent in some video cameras 
without losing any signal components that may otherwise be obscured due to 
noise. 
It is also an object of the present invention to provide a method and 
apparatus for correcting baseline shading or tilt inherent in some video 
cameras without losing any signal components. 
It is a further object of the present invention to provide a method and 
apparatus for correcting both baseline offset error and baseline shading 
or tilt without losing any signal components that may otherwise be 
obscured due to noise. 
It is a still further object of the present invention to provide a method 
and apparatus for correcting pixel to pixel fixed pattern offset present 
in some video cameras. 
It is yet another object of the present invention to provide a method and 
apparatus for correcting errors involving less than one digital count 
which are present in some video cameras used for making dimensional 
measurements. 
It is another object of the present invention to provide a method and 
apparatus for improving the results of using video cameras in laser beam 
diagnostics and performance evaluations. 
It is a further object of the present invention to provide a method and 
apparatus for improving the linear measurement of a laser beam profile 
using a video camera where the laser beam exhibits low intensity wings. 
It is another object of the present invention to provide a method and 
apparatus for correcting baseline offset error and poor signal-to-noise 
ratio inherent in some video cameras without losing any signal components 
that may otherwise be obscured due to noise. 
It is an additional object of the present invention to provide a method and 
apparatus for improving dimensional measurements made using optical 
techniques and video cameras. 
These and other objects and advantages of the invention will become more 
fully apparent from the description and claims which follow, or may be 
learned by the practice of the invention. 
The present invention provides a method and apparatus for improving the 
performance of video cameras used to make dimensional measurements. The 
present invention is particularly well suited for improving dimensional 
measurements of laser beams using cameras such as CCD cameras and vidicon 
cameras. Such cameras have particular drawbacks including baseline offset 
error, shading error, and poor signal-to-noise ratio. 
One preferred method of the present invention provides that the baseline 
offset error is corrected without the loss of any signal components that 
may otherwise be obscured due to noise. The method of the present 
invention is carried out using a video camera and a digitizer which 
processes the output of the video camera. 
According to one aspect of the preferred correction method, the baseline of 
the signal is raised above the digitizer's zero level, an average baseline 
without an input signal present is determined, and then the average 
baseline is subtracted from the subsequently obtained signals. The 
subtraction process is performed such that the subsequent frames in memory 
retain all negative signal components, as well as positive signal 
components, and thus there is no loss of the desired signal. Any 
subsequent quantitative measurements consider both the positive and 
negative signal components and thus measurements are made as accurately as 
possible. After the baseline error is corrected, one or more of a number 
of noise reduction methods are implemented to improve the signal-to-noise 
ratio. 
One method for improving the signal-to-noise ratio comprises the steps of 
obtaining a plurality of frames each including both noise and an optical 
signal and then averaging the plurality of frames together to provide a 
representative signal wherein the optical signal is extracted from the 
noise. Another method for improving the signal-to-noise ratio comprises 
the steps of determining a plurality of resulting spatial measurement 
representations based upon the noise and optical signal and averaging the 
plurality of resulting spatial measurement representations wherein the 
signal is extracted from the noise. 
In accordance with another aspect of the present invention, the 
deficiencies of baseline offset error, shading, fixed pattern and random 
temporal noise can all also be corrected. The method includes the steps of 
blocking all optical signals from reaching the camera and adjusting the 
zero baseline of the signal output by the camera such that all noise is 
represented by positive counts output from the digitizer and a baseline 
signal is produced. Next, a plurality of frames containing the baseline 
signal are averaged to obtain a reference frame. This reference frame is 
referred to as a sub-count reference frame since the averaging process 
provides a resulting reference frame having an accuracy which is less than 
one digitizer count or "sub-count." 
The optical signal, for example a laser beam, is allowed to reach the 
camera and subsequent digitized frames are generated. The reference frame 
is then subtracted from each of the subsequent digitized frames to correct 
for deficiencies in the signal output from the video camera to produce 
accurate dimensional measurements. Such deficiencies include baseline 
offset, shading, and fixed patterns in pixel offsets. The correction 
occurs such that no received optical signal components are lost. Further 
steps are preferably carried out to improve the signal-to-noise ratio of 
the signal output from the video camera. 
In another aspect of the present invention, an apparatus for improving the 
performance of video cameras used for making dimensional measurements is 
provided. The video camera provides an output signal comprising both noise 
and an optical signal. A means for acquiring the output signal from the 
video camera and a means for digitizing the optical signal contained in 
the output signal is provided. Also provided are means for adjusting the 
baseline of the received signal such that all noise is represented by 
positive going counts and such that the mean of the baseline is centered 
at a predetermined positive integer digital count to arrive at a reference 
baseline level and means for subtracting the reference baseline from 
subsequent frames to correct for baseline error. 
Also provided to improve the signal-to-noise ratio of the output signal are 
a means for obtaining a plurality of frames each representing both noise 
and an optical signal and means for averaging the plurality of frames 
together to provide a representative signal wherein the optical signal is 
extracted from the noise. 
Another apparatus to improve the signal-to-noise ratio comprises means for 
determining a plurality of resulting measurement representations based 
upon the noise and optical signal and means for averaging the plurality of 
linear measurement representations wherein the optical signal is extracted 
from the noise. 
Still another apparatus to improve the signal-to-noise ratio of the output 
signal comprises means for providing an aperture around the optical 
signal. Yet another apparatus for improving the signal-to-noise ratio of 
the output signal comprises means for averaging a line in the video 
camera. An even further apparatus for improving the signal-to-noise ratio 
of the output signal comprises means for averaging an area in the camera. 
In accordance with another aspect of the present invention, a further 
apparatus for improving the performance of video cameras used for making 
dimensional measurements is provided. In one preferred embodiment, a means 
for acquiring the received signal from the video camera and a means for 
digitizing the optical signal contained in the received signal is 
provided. A means for adjusting the zero baseline of a signal output by 
the camera is included such that all noise is represented by positive 
counts output from the digitizer and a baseline signal is produced. Also 
furnished is a means for averaging a plurality of frames containing the 
baseline signal to obtain a reference frame and a means for subtracting 
the reference frame from subsequent digitized frames to correct for 
deficiencies in the signal output from the video camera to produce 
accurate dimensional measurements. 
The preferred means for averaging a plurality of frames containing the 
baseline signal comprises a means for summing a plurality of frames and a 
means for dividing a value obtained for each pixel in the plurality of 
frames by the number of frames to arrive at a sub-count reference frame. 
The deficiencies which are corrected include baseline offset, shading, and 
fixed patterns in pixel offsets. The correction takes place such that no 
received optical signal components are lost. The preferred apparatus also 
includes means for improving the signal-to-noise ratio of the received 
signal such as previously described. The apparatus of the present 
invention is preferably carried out using digital processing techniques 
and equipment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following description, measurement of the width of a laser beam will 
be used to explain the present invention. Measuring the width of a laser 
beam using video-cameras often encounters serious problems such as 
baseline offset error, shading, and fixed patterns in pixel offsets, and 
noise. Significantly, the method and apparatus of the present invention 
apply to other quantitative dimensional measurements made with various 
video cameras, for example, CCD cameras or vidicon cameras, accompanied by 
appropriate signal processing apparatus and techniques. For example, the 
present invention can be applied in many applications using video cameras 
to optically make quantitative dimensional measurements. 
With regard to CCD cameras and vidicon cameras, CCD cameras are generally a 
good choice for use in laser beam analysis systems. CCD cameras have many 
desirable characteristics that make them a good choice for spatial profile 
measurements of laser beams. Some of these desirable characteristics are: 
Simultaneous matrix, whole beam measurement; Relatively high resolution, 
as low as 9 .mu.m per pixel; Linear output signal vs. input power; Wide 
spectral coverage from 190 nm to 1.1 .mu.m; Relatively uniform 
responsitivity over the light sensing surface; A CCD apparatus is a rugged 
solid-state device; and, Commercially available CCD cameras are relatively 
inexpensive and provide high performance. 
An apparatus for laser beam analysis generally includes a video camera with 
associated optics and signal processing electronics including devices such 
as a digitizer, general purpose processor, a display, and other devices 
known in the industry. 
As indicated earlier, video cameras such as CCD cameras inherently have 
deficiencies that limit their ability to provide high precision 
measurements of laser beam characteristics. In order to obtain accurate 
laser beam measurements, such deficiencies which heretofore have been 
unrecognized or accepted as inevitable in the industry, must be overcome. 
One principal deficiency of CCD cameras is the relatively low saturation 
level of the light sensing device. The relatively low saturation level 
results in a low signal-to-noise ratio, even when the signal is close to 
the saturation level of the CCD camera. The problem of low signal-to-noise 
ratio becomes even more acute under conditions where the signal cannot be 
adjusted so that it is close to the saturation of the camera or when 
gathering information on the relatively low intensity wing portions of a 
laser beam. 
CCD cameras typically have a signal-to-noise ratio of about 300 when 
comparing peak signal at saturation to RMS noise. In this case, the RMS 
noise is equivalent to the 1 sigma level of the standard deviation of the 
noise distribution. While a signal-to-noise ratio of 300 appears adequate 
for many applications, in laser beam measurement applications such a 
characteristic translates to a peak-to-peak signal-to-noise ratio of only 
about 50, where the peak-to-peak signal level is typically plus or minus 
three standard deviations. 
This high noise level, equivalent to 2% of saturation of the laser beam, 
inhibits accurate measurements of beam characteristics especially in the 
wings of the laser beam where signal levels are low. An adequate 
signal-to-noise ratio is particularly important when making laser beam 
diameter measurements since the diameter of a laser beam is in some cases 
defined to be the 1/e.sup.2 point, or the point on the laser beam profile 
in the wing where the intensity is only 13.5% of the peak intensity. As 
explained earlier, other definitions of laser beam width present a similar 
situation. 
Significantly, if the signal-to-noise ratio is 50 at the peak, the 
signal-to-noise ratio at the 1/e.sup.2 point of the laser beam may be as 
low as about 7. If the peak signal were only 50% of saturation, the 
signal-to-noise ratio at the 1/e.sup.2 point would deteriorate to as low 
as 3. This low signal-to-noise ratio can have a serious effect on the 
measurement of laser beam diameter. 
The inherent high noise level present in CCD cameras, in addition to 
influencing the accuracy of measurements, also compels the use of 
cumbersome beam attenuation optics. Since both the signal-to-noise ratio 
and the dynamic range of CCD cameras is low, it is generally necessary to 
adjust the laser beam energy to be very near saturation for each 
measurement. If a series of measurements are being made, wherein the beam 
intensity changes significantly, then adjusting the beam close to 
saturation can become very time consuming and cumbersome. 
Another serious drawback of CCD cameras is variation in the camera baseline 
offset, or zero signal level. Since all signal levels are measured riding 
on the baseline offset, errors in the offset adjustment can directly 
affect measurement of laser beam profile properties. 
The baseline offset adjustment of a CCD camera influences measurements with 
the camera for a number of reasons. For example, the camera offset level 
drifts with time, with temperature of the environment, with aging, and 
especially during the first 1-2 hours as the camera heats up after being 
turned on. 
Any shift or drift in offset is especially serious in the case of two laser 
beam measurement conditions. The first measurement condition where shift 
or drift in offset is especially serious is when the laser beam is low 
intensity thus resulting in low level signals. Under low signal levels the 
signal-to-noise ratio is poorer and the effect of offset errors are 
multiplied on any dimensional measurements which are made. 
The second measurement condition occurs when very small laser beams, that 
cover only a few pixels, are being measured. In the case of very small 
laser beams, a small error in the baseline, averaged over all the pixels, 
can create a greater signal than the laser beam itself. 
The problems caused by baseline offset and noise encountered when measuring 
laser beam intensity profiles and dimensions will be explained by 
referring to FIGS. 1-4. FIG. 1 illustrates a typical profile of a laser 
beam assuming a perfect video camera with zero noise. In FIGS. 1-4 and 6, 
the diamonds represent the plot of a Gaussian laser beam while the 
vertical axis represents the laser beam intensity and the horizontal axis 
represents the beam dimension. Also, FIG. 1 assumes that the baseline is 
flat and adjusted perfectly so that measurements can be made precisely on 
the energy in the laser beam. In FIG. 1, as well as in FIGS. 2-4, and 6 
the laser beam signal is represented by the curve 10 and the digitizer 
baseline is represented by the horizontal line 12. 
FIGS. 2 and 3 illustrate a laser beam in the presence of baseline offset. 
In FIG. 2 the baseline offset, represented by gap 14, is adjusted too low 
and the low intensity wings 16 of the laser beam are suppressed below the 
digitizer zero cutoff. Thus, information in the wings 16 is suppressed, 
and any resulting measurement of beam width will be smaller than the 
actual laser beam width. 
In FIG. 3, the baseline offset, represented by gap 18, is adjusted too high 
so that even where there is zero actual signal the digitizer would output 
a positive count or value. In the case represented by FIG. 3, any 
resulting dimensional measurement on a laser beam would result in a beam 
width measurement which is larger than the actual beam width. 
FIG. 4 illustrates the situation where the baseline offset is correctly 
adjusted but noise inherent in video cameras, for example a CCD camera, is 
present. With the presence of noise, some components (represented at 20) 
of the laser beam signal 10 are suppressed below the digitizer baseline 12 
and thus are lost. 
Techniques have been proposed in the past to partially compensate for 
baseline offset and noise encountered when measuring laser beam 
dimensions. One technique is referred to in the art as "reference frame 
subtraction" or "baseline reference subtraction" and another technique is 
referred to in the art as "signal averaging." 
In the case of FIG. 3, where the baseline offset is too high, reference 
frame substraction requires that data representing a signal frame with no 
laser beam signal present be stored in memory and that frame be subtracted 
from every subsequent frame which contains the laser beam signal. 
Disadvantageously, the previously available reference frame subtraction 
techniques only result in the case of FIG. 4 wherein noise is present and 
the negative noise components are discarded. This causes laser beam signal 
levels 20 below the baseline 12 to be suppressed and lost. 
In addition, the reference frame subtraction technique works to correct 
baseline offset only when the baseline offset is too high as in the case 
represented in FIG. 3. In the case of the baseline offset being too low, 
as represented in FIG. 2, it is not possible to digitize signal portions 
below the zero baseline 12, and therefore there is no possibility of 
subtracting a negative baseline offset. 
At best, reference frame subtraction can only remove gross errors in the 
baseline offset if the errors are in the positive direction. That is, only 
if the baseline level is higher than the zero digitizer level. If, 
however, the baseline is in the opposite direction, that is, it is 
negative from the zero digitizer level, then it is impossible to detect 
how much baseline offset exists, and the previously available reference 
frame subtraction technique cannot correct for such offset. 
When signal averaging is used for improving signal-to-noise ratio in laser 
beam analysis, the laser beam signal is averaged over multiple frames 
resulting in a suppression of noise. Disadvantageously, whenever the 
averaging is performed with ideal baseline adjustment as in FIG. 4, many 
of the negative signal components are still lost and the averaging does 
not adequately correct for any signal components below zero. 
Thus, with some video cameras, such as standard CCD cameras and vidicon 
cameras, and standard digitizers commonly used in the industry, there is 
no way to adequately correct for baseline offset and noise, especially 
when accurate dimensional measurements are desired. This is especially 
true in the case of measurement of laser beams where a significant amount 
of information in the beam can be contained in the low intensity wings. 
The low intensity information in the wings cannot be recovered by 
previously available averaging or baseline subtraction techniques due to 
loss of the negative signal components in the digitizer. 
In accordance with one aspect of the present invention, a method and 
apparatus is provided which compensates for both low signal-to-noise ratio 
and baseline offset error inherent in CCD cameras and other video cameras. 
It is preferred that digital signal processing techniques be used to 
implement the present invention and minimize the effects of poor 
signal-to-noise ratio and baseline offset error. Those skilled in the art 
will be able to use the information provided herein to arrive at 
appropriate programming code for use in digital processors to carry out 
the present invention. Using the method and apparatus described herein, it 
becomes possible to make accurate dimensional measurements not previously 
possible. By use of the described methods of the present invention, CCD 
cameras and vidicon cameras can be used to make accurate dimensional 
measurements which would not otherwise be possible with such cameras. 
In accordance with the present invention, both baseline error correction 
and reduction of noise (improvement of signal-to-noise ratio) is 
advantageously provided. The method and apparatus of the present invention 
corrects for baseline offset error and for low signal-to-noise ratio while 
preserving the signal components which are buried in the noise. By 
preserving the signal components which would otherwise be obscured by the 
noise, such signal components can be recovered and used to make 
measurements. 
In accordance with the method of the present invention, the steps of one 
preferred method for correcting the baseline error are represented in FIG. 
5. To correct the baseline offset error inherent in the camera, all 
optical signals, i.e., the laser beam, are first blocked from the video 
camera (Step 102). Blocking of the optical signals allows the baseline to 
be corrected in the absence of any laser beam signal. 
The baseline is next raised, including noise, until all signals are above 
the zero digitizer level, that is, until there are no zero digitizer 
counts (Step 104). Thus, all signal components are obtained when 
digitizing the signal. This is in contrast to the result of losing low 
level signals when using the previously available techniques. 
In accordance with the present invention, a preferred method used to 
calibrate the amount the offset should be raised will be described. One 
preferred calibration method involves adjusting the baseline offset and 
then measuring the distribution of noise counts (Step 106). The noise 
found in the output of a CCD camera typically exhibits a Gaussian 
distribution and is centered at a given count with a plus and minus 
deviation above and below the mean. The baseline is adjusted 
(incrementally either by adjusting the camera or using digital processing) 
until the mean is centered at an integer digital count with no zero counts 
(Step 108). Stated another way, the baseline is adjusted such that there 
is an equal number of pixel counts (representing noise only) above the 
mean as there are below the mean. The mean of the noise counts, n, is then 
subtracted from each frame to obtain an average zero count in the 
measurement of the entire frame. This subtraction process is done, 
maintaining minus noise component levels, so that no information component 
is lost. Once the baseline offset is adjusted so that the mean is centered 
at an integer digital count, it becomes possible to subtract this mean 
digital count from all signal components. The optical signal is then 
unblocked (Step 110) and data for measurements is gathered to form 
additional frames and the baseline offset is subtracted therefrom (Step 
112). Stated another way, the baseline offset is subsequently subtracted 
from all subsequent signal frames to correct the baseline offset error. 
In contrast to the previously available techniques, the subtraction step of 
the present invention preserves negative signal counts as well as positive 
signal counts, and thus none of the laser beam signal is lost. With the 
baseline properly adjusted, measurements can be made on beam widths with 
all components, both positive and negative, being preserved in memory 
following the subtraction process. 
It will be appreciated by those skilled in the art that the baseline error 
correction method of the present invention is most preferably carried out 
using digital signal processing techniques. It is, however, within the 
scope of the present invention to carry out the method of the present 
invention using analog signal processing techniques or a combination of 
digital and analog processing techniques. 
In addition to baseline error correction, it is within the scope of the 
present invention to carry out further steps to improve the 
signal-to-noise ratio. Similarly to the method of correcting baseline 
error, the subsequent signal processing carried out to improve the 
signal-to-noise ratio is conducted so that substantially no laser beam 
signal components are lost. 
Once the baseline error has been corrected in accordance with the present 
invention without suppressing any negative laser beam signal components, 
other techniques for improving signal-to-noise ratio, and thus dimensional 
measurement accuracy, can be carried out within the scope of the present 
invention. The preferred steps which can be carried out to improve the 
signal-to-noise ratio which are combined with the steps for correcting 
baseline error will be explained below. It will be appreciated that while 
some of the steps carried out to improve the signal-to-noise ratio have 
been proposed in the art, their combination with the baseline error 
correction method of the present invention provides results not heretofore 
available or suggested in the art. 
As is known, the noise which is present on a given pixel is random and has 
a normal distribution in its magnitude over time. Also, there is a random 
noise component of all the pixels in a given frame that is also a normal 
distribution about a mean noise level. That is, summing all the pixels in 
the entire frame will yield a mean with a normal distribution. The laser 
beam signal to be measured effectively rides on top of the baseline offset 
and is immersed in the random noise pattern. 
One preferred method for use in combination with the preferred baseline 
error correction method of the present invention is referred to as frame 
averaging. Referring to FIG. 5A, the preferred steps for carrying out 
frame averaging for improving signal-to-noise ratio are represented. 
Each frame from the CCD camera with signal present is input to the 
digitizer (Step 120). The baseline offset level is subtracted from the 
signal and the resulting information is stored in memory (Step 122). With 
the preferred frame averaging method of the present invention, the above 
steps are repeated for a plurality of frames (Step 124), each of the 
frames retaining the positive as well as all negative signal components, 
and the data for the multiple frames is averaged (Step 126). Thus, the 
frame averaging method is able to improve the signal-to-noise ratio 
without losing any signal components. Typically, the frame averaging 
method of the present invention described herein can be expected to 
improve the signal-to-noise ratio by the square root of the number of 
frames averaged. 
Regarding the use of the frame averaging method, when a laser beam is 
jittering in position, but not in size, the frame averaging method can 
result in errors. Typically, a laser beam which is jittering in position 
but not in size and on which frame averaging is carried out would result 
in measurements indicating a larger beam dimension than was actually 
present. 
Another preferred noise reduction method for use in combination with the 
described baseline error correction method of the present invention is 
referred to as results averaging. Referring to FIG. 5B, the preferred 
steps for carrying out the results averaging method for improving 
signal-to-noise ratio are represented. The results averaging method 
provides more accurate results than the frame averaging method when the 
laser beam is jittering in position, but not in size. 
Using results averaging, the laser beam dimensions of each individual frame 
are calculated (Step 130) and the calculated laser beam dimensions for 
each individual frame are stored (Step 132). The stored laser beam 
dimensions are then averaged (Step 134). The results averaging method of 
the present invention described herein can be expected to improve the 
signal-to-noise ratio as the square root of number of frames averaged. 
A further preferred method for use in combination with the preferred 
baseline error correction method of the present invention is referred to 
as area averaging. The area averaging method can be used to improve many 
measurements made on laser beams. For example, a beam diameter measurement 
may sum all signal components above a certain threshold level and a 
diameter is calculated from the sum of pixels. In this case, the noise is 
suppressed by the averaging process that comes from summing areas. Using 
the area averaging method, the signal-to-noise ratio improves as the 
square root of the number of pixels in the area. 
A still further preferred method for use in combination with the preferred 
baseline error correction method of the present invention is referred to 
as line averaging. The line averaging method measures, for example, an 
equivalent knife edge passing across the laser beam. The line averaging 
method of the present invention includes the average of all the pixels in 
a given line which is used to calculate the dimensions of the laser beam. 
With the line averaging method, the signal-to-noise ratio improves as the 
square root of the number of pixels in the line. 
Yet another preferred method for use in combination with the preferred 
baseline error correction method of the present invention places an 
aperture about the area to be measured. For example, apertures drawn 
around small beams can remove additional noise found beyond the wings. 
The methods of frame averaging, line averaging, area averaging, and results 
averaging all reduce the influence of noise which can cause errors in 
laser beam dimension measurements. 
It will also be appreciated that it is within the scope of the present 
invention to use a combination of the above described methods. For 
example, line averaging and area averaging can be used in conjunction with 
frame averaging or results averaging, even further in combination with 
aperture methods, to obtain maximum improvement possible in 
signal-to-noise ratio and dimensional measurement accuracy. Those skilled 
in the pertinent art will be able to determine the extent and quantity of 
data which should be considered in any averaging method such as those 
described above. 
With the methods described above, the dimensional measurements made using 
video cameras, such as CCD cameras, are much more accurate than previously 
possible using such cameras. By making these dimensional measurements 
using both the positive and negative signal components, the measurement of 
laser beams can be made to a high degree of accuracy with no loss of 
signal which represents the wings of the laser beam. 
In addition to the problems of baseline offset and noise as explained 
above, many video cameras, also exhibit problems described as "shading" or 
"tilt." One type of shading is fixed shading which is defined as a slope 
of the baseline from one side of the light sensing surface to the other 
side of the light sensing surface of the video camera. Fixed shading can 
also be expressed as a curved slope which might be high on both ends and 
low in the middle. In addition, there may also be the problem of fixed 
pattern offset from pixel to pixel. Such pixel to pixel fixed pattern 
offset is typically low in CCD cameras and much more severe in video 
cameras such as self-scanned FET array video cameras. 
In the case of CCD cameras, in addition to offset drift, shading is 
generally also present which distorts the measurements. As mentioned, 
shading can be described as a slope in the baseline level from one portion 
of light sensitive surface in the video camera to another. For example, 
typical shading in CCD cameras from top to bottom is minimal but from side 
to side is often significant. Moreover, shading in vidicon cameras is far 
more serious than encountered in CCD cameras. Thus, vidicon cameras using 
PbO-PbS sensors and vidicon cameras using pyroelectric sensors for 
detection of infrared energy are much more prone to exhibit shading and 
can significantly benefit from the present invention just as CCD cameras. 
FIG. 6 is a graphical representation of a laser beam in the presence of 
fixed baseline shading. In FIG. 6, the baseline offset is too low on one 
side, represented by gap 14A, and a portion of the low intensity wings 16A 
of the laser beam are suppressed below the digitizer zero cutoff. The 
baseline offset is adjusted too high on the other side, represented by gap 
18A, so that even where there is zero actual received optical signal the 
digitizer would output a positive count or value. 
It will be appreciated that where fixed shading or fixed pattern offset 
from pixel to pixel is a problem, the previously described method of the 
present invention may not correct for all of the deficiencies. Thus, a 
further aspect of the present invention will be described to provide 
additional improvement when making dimensional measurements using video 
cameras. 
As in the previously described preferred methods, it will be appreciated 
that those skilled in the art will be able to use the information provided 
herein to arrive at appropriate programming code for the apparatus of the 
present invention to make a variety of dimensional measurements. 
Referring now to FIG. 7, the steps of another preferred method of the 
present invention will be described. As represented at Step 150 in FIG. 7, 
all optical signals, e.g., the laser beam, are first blocked from the 
video camera. Then the signal which is output from the video camera is 
then digitized (Step 152) and the baseline is adjusted high enough above 
zero so that there are no zero counts (Step 154). 
Somewhat similarly, to the method explained in connection with FIG. 5, 
raising the baseline ensures that there are also no negative noise counts 
that would be missed by the digitizer. That is, if there were a 
peak-to-peak of 7 counts of noise in the digitizer, the digital baseline 
level would be set at 4 counts, so that the minimum count would be 1, and 
the maximum count would be 7. 
In the method represented in FIG. 7, however, it is not necessary to 
precisely set the digitizer baseline at an integer count, rather, only to 
ensure that no pixel has a noise count of zero. Those skilled in the art 
will appreciate that the term noise is being used herein in its broadest 
sense, that is, noise is intended to include all components contained 
within the 7 counts including temporal noise, fixed pattern offset, and 
shading. 
Still referring to FIG. 7, Step 156 requires averaging multiple frames of 
the baseline signal to obtain a reference frame. The reference frame will 
contain the mean, or average offset of each individual pixel of the 
camera. 
The preferred arrangement for obtaining an average of multiple frames will 
now be explained. Even though the video camera and the digitizer are 
accurate to only 8 bits, the signal processing arrangement following the 
digitizer is preferably capable of processing greater than 8 bits, for 
example, 16 bits. By use of a 16-bit signal processing arrangement 
following an 8-bit digitizer allows averaging of the baseline signal to 
obtain an accuracy much greater than 1 digital count. For example, by 
summing 256 frames and then dividing by 256, the mean value of each pixel 
baseline offset can be resolved to within 1/256 of a digital count. With a 
16-bit memory this 256 levels of sub-count accuracy can be obtained. Thus, 
the mean baseline offset of a pixel can be known to an accuracy of 1/256th 
of a digital count. For ease of reference, data which has been expanded by 
this frame averaging process will be referred to as data which contains 
"sub-count" resolution. Thus, by finding the mean baseline offset of each 
individual pixel, any fractional baseline shading or single pixel offsets 
will become identified in the reference frame. 
In summary, baseline frame averaging is used to remove noise from the 
baseline reference frame. This process retains fractions of baseline 
offset to improve precision of subsequent signal processing. This 
sub-count reference frame is then stored in memory for use in further 
processing. 
Still referring to FIG. 7, the next preferred Step 158 is to allow the 
optical signal, that is, the laser beam or any other optical signal, to 
impinge upon the light sensitive surface of the video camera. The next 
Step 160 is to subtract the sub-count reference frame from each subsequent 
digitized signal frame. Even though the digitized signal frame is not 
sub-count at this stage, the subtraction of the sub-count reference frame 
means that additional error is not introduced due to subtracting a noisy 
baseline. Thus, the sub-count accuracy is retained in the subtraction 
step. 
Additionally, since the baseline is subtracted from a noisy signal, there 
will be negative signals generated in the subtraction process. In the 
method described in FIG. 7, these negative signals are advantageously 
retained since the true signal rides on this noise. It is important in the 
accuracy of the processing to retain these negative numbers because they 
average against positive noise contributions to obtain a more precise 
baseline. Thus, the memory following the 8-bit digitizer not only retains 
sub-count signal levels, but also contains negative numbers. 
For further improvement in the signal-to-noise ratio it is now possible to 
perform additional noise reduction signal processing on the resulting 
signal (Step 162) and obtain a resulting measurement with much greater 
accuracy than previously possible. For example, the noise reduction 
techniques described earlier, and particularly those described in 
connection with FIGS. 5, 5A, and 5B, are preferred for use in Step 162. In 
particular, when performing signal averaging to improve the 
signal-to-noise ratio, the signal is also maintained to sub-count 
accuracy. As an example, with a 16-bit memory, it is possible to average 
the signal until it is known to an accuracy of 1/256th of a single digital 
count. When the sub-count reference frame is subtracted from the sub-count 
frame, the resultant signal is also accurate to the sub-count level of the 
baseline and optical signal. This sub-count accuracy includes both 
corrections for baseline offset error, shading, fixed pattern offset and 
random temporal noise. 
In calculations for laser beam width and other parameters, the sub-count 
accurate averages for the signal level can be used and thus much higher 
accuracy measurements of beam widths and other spatial measurements can be 
performed. Additionally, negative signals that are still present after 
averaging, which result from the subtraction process, are included in the 
calculations to preserve their self-canceling influence upon measurement 
accuracy. 
The preferred methods of the present invention provide a significant 
improvement in baseline offset correction. This improvement in baseline 
offset results in offset correction performed to a sub-count accuracy. In 
addition, any shading or individual pixel fixed pattern offsets in the 
background are also corrected, and corrected to sub-count accuracy. This 
allows correction of shading error involving less than 1 digital count and 
also allows information to be extracted that would be otherwise 
inaccessible without the method of the present invention. 
The preferred methods of the present invention also provide averaging of 
the optical signal and maintaining the average to sub-count accuracy and 
then subtracting the sub-count reference frame. The method of the present 
invention gives extremely high precision in the resultant signal for 
subsequent processing. 
The methods of the present invention also maintain negative results from 
the subtraction step which improves the accuracy of subsequent 
calculations. Significantly, by keeping negative numbers in the 
calculations their contribution to area and line averages used in the 
second moment definitions of laser beam width calculations are preserved. 
Using the above techniques, in the case of small area laser beams and/or 
for low intensity laser beams, the measurement accuracy which is obtained 
is much greater than possible using prior art techniques. Using the 
methods of the present invention, signal levels which are ten times 
smaller than those required by prior art techniques can be used to obtain 
accurate measurements. This improvement in accuracy is extremely 
significant in the ability to precisely measure laser beams. Using the 
methods of the present invention, accuracy of approximately 1% is possible 
when measuring laser beams. Similar accuracy can also be obtained when 
making spatial measurements in other applications. 
Reference will next be made to FIG. 8 which is a block diagram of the 
presently preferred apparatus of the present invention. It will be 
appreciated that those skilled in the art can derive many different 
particular embodiments of the present invention utilizing the block 
diagram of FIG. 8 and the other information contained herein. 
In FIG. 8 a laser 200 is represented. The laser 200 can be any one of a 
number of laser devices such as continuous wave or pulsed lasers operating 
in any of a number of frequency ranges. The laser beam 202 emitted from 
the laser 200 is subjected to a beam splitter 204 to attenuate the 
intensity of the laser beam 202 and to perform any other necessary optical 
operations on the laser beam 202. 
A portion of the laser beam 203 is diverted to a video camera 206. The 
video camera 206 can be any one of a number of available video cameras 
including CCD cameras, vidicon cameras and pyroelectric vidicon cameras. 
The data output from the camera 206 is input to a processor 208. The 
processor 208 preferably includes a digital microprocessor, digital 
memory, a non-volatile mass storage device, and user input devices as are 
known in the art. An 8-bit analog to digital convertor, also referred to 
as a digitizer, which generates 256 digital levels, is preferably included 
in the processor 208. The 8-bit analog to digital convertor is preferably 
a single-ended 8-bit digitizer, which digitizes only positive going 
signals, as is commercially available. Other digitizers and other 
electronic devices can also be used within the scope of the present 
invention. 
The processor 208 is preferably provided with means for communicating with 
other devices such as a personal computer and a printer (not illustrated). 
A video monitor 210 provides real time graphical representation and 
graphical data derived from the laser beam. Information such as beam 
energy, beam location, beam dimensions, elliptical dimensions, whole beam 
Gaussian fit, elliptical beam Gaussian fit, top hat measurements, 
divergence measurement, aperture calculation and display. 
In view of the foregoing, it will be appreciated that the present invention 
provides a method and apparatus for correcting deficiencies in video 
cameras used for performing dimensional measurements and for correcting 
baseline offset inherent in some video cameras without losing any signal 
components that may otherwise be obscured due to noise. The present 
invention also provides a method and apparatus for improving the results 
obtained using video cameras in laser beam diagnostics and performance 
evaluations as well as a method and apparatus for improving the spatial 
measurement of a laser beam using a video camera where the laser beam 
exhibits low intensity wings. The present invention also provides a method 
and apparatus for correcting baseline offset error and poor 
signal-to-noise ratio inherent in some video cameras without losing any 
signal components that may otherwise be obscured due to noise. The present 
invention also provides a method and-apparatus for correcting baseline 
shading or tilt inherent in some video cameras without losing any signal 
components and which corrects for pixel to pixel fixed pattern offset. 
The present invention may be embodied in other specific forms without 
departing from its spirit or essential characteristics. The described 
embodiments are to be considered in all respects only as illustrative and 
not restrictive. The scope of the invention is, therefore, indicated by 
the appended claims rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope.