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The MicroImage VMU-300 and VMU-400 are
capable of measuring to an accuracy of better than 0.2% under optimal conditions. This
accuracy, however, is highly dependent on a number of factors. Understanding these factors
will help you to minimize their impact on your readings.
The minimum line resolution of the VMU-700 operating with an NTSC RS-170
compatible signal
is 1:1280 in the horizontal dimension and 1:480 in the vertical. Using a PAL video source
on a VMU-700P offers a horizontal line resolution of 1:1536, compared to a vertical of
1:576. As a result, a horizontal measurement can be more than twice as accurate as one
taken vertically. This lower vertical resolution is due to inherent limitations of the
NTSC and PAL video standards. An inaccuracy of + or - 1 pixel, due to this minimum
resolution, is unavoidable. Other sources of error, such as cumulative rounding errors and
control non-linearities, have been minimized by the use of extra precision digits for all
internal calculations, and by the use of rotary digital encoders rather than analog
potentiometers for user controls.
The VMU-700 allows the user to set a reference distance, either vertical or horizontal,
and assign it a numeric dimension. A larger reference distance allows for greater
accuracy. If a horizontal reference distance of 10 pixels (out of the 1280 available) is
chosen, the best possible accuracy is only 10%, since a single pixel change is 10% of the
total reference. If a reference distance of 1000 pixels is chosen, a single pixel is only
0.1% of the 1000, giving a far more accurate reference. Generally, measurements made over
a larger portion of the screen can have a higher degree of accuracy than those over only a
few pixels. Assuming ideal optics, keeping both the reference distance and the actual
measurements to their largest possible sizes on screen offers the highest degree of
accuracy.
All camera lenses create some distortion and non-linearity of the displayed image. This is
due to a series of factors, such as the difference in distance between lens and subject at
the center versus at the edge of the display. These distortions consist of a combination
of spherical distortion, pincushioning, and barrel distortion, and usually become most
pronounced near the edges of the screen, particularly with wider-angle lenses.
Fortunately, lens distortion will have a similar effect on the measurement of both the
reference object and the subject itself. Because of this, optical errors can be partially
negated by making both the reference and actual measurements at roughly the same part of
the display, and by selecting a reference object similar in size to the actual object to
be measured. A WORST case scenario would be to set a reference dimension using a small
object near the center of the display, and then to measure a large object located along
one edge.
The camera itself may be a source of linearity errors. The regular spacing of pixels on a
CCD or CMOS image sensor vary very slightly across its surface, and older tube camera
sensors are notorious for severe nonlinearity. Because of this, the use of tube cameras
for measurement purposes is not recommended.
As the distance between subject and camera changes, so does the apparent size of the
displayed image. If two objects, identical in all but thickness, are alternately placed
against the same background, the thicker object will appear larger, since its viewed
surface is closer to the camera. To maintain a reasonable degree of accuracy, both the
reference object and the subject to be measured must be at the same distance from the
camera. A lens with a wide angle field of view will be more susceptible to this effect,
while a concentric lens (in which the field of view may be represented by nearly parallel
lines) is relatively immune.
Any time the camera is refocused, it changes the apparent size of the
displayed object, and
requires that the VMU be recalibrated for an accurate measurement. There is a narrow band
of distance from the camera within which objects will appear in focus, and moving either
closer to or farther from the camera will cause them to blur. The size of this band is
known as depth of field. The aperture size of the lens has an inverse effect on the depth
of field. As the lens aperture is reduced (higher f stop number), the amount of light will
decrease, but the image will be sharper over a wider depth of field. A shallow depth of
field is more difficult to work with, because of the blurring of objects out of the plane
of focus, but serves as a better indication of what is within range of the current
calibration.
After setting the crosslines on the VMU to a known reference dimension, the user can enter
that dimension using the SET button and the numeric keypad. The on-screen display will
then provide a numeric readout of the distance between lines. Through the menu, the user
can choose the number of digits which the VMU will display. The ability to display
dimensions using less than the maximum number of digits available allows the user to
reduce the impact of the numeric readout on the active display area. Obviously, choosing
too low of a number of digits of resolution will limit accuracy. On the other hand,
displaying too many digits reduces readability, and may provide a false sense of accuracy.
For each pixel that the distance between lines changes, the display will change by an
increment equal to the reference dimension divided by its size in terms of pixels. For
example, a 50 pixel wide reference calibrated to a dimension of 2.000 would change by
2.000 divided by 50, or 0.040 per pixel. While the resolution of the numeric readout would
appear to be 0.001, or 0.05% accuracy, a typical reading in this case would only be to
within 0.040, or 2%. The last displayed digit would not be meaningful, and the next would
only be partially significant.
Reference dimensions may be chosen within the range of 0.0001 to 9999, although this
choice of dimension may impose other limitations on the accuracy of the readout. The VMU
is limited to displaying no more than 5 significant digits per dimension. If, for
instance, the user set a large reference distance of 500 pixels and entered a dimension of
0.001, the best accuracy obtainable would be 10%, or 50 pixels, since any change less than
that would be under the 0.0001 minimum which the VMU could display. Choosing a reference
number closer to 1 (between 0.1 and 100) should prevent this from occurring. For example,
rather than enter a reference measurement as 0.003mm, using 3.00nm would keep the numbers
well above the VMU's lower boundary. |
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