Method and apparatus for performing modulation transfer function tests on endoscopes

In a method and apparatus for performing an MTF test on endoscopes, a beam of light is filtered through a sinusoidally-varying medium defining a transmittance which is approximately uniform along a first direction within the medium, and which varies sinusoidally along a second direction within the medium thereby generating a beam defining an intensity which varies sinusoidally in a predetermined direction. The filtered beam is transmitted through an endoscope from a first end through a second end of the endoscope, and a video system generates signals indicative of the optical intensity of the transmitted beam at each of a plurality of predetermined locations within the beam. Noise is filtered from the intensity signals, and the filtered intensity signals are in turn used in determining the MTF at each of a plurality of predetermined locations within the beam, thereby providing an indication of the optical performance characteristics of the endoscope.

FIELD OF THE INVENTION 
The present invention relates to methods and apparatus for testing optical 
systems, and more particularly, to such methods and apparatus for 
performing a modulation transfer function ("MTF") test on fiber optic 
endoscopes. 
CROSS REFERENCE TO RELATED APPLICATIONS 
Some of the matter contained herein is disclosed in U.S. patent application 
Ser. No. 08/821,112, entitled "METHOD AND APATUS FOR EVALUATING THE 
PERFORMANCE CHARACTERISTICS OF ENDOSCOPES" (Attorney Docket No. 5509-01); 
U.S. patent application Ser. No. 08/831,601, entitled "APATUS FOR 
EVALUATING THE PERFORMANCE CHARACTERISTICS OF ENDOSCOPES" (Attorney Docket 
No. 5509-03); and U.S. patent application Ser. No. 08/822,282, entitled 
"AUTOMATED METHOD AND APATUS FOR EVALUATING THE PERFORMANCE 
CHARACTERISTICS OF ENDOSCOPES" (Attorney Docket No. 5509-04), each of 
which is being filed on even date herewith, is assigned to the Assignee of 
the present invention, and is hereby expressly incorporated by reference 
as part of the present disclosure. 
BACKGROUND OF THE INVENTION 
A typical endoscope comprises a cylindrical stainless steel case enclosing 
a bundle of optical fibers extending between a distal end (tip end) and 
proximal end of the endoscope for transmitting light through the 
endoscope. A fiber optic cable delivers light from a light source into the 
case through an aperture situated near the proximal end of the endoscope. 
The optical fibers transmit the light through to the distal end, where the 
light exits the endoscope and illuminates the area near the distal end. 
The endoscope in turn transmits an image of that area through the optical 
fibers to an eyepiece lens at the proximal end. A video camera coupled to 
the eyepiece converts the image into electronic signals and transmits the 
signals to a video monitor, where the image is displayed. 
Endoscopes are used most often in "minimally invasive surgery", in which an 
endoscope is inserted into a patient, allowing a surgeon to illuminate and 
view the interior of the patient with minimal penetration. The use of 
endoscopic surgery is growing, in large part because it is generally safer 
and less expensive than conventional surgery, and patients tend to require 
less time in a hospital after endoscopic surgery. Conservative industry 
experts estimate that about 4 million minimally invasive procedures were 
performed in 1996. As endoscopic surgery becomes more common, there is an 
increasing need to accurately evaluate the performance characteristics of 
endoscopes. 
It has been recognized that the endoscope lens should be evaluated in order 
to obtain a true measure of the performance of an endoscope. The lens may 
distort images or blur the sharpness of image colors. These and other 
shortcomings in the optical performance of endoscopes may be the result of 
imperfections in the manufacturing process and/or may develop as the 
endoscope is used over time. 
The Modulation Transfer Function (MTF) is commonly used in assessing the 
degree to which an optical system reduces the sharpness (contrast) of 
transmitted images. An image typically has an intensity which varies, and 
the modulation of the image measures this variation. The modulation of the 
image is correlated with the contrast (sharpness) of the image. 
The modulation at a location in an image is defined by the maximum 
intensity and the minimum intensity at that location as follows: 
modulation=(maximum intensity-minimum intensity).div.(maximum 
intensity+minimum intensity). 
As used herein, the terms "maximum intensity" and "minimum intensity" refer 
to the local maxima and minima, respectively, of a cycle in a 
periodically-varying intensity. 
It has been recognized that an endoscope reduces the modulation of a 
transmitted image, so that the transmitted image is not as sharp as the 
image received at the endoscope tip. The Modulation Transfer Ratio (MTR) 
is a measure of the change in an image's modulation after transmission 
through an endoscope, and is defined as follows: 
MTR=modulation of image after transmission.div.modulation of image before 
transmission. 
It has been further recognized that measuring the MTR of an endoscope while 
it transmits an image having an intensity which varies along a direction 
at a predetermined spatial frequency is equivalent to measuring the 
modulation transfer function (MTF) of the endoscope at the predetermined 
spatial frequency. 
The MTF of an optical system is defined by the system's response to a 
sinusoidal intensity variation. However, in conventional methods for 
determining the MTF of an optical system, the system's response to a 
square-wave intensity variation is measured, and this response is used to 
indirectly determine the MTF. A square-wave intensity variation is 
frequently used because it is a much easier variation to generate. The 
system's response to the square-wave intensity variation is known as the 
contrast transfer function (CTF), and may be used to determine the MTF 
using the following infinite series: 
EQU MTF.sub.n =(.pi./4) (CTF.sub.n +(1/3)CTF.sub.3n -(1/5)CTF.sub.5n 
+(1/7)CTF.sub.7n -. . . ) 
wherein: 
MTF.sub.i is the MTF at a frequency i (e.g., cycles per millimeter); and 
CTF.sub.i is the CTF at a frequency i (e.g., cycles per millimeter). 
Unfortunately, determining an MTF from a CTF involves evaluating the above 
infinite series, or approximating it by evaluating a subset of terms from 
the infinite series. Furthermore, although the MTF of a combination of 
components is the product of the MTF values of each component, the 
corresponding CTF values of the components cannot be cascaded in this 
manner since the CTF of the combination of components is a nonlinear 
function of the component CTFs. Thus, it is extremely difficult to 
precisely determine the MTF from a set of CTFs in an optical system, such 
as an endoscope, having several optical components. It is likewise 
difficult to obtain the MTF of a single component based on the CTF of the 
entire combination of optical components. 
A further complication which arises in performing an MTF test on endoscopes 
is the introduction of noise into the test results. Typically, a camera 
coupled to the endoscope records the image transmitted through the 
endoscope during an MTF test. All electrical devices such as cameras 
introduce noise into the signals generated by the camera, and thus the 
noise affects the calculation of the MTF. Such noise signals cannot easily 
be attenuated without also attenuating the test result signals and skewing 
the determination of the MTF. It would be ideal to provide an MTF test for 
use with endoscopes which is not noticeably susceptible to degradation 
from noise signals. 
Color video cameras introduce even greater noise than black-and-white 
cameras, but are preferable over black-and-white cameras because they 
reproduce higher quality images even under poor illumination. Accordingly, 
it would also be desirable to provide an MTF test for use with endoscopes 
which is not noticeably susceptible to degradation from noise signals 
generated by color video cameras. 
Accordingly, it is an object of the present invention to provide an 
effective method and apparatus for performing an MTF test on endoscopes, 
such as fiber optic endoscopes used in medical applications. 
SUMMARY OF THE INVENTION 
The present invention is directed to an apparatus and method for performing 
an MTF test on endoscopes, wherein a beam of light is transmitted through 
a diffusing medium, creating a beam of substantially uniform intensity 
which is itself filtered through a semi-transparent medium having a 
transmittance which is substantially constant in a first direction along 
the medium, and varies approximately sinusoidally in a second direction 
transverse to the first direction. As a result, the filtered beam defines 
an intensity which is substantially constant in the first direction and 
varies approximately sinusoidally in the second direction. 
The filtered beam is transmitted through the endoscope from a first end of 
the endoscope through a second end, and a video system generates signals 
indicative of the optical intensity of the transmitted beam at each of a 
plurality of predetermined locations within the beam. In the preferred 
embodiment, the predetermined locations are at the approximate center of 
the transmitted beam, and other select locations on the periphery of the 
beam, including above center, below center, to the right of center, and to 
the left of center. 
The intensity signals are filtered to remove or substantially attenuate 
noise signals. Preferably, this is accomplished by selecting an angle 
between a horizontal scan direction of the video system and the second 
direction, and rotating the semi-transparent medium relative to the video 
system's scan direction accordingly. Different angles between the scan 
direction and the second direction define different frequencies of the 
intensity signals. The angle is selected such that the intensity signals 
define a frequency which is sufficiently different from the frequency of 
the noise signals to allow the noise to be substantially attenuated 
without noticeably attenuating the intensity signals. 
A second set of signals indicative of the MTF of the endoscope at the 
predetermined locations are generated in response to the intensity 
signals. The second set of signals thus provide an indication of the 
optical performance characteristics of the endoscope. Preferably, 
graphical, alphanumeric and/or numerical indicia are generated which are 
indicative of the performance characteristics. 
One advantage of the apparatus and method of the present invention is that 
the lens of an endoscope, such as a fiber optic endoscope used for medical 
applications, may be tested and evaluated in order to accurately and 
quantitatively assess the performance characteristics of the endoscope. 
Other advantages of the present invention will become apparent in view of 
the following detailed description and accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, an apparatus embodying the present invention for performing MTF 
tests on endoscopes and for evaluating the optical performance of the 
endoscopes in dependence on the results of the tests is indicated 
generally by the reference numeral 10. The apparatus 10 comprises a test 
station 12 including an adjustable mounting arm 14 for receiving and 
retaining an endoscope to be tested, and a carrier 16 for positioning a 
target under the distal end (tip) of the endoscope for performing the MT 
test. 
A variable, high-intensity light source 18, such as a xenon arc lamp or a 
halogen bulb, delivers light along a fiber optic cable 20 which is 
detachably connected to directly illuminate the target on the carrier 16. 
In the currently preferred embodiment, the light source 18 is a variable 
xenon short-arc lamp, such as the 150 watt lamp sold under model no. 610 
by Karl Storz Endoscopy-America Inc. of Culver City, Calif. 
A video system 22 generates signals indicative of the image which is 
projected through the eyepiece at the proximal end of the endoscope. In 
the preferred embodiment, the video system 22 comprises a charge-coupled 
device (CCD) video camera 24 coupled to the eyepiece and a video signal 
processor 26 coupled to the camera. As is known in the art, the CCD video 
camera records an image by storing charges in a plurality of semiconductor 
potential wells, thereby defining a two-dimensional array of charges which 
each correspond to the intensity at a point in the transmitted image. The 
video signal processor 26 transfers the charges out of the wells and 
thereby generates time-varying video signals indicative of the recorded 
image. The CCD video camera 24 is coupled to the endoscope eyepiece with 
an adjustable vice 28 having a lens system with both zoom and focus 
control rings. Thus, the projected image may be properly zoomed and 
focused through the vice lens system 28 before it is recorded by the 
camera 24. 
The video system 22 transmits the signals indicative of the image through a 
BNC connector to a standard video monitor 30 which displays the image, and 
to a desktop computer 32 which processes the signals in accordance with 
the present invention, as is described further below. In the 
currently-preferred embodiment, the computer 32 is an Intel Pentium.TM. 
microprocessor-based desktop computer which includes known computer 
software and peripheral devices as is necessary for its operation, such as 
an operating system, a keyboard, a hard disk, random access memory (RAM), 
a computer monitor and a mouse. The computer 32 further includes a frame 
grabber card (not shown), which is an analog-to-digital converter for 
receiving the image signals from the video system 22 and generating in 
dependence thereon digital signals indicative of the image. The frame 
grabber card thus translates the image signals from the format of the 
video system 22 to a digital format which the computer's microprocessor 
can accept and manipulate. The digital signals generated by the frame 
grabber card are preferably in the format of a 512 by 512 array of pixel 
intensities, and thus the number of pixels generated by the card is 
approximately determined as follows: 512.times.512=262,144 pixels. 
The frame grabber card is preferably a "plug-in card" which is detachably 
connected to the system bus of the computer 32 in a known manner. The 
frame grabber card may be implemented with an "RT Mono".TM. video capture 
board, sold by Digital Vision, Inc. of Dedham, Mass., and driver software 
sold by ViewPoint Solutions of Rochester, N.Y. for providing an interface 
between the video capture board and the standard Pentium.TM.-based 
computer. Alternatively, the frame grabber card may be implemented with an 
IMAC PCI-1408 video capture board, sold by National Instruments of Austin, 
Tex. IMAQ Vision software and NI-IMAQ driver software, also sold by 
National Instruments, may provide the interface between the IMAC PCI-1408 
video capture board and the standard Pentium.TM.-based desktop computer. 
The computer 32 is coupled in a known manner to a standard printer 34 for 
printing images processed by the computer. In the preferred embodiment, 
the printer 34 is a laser printer having a resolution of at least about 
600 dots per inch (dpi). 
Turning to FIG. 2, the test station 12 comprises a frame 36 including an 
upstanding arm support 38, and a carrier support 40 defining a horizontal 
support surface. As indicated by the arrows in FIG. 2, the mounting arm 14 
is pivotally coupled to the arm support 38 and the angular position of the 
arm is adjustable relative to the carrier support 40 in order to 
accommodate endoscopes of all possible tip angles. In the preferred 
embodiment, the mounting arm 14 accepts endoscopes having outer diameters 
within the range of approximately 1.9 through 10.0 millimeters, lengths 
within range of approximately 4 inches through 13 inches, and tip angles 
within the range of approximately 0 degrees through 120 degrees. 
As shown in FIG. 2, the carrier 16 is mounted on the carrier support 40 
adjacent to the mounting arm 14 and is moveable relative to the mounting 
arm along rails 31 in order to adjust the position of the carrier relative 
to an endoscope on the mounting arm. As is described further below, the 
carrier 16 includes a for performing an MTF test in accordance with the 
invention in order to evaluate the optical performance of the endoscopes. 
As described in the above-mentioned co-pending patent applications, the 
carrier may include additional targets (not shown), in order to perform 
additional tests and thereby more fully assess the performance of the 
endoscopes, such as a light loss test, reflective symmetry test, geometric 
distortion test, and a lighted fibers test. The carrier support 40 defines 
an aperture 42 extending through the support surface immediately below the 
base of the mounting arm. In the preferred embodiment of the test station, 
the aperture 42 is approximately circular and defines a diameter of 
approximately 1.9 inches. 
As shown schematically in FIG. 1, directly beneath the aperture 42 and 
fixed to the underside of the carrier support 40 is a fiber optic cable 
holder 44 of a type known to those of ordinary still in the pertinent art. 
The cable holder 44 receives and retains the fiber optic cable 20 in order 
to transmit a beam of light from the light source 18, through the aperture 
42, the MTF target aligned with the aperture, and in turn into the distal 
end or tip of an endoscope being tested, as is described further below. A 
collimating lens 46 is fixed to the underside of the carrier support 40 
and covers the aperture 42 in order to collimate the light beam projected 
through the aperture. 
As shown in FIG. 3, a target 48 is seated over an aperture 49 extending 
through the carrier 16 for performing the MTF test. The target 48 is 
formed by a semi-transparent medium, such as a film, defining a 
transmittance which varies sinusoidally along a predetermined direction 
within the medium. A beam of light which is filtered through the 
sinusoidally-varying medium 48 will therefore have an intensity which 
varies approximately sinusoidally along the predetermined direction within 
the beam. In the preferred embodiment, the semi-transparent medium 48 has 
a transmittance which varies approximately sinusoidally along the medium 
in a first direction, and which is substantially constant along the medium 
in a second direction transverse to the first direction. Also in the 
preferred embodiment, the sinusoidally-varying transmittance of the medium 
defines a spatial frequency of one cycle per millimeter (cycles per unit 
length), and is preferably of the type sold by Sine Patterns, Inc. of 
Penfield, N.Y. 
As further illustrated in FIG. 4, a diffusing opal glass plate 55 is 
superimposed over the aperture 49 in the carrier 16, a white plastic 
translucent sheet 57 is superimposed over the glass plate, and the 
sinusoidally-varying medium 48 is seated over the plastic sheet. As is 
described further below, when performing the MTF test the carrier 16 is 
positioned to align the circular aperture 49 with the aperture 42 of the 
carrier support 40, and the free end of the fiber optic cable 20 is 
mounted beneath the aperture 42 to transmit a beam through the apertures, 
and in turn through the glass plate 55, white plastic sheet 57 and 
sinusoidally-varying medium 48. The glass plate 55 and plastic sheet 57 
together diffuse the beam from the fiber optic cable, creating a beam of 
substantially uniform intensity, and the sinusoidally-varying medium 48 in 
turn filters the substantially uniform intensity beam to generate a beam 
which varies approximately sinusoidally along the first direction. 
A black paper mask 59 defining a plurality of apertures is preferably 
inserted between the sinusoidally-varying medium 48 and the white plastic 
sheet 57 in order to block light transmission through portions of the 
medium, and thereby enhance selected points in an image corresponding to 
the non-masked (illuminated) portions of the medium. As is further 
described further below, each aperture of the black paper mask 59 defines 
a respective portion of the transmitted beam image within which the MTF is 
measured. 
TEST PERFORMANCE 
FIG. 5 illustrates a flow chart 70 for performing the MTF test for 
measuring the modulation transfer function (MTF) of the endoscope at a 
predetermined spatial frequency, which is equivalent to measuring the 
modulation transfer ratio (MTR) of the endoscope while it transmits an 
image having an intensity which varies along a direction at the 
predetermined spatial frequency. 
FIGS. 6 and 7 illustrate the maximum and minimum intensities in an image 
before and after transmission through an endoscope, respectively. The 
graph 80 in FIG. 6 represents the sinusoidally-varying intensity of an 
image which is received at the endoscope tip (i.e. before transmission 
through the endoscope). Points 82 and 84 have a maximal intensity 86, 
while a point 88 has a minimal intensity 90. The intensity variation 92 in 
the image before transmission is the difference between the maximal 
intensity 86 and the minimal intensity 90. 
The graph 100 in FIG. 7 represents the sinusoidally-varying intensity of an 
image after transmission through the endoscope. Points 102 and 104 have a 
maximal intensity 106, while a point 108 has a minimal intensity 110. The 
points 102, 108 and 104 of the transmitted image (FIG. 7) correspond 
respectively to the points 82, 88 and 84 of the received image (FIG. 6). 
The intensity variation 112 in the image after transmission is the 
difference between the maximal intensity 106 and the minimal intensity 
110. 
The intensity variation 112 in the image after transmission is less than 
the intensity variation 92 in the image before transmission. In other 
words, transmission through the endoscope reduces the sharpness of the 
image. The MTF test is directed to measuring this reduction in sharpness 
at different locations in the transmitted image. 
Referring again to FIG. 5, in performing the MTF test, the light source 18 
(FIG. 1) is activated (step 72) and set to a selected intensity level. The 
selected intensity level is an intensity which results in a transmitted 
image, described below, defining intensities within the preferred 
measurement range of the device for measuring the intensity, which is the 
video system 22 in the preferred embodiment. For example, a selected 
intensity level which is too low will result in a transmitted image 
defining a majority of dark (unilluminated) locations, while a selected 
intensity level which is too high will result in a transmitted image 
defining intensities too high for the video system to measure. 
The free end of the fiber optic cable 20 (FIG. 1) is attached to the fiber 
optic cable holder 44 (FIG. 1) mounted below the aperture 42 of the 
carrier support 40 (step 74), in order to transmit a light beam through 
the aperture 42. The carrier 16 is then positioned so that the 
sinusoidally-varying medium 48 is positioned directly underneath the 
endoscope tip and above the carrier support aperture 42 (step 76), thereby 
filtering the beam projected from the fiber optic cable and transmitting 
the filtered beam through the endoscope. The frame grabber card stores the 
transmitted image and generates digital signals indicative of the image 
(step 78). 
The MTF test described above provides test results comprising a set of 
signals indicative of intensities at predetermined locations in a beam of 
light transmitted through the endoscope. As hereinafter described, the 
system of the invention further generates a second set of signals 
responsive to the test result signals which is indicative of the 
performance characteristics of the endoscope. 
TEST RESULT EVALUATION 
The computer system 32 (FIG. 1) receives the signals indicative of the beam 
intensities, and generates the second set of signals in accordance with 
the intensity signals. FIG. 8 shows an exemplary display 120 of a 
graphical user interface (GUI) generated and displayed on the monitor of 
the computer system 32. As is known in the art, a GUI provides both a 
means for user input and a means for the computer system to display 
information. The GUI provides signals indicative of the type of test, 
typically in accordance with user commands such as the actuation of 
graphical buttons and switches. In the preferred embodiment, the GUI is 
implemented using LabVIEW.RTM. software sold by National Instruments of 
Austin, Tex. Accordingly, the various types of displays and input methods 
of the GUI described herein are those which are most easily implemented 
with LabVIEW.RTM. software. However, those skilled in the art will 
recognize that other types displays and input methods may be implemented, 
including other forms of graphical, textual and audio input and output. 
The display 120 includes a region 122 showing a graphical display of an 
intensity pattern of the transmitted beam. In the preferred embodiment, 
the region 122 defines a two-dimensional array of pixels, and each pixel 
has a respective color (e.g., a shade of gray) indicating the intensity of 
the transmitted beam at a location in the beam corresponding to the 
location of the pixel. Also in the preferred embodiment, a substantially 
white pixel indicates a location in the beam with the highest intensity 
relative to all pixels, while a substantially black pixel indicates a 
location in the beam with the lowest intensity relative to all pixels. 
With reference to the display region 122, the apertures of the black paper 
mask 59 (FIG. 4) each define a respective region 359 of the transmitted 
beam image within which the MTF is measured. In the preferred embodiment, 
and as shown in FIG. 19, the MTF is measured at the approximate center of 
the transmitted image, and at select locations on the periphery of the 
transmitted image, including above center, below center, to the right of 
center, and to the left of center. 
Referring to the exemplary graphical image shown in the region 122 of FIG. 
8, the region 122 defines five illuminated regions 124, each corresponding 
to a respective aperture of the black mask 59 (FIG. 4), and defining an 
intensity which varies approximately sinusoidally across the respective 
region, as indicated by the variations in color (or shading). As shown, 
the pixels sinusoidally progress across each region from darker shades, 
corresponding to lower intensities, to lighter shades, corresponding to 
higher intensities. 
The region 122 further includes a vertical indicator bar 126 and a 
horizontal indicator bar 128 which may be moved horizontally and 
vertically, respectively, within the region 122 by appropriate user 
command (e.g. a mouse action or keyboard key press). Each bar 128 and 126 
defines a plurality of collinear pixels in the display and, thus, 
corresponds to a plurality of approximately collinear locations within the 
transmitted beam. 
The display 120 further includes a vertical cross-section display region 
130 showing a graphical display indicative of the intensity of the 
transmitted beam in the approximately collinear locations defined by the 
vertical bar 126 of the region 212. The intensity curve shown in the 
region 130 is formed by a plurality of pixels each having a vertical 
position corresponding to the vertical position of a pixel in the region 
122 along the vertical bar 126, and a horizontal position corresponding to 
the intensity of that pixel. 
Similarly, the display 120 further includes a horizontal cross-section 
display region 132 showing a graphical display indicative of the intensity 
of the transmitted beam in the approximately collinear locations defined 
by the horizontal bar 128 of the region 122. The intensity curve shown is 
formed by a plurality of pixels each having a horizontal position 
corresponding to the horizontal position of a pixel in the region 122 
along the horizontal bar 128, and a vertical position corresponding to the 
intensity of that pixel. 
The three regions 122, 132 and 130 (FIG. 8) each provide an indication of 
the MTF values at the predetermined locations. A user may be able to 
evaluate an endoscope based on this type of graphical feedback alone, and 
determine whether or not the endoscope is acceptable. However, as 
discussed below, the computer system 32 of the invention further provides 
an explicit indication of whether the tested endoscope passes a threshold 
standard for the MTF test. 
Upon actuation of a button 134 in the display 120, the GUI provides a 
display 140 of the type shown in FIG. 9. The display 140 includes 
indicators 142, 144, 146, 148 and 150 which each indicate the MTF at a 
respective one of the selected locations within the transmitted beam. 
As discussed above, the sinusoidally-varying medium 48 causes the intensity 
of the transmitted beam to vary sinusoidally along a predetermined 
direction in the beam, and the video signals generated by the video system 
22 correspondingly vary sinusoidally with respect to time and thus are 
indicative of the intensity pattern of the transmitted image. The maximum 
and minimum intensities used in calculating the modulation are each 
measured with respect to an intensity of the video signal corresponding to 
a "black" (dark or unilluminated) intensity. Those skilled in the art will 
note that the signal intensity corresponding to "black" is different from 
the "back porch" level of the video signal, which is typically lower. 
FIG. 10 depicts a graph 160 illustrating the definition of modulation at a 
location within a light beam. The graph 160 includes a curve 162 
representing an intensity which varies sinusoidally along a predetermined 
direction. At a location defined by a range 164 along the predetermined 
direction, the intensity varies from a maximum intensity 166 at a distance 
168 along the predetermined direction to a minimum intensity 170 at a 
distance 172 along the predetermined direction. Thus, the modulation at a 
location defined by the range 164 may be determined from the maximum 
intensity 166 and minimum intensity 170 in the manner defined above. In 
another embodiment, the maximum intensity used in the modulation 
determination is the average of the maximum intensities of two consecutive 
cycles, such as the intensities at distances 168 and 174. Using an average 
of two maximum intensities accounts for maximum intensities which vary 
significantly among consecutive cycles, such as is common at the periphery 
of the transmitted image where the intensity sharply declines. 
The MTF value of a system is equal to the product of the MTF values of the 
components of the system. The above-described MTF values are generated in 
dependence on the signals generated by the frame grabber card, and 
represent the MTF value of the combination of the endoscope and the video 
system 22. However, since the video system is typically optically superior 
to the endoscope, a fixed correction factor can be calculated for the 
video system and used in determining the MTF of the endoscope alone. 
According to "Video Engineering" by Andrew Inglis, McGraw Hill, 1993, 
which is hereby expressly incorporated by reference as part of the present 
disclosure, a correction factor of 0.96 is appropriate for video systems 
such as those used in the present invention. Furthermore, the precision 
micro-densitometer modulation calibration of the sinusoidally-varying 
medium 48 also affects the MTF of the entire endoscope evaluation 
apparatus. The sinusoidally-varying medium 48 provided by Sine Patterns, 
Inc. has a modulation calibration of 0.81366. Accordingly, the MTF value 
for the endoscope alone is determined by dividing the above-calculated MTF 
(which is equivalent to the MTR) by the following product: 
(0.96)*(0.81366). 
Referring again to FIG. 9, each of the indicators 142, 144, 146, 148 and 
150 further indicates whether an MTF value indicates either a "pass" or 
"fail" condition. In the preferred embodiment, each MTF value is compared 
to a predetermined MTF threshold. MTF values which are below the 
predetermined MTF threshold are considered "fail" values. If at least one 
of the plurality of MTF values is a fail value, the endoscope has failed 
the MTF test. Textual displays 152 (FIG. 9) and 136 (FIG. 8) indicate 
whether the endoscope has passed the MTF test. In the preferred 
embodiment, for 10 millimeter diameter endoscopes with 0 degree tip 
angles, the MTF threshold at the center of the transmitted beam is 
approximately 0.70, and at other locations in the transmitted beam is 
approximately 0.20. 
As is known in the art, a spatial frequency is defined by the number of 
cycles (transitions from a maximum intensity to a minimum intensity and 
back again) per unit of length. The sinusoidally-varying medium 48 (FIG. 
3) is preferably implemented as the above-mentioned sinusoidally-varying 
medium having a spatial frequency of one cycle per millimeter. Thus, 
filtering a beam of light through this medium during the MTF test 
generates a beam having an intensity which varies approximately 
sinusoidally along a first direction in the generated beam at a spatial 
frequency of about one cycle per millimeter, and which is approximately 
constant along a second direction in the generated beam. The generated 
beam is transmitted through the endoscope and recorded by the video system 
22 (FIG. 1) as set forth above. 
The video system 22 records the transmitted image in a conventional manner: 
the CCD camera sequentially scans in a horizontal scan direction from one 
end of the transmitted image to the other, defining a scan row, and the 
video system generates time-varying video signals indicative of a row of 
pixels for the scanned row. In other words, the video signals generated by 
the video system 22 correspondingly vary sinusoidally with respect to time 
and thus are indicative of the intensity pattern of the transmitted image 
along the scan row. After scanning a row, the CCD camera proceeds to scan 
the next row until all rows have been scanned and all pixels have been 
generated. 
With the sinusoidally-varying medium described above, each row across the 
transmitted beam has an approximately-sinusoidal intensity variation. The 
frequency of the transmitted beam, in units of cycles per degree of the 
apparent field of view of the endoscope, is defined not only by the 
spatial frequency of the medium but also by the distance between the 
medium and the endoscope tip. As is described below, this distance is 
selected so that the frequency of the transmitted beam is approximately 
six cycles per degree of apparent field of view. In accordance with a 
further aspect of the invention, it has been determined that MTF 
measurements at a single spatial frequency of approximately six cycles per 
degree of apparent field of view are an accurate indication of optical 
instrument performance. 
FIG. 11 is a schematic illustration provided for purposes of determining 
the frequency of the transmitted beam. The tip 180 of an exemplary 
endoscope 182 is spaced a predetermined distance "d" from a 
sinusoidally-varying medium 184. The exemplary endoscope 182 has an actual 
field of view of seventy degrees, which is an angle 185 of thirty five 
degrees to each side. The actual field of view is defined as the angular 
extent of an object visible through the endoscope subtended at the tip of 
the endoscope. Thus, the length "x" of the medium visible in each thirty 
five degree field of view is determined as follows: 
EQU x=(tan 35.degree.)(d)=(0.7)(d) millimeters (1) 
Accordingly, the length of the medium which lies in the entire seventy 
degree field of view is twice the length "x" as defined in equation (1): 
EQU 2x=2(d) (tan 35.degree.)=(1.4)(d) millimeters (2) 
In the preferred embodiment, since the medium 184 defines a spatial 
frequency of approximately one cycle per millimeter, the number of cycles 
across the medium in this direction is determined approximately as 
follows: 
______________________________________ 
(3) total cycles = (1 cycle/millimeter) 
(2.times. millimeters) 
= (1 cycle/millimeter) 
((1.4) (d) millimeters) 
= (1.4) (d) cycles in the 
apparent field of view. 
______________________________________ 
As discussed above, in the preferred embodiment the desired frequency of 
the transmitted beam is six cycles per degree of apparent field of view. 
The exemplary endoscope 182 has an apparent field of view 186 of twelve 
degrees. The apparent field of view is the angle between the right and 
left sides of the image as subtended by the eye. The desired number of 
cycles across the medium 184 in the apparent field of view is determined 
approximately as follows: 
EQU (6 cycles/degree)(12 degrees)=72 cycles in the apparent field of view. (4) 
Accordingly, the value of "d" is determined from the equality of 
relationships (3) and (4) for the desired number of cycles in the field of 
view: 
______________________________________ 
(5) (1.4) (d) = 72 
(6) d = 72/1.4 
= 51.4 millimeters 
= 2.02 inches 
______________________________________ 
Thus, the tip 180 of the endoscope 182 is spaced approximately two inches 
from the medium 184. Since the distance d is known, the length of the 
medium which lies in the field of view is determined as follows: 
______________________________________ 
(7) 2.times. = (1.4) (d) 
= (1.4) (2.02 inches) 
= 2.83 inches 
______________________________________ 
Conventional video systems scan at a rate of approximately one frame every 
1/30th of a second. Since each frame comprises a two-dimensional array 
which is 512 pixels.times.512 pixels, the video system 22 scans 512 rows 
every 1/30th of a second. Thus, the video system scans one row every 
1/15360th of a second (about one row every 65 microseconds). Since, from 
relationship (4) above, there are seventy two cycles in the field of view, 
the video system 22 scans 72 cycles in 65 microseconds if the horizontal 
scan direction is substantially parallel to the direction along the medium 
which varies at one cycle per millimeter. 
The scan rate of 72 cycles in 65 microseconds defines a video frequency of 
the time-varying video signals which the video system generates: 
72 cycles/65 microseconds=1.1 megahertz (MHz) 
Of course, the video frequency is easily determined for video systems which 
scan at a rate different than one frame every 1/30th of a second, and for 
video systems which scan at a number of cycles in the apparent field of 
view different than 72 cycles. 
As set forth above, the time-varying video signals, like most signals, are 
at least partially corrupted by noise. In conventional video systems, the 
predominant form of noise is a color subcarrier signal having a frequency 
of approximately 3.57 MHz. The above-stated video frequency of 1.1 MHz is 
so close to the noise frequency of 3.57 MHz that a conventional low-pass 
filter, such as a Butterworth filter implemented in the frame grabber 
card, will not be able to substantially attenuate the noise signals 
without also attenuating the video signals. Thus, the filter will not be 
able to improve the signal-to-noise ratio. 
Two quantities which determine the video frequency, the spatial frequency 
of the sinusoidally-varying medium 48 and the scan time of the video 
system 22, cannot easily be adjusted. The spatial frequency of the 
sinusoidally-varying medium is selected for efficiently testing the 
endoscope, and the scan time of the video system cannot be changed without 
significant hardware redesign. Accordingly, the video frequency is 
preferably not adjusted by adjusting either of these two values. However, 
if the horizontal scan direction is transverse to the direction along the 
medium in which the intensity varies at one cycle per millimeter, then the 
video system scans less than 72 cycles in 65 microseconds. The number of 
cycles scanned in a row decreases as the scan direction becomes less 
parallel and more perpendicular to the direction of variance in the 
medium. As will be seen below, the frequency of the video signal will 
likewise decrease, further separating the video frequency from the noise 
frequency and thereby facilitating noise filtering. 
FIGS. 12 and 13 illustrate the manner in which the total number of cycles 
scanned depends on both the horizontal scan direction and the direction 
along the medium which varies at one cycle per millimeter. FIG. 12 shows 
in schematic form an exemplary medium 190 in which the transmittance 
varies sinusoidally along a direction 192. Maxima (or cycles) in the 
transmittance are indicated by reference numerals 194, 196, 198, 200, 202 
and 204. Thus, the medium 190 illustrated in FIG. 12 includes six cycles 
along the direction 192. A horizontal scan direction 206 is substantially 
parallel to the direction 192 of the medium 190. Accordingly, a scan 
across the scan direction 206 crosses six cycles from one end of the 
medium 190 to the other across a distance 208. 
FIG. 13 shows in schematic form another exemplary sinusoidally-varying 
medium 210 which is substantially identical to the medium 190 of FIG. 12 
but rotated with respect to the horizontal scan direction. The medium 210 
defines a transmittance which varies sinusoidally along a direction 212. 
Maxima (or minima) in the transmittance are indicated by reference 
numerals 214, 216, 218, 220, 222 and 224. Thus, the medium 210 includes 
six cycles along the direction 212. As shown, a horizontal scan direction 
226 is transverse to the direction 212 of the medium 210. Accordingly, a 
scan across the scan direction 226 crosses less than six cycles from one 
end of the medium 210 to the other across a distance 228 equal to the 
distance 208 in FIG. 12. 
Thus, decreasing the number of cycles scanned in a row by selecting a scan 
direction transverse to the direction of variance in the medium will 
likewise decrease the video frequency without changing the spatial 
frequency of the image. In fact, the angle between the scan direction and 
the direction of variance in the medium defines the decrease in the video 
frequency. The number of cycles scanned in a row is determined by the 
relationship: 
Cycles scanned=(Max Cycles) (cos .theta.) 
wherein: 
.theta. is the angle between the scan direction and the direction of 
variance in the medium; and 
Max Cycles is the number of cycles scanned when .theta.=0. 
Similarly, the frequency of the video signal is determined by the 
relationship: 
Frequency=(Max Frequency) (cos .theta.) 
wherein: 
Max Frequency is the frequency when .theta.=0. 
Thus, to divide the video frequency of 1.1 MHz by a factor of three, the 
angle .theta. is selected in accordance with the following relationship: 
Frequency=(Max Frequency) (cos .theta.) 
1.1/3=(1.1) (cos.theta.) 
1/3=cos.theta. 
.theta.=71 degrees 
Thus, orienting the sinusoidally-varying medium relative to the horizontal 
scan direction of the video system to form a 71 degree angle between the 
scan direction and the direction of variance in the medium reduces the 
video frequency approximately as follows: 1.1 MHz/3=0.37 MHz. This reduced 
video frequency is more easily distinguished from the noise frequency of 
3.57 MHz by a low pass filter. For example, a fourth-order Butterworth 
filter having a "cut-off" frequency of 0.66 MHz substantially attenuates 
the 3.57 MHz noise signal while attenuating the 0.37 MHz video signal by 
less than 0.1 dB. 
Of course, other values for the angle .theta. will reduce the video 
frequency by different amounts. For example, an angle of 40 degrees yields 
a decrease by a factor of 1.4, while an angle of 89 degrees yields a 
decrease by a factor of 57. Greater decrease factors may be produced by 
selecting angles which are even closer to 90 degrees, but selecting an 
angle of 90 degrees results in a frequency of zero. 
As is known in the art, the fourth-order Butterworth filter has overshoot 
and ringing characteristics for step input signals. However, according to 
the "Handbook of Filter Synthesis" by Anatol Zverev, John Wiley and Sons, 
Inc., 1967, which is hereby expressly incorporated by reference as part of 
the present disclosure, the overshoot and ringing is reduced to about 1% 
after a delay of 10 normalized periods of the filter cut-off frequency. 
This delay is, in this filter, only approximately 2.4 microseconds, which 
is less than 4.4% of the horizontal scan time and thus not a significant 
source of error. 
FIG. 14 depicts an exemplary graph 230 illustrating in simplified form the 
attenuation characteristics of a low-pass filter. The graph 230 defines a 
first set of frequencies 232 which are virtually unattenuated, and a 
second set of frequencies 234 which are substantially completely 
attenuated. A frequency 236 is the video frequency, such as 1.1 MHz, when 
the horizontal scan direction is parallel to the direction of variance in 
the medium. A frequency 238 is the video frequency, such as 0.37 MHz, 
after selecting a horizontal scan direction transverse to the direction of 
variance in the medium. A frequency 240 defines a threshold below which 
the filter does not substantially attenuate signals. Another threshold 
frequency 242 defines the frequencies above which the filter substantially 
completely attenuates signals. Finally, a frequency 246 is the noise 
frequency, such as 3.57 MHz. The filter substantially attenuates the noise 
frequency 246 since it is greater than the frequency 242. In addition, the 
filter passes substantially unattenuated the frequency 238 since it is 
less than the frequency 240. 
In response to a user command input through the GUI, the computer system 32 
transmits any of the above-described displays to the printer 34 of FIG. 1. 
The printed display is useful in the analysis of test results, and thus in 
evaluating the performance characteristics of tested endoscopes, in that 
it may be transmitted (i.e., sent by mail, electronic mail or facsimile) 
to distant users unable to view, or otherwise access the monitor or 
computer system of FIG. 1. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof, it would be understood by those skilled in 
the art that other various changes, omissions and additions thereto may be 
made without departing from the spirit and scope of the invention as 
defined in the appended claims. For example, many types of optical systems 
besides endoscopes may benefit from the above-described method and 
apparatus for filtering noise which is introduced during the performance 
of an MTF test. 
In addition, there are other fiber-optic scopes, known as bore scopes, 
which are similar to endoscopes for use in medical applications, but have 
much larger length-to-diameter ratios. Bore scopes are used to examine the 
internal subassemblies in large engines, compressors and turbine 
machinery. Such bore scopes may be tested and evaluated using the system 
of the present invention with minimal modification of the preferred 
embodiment presented herein. 
Accordingly, this detailed description of a preferred embodiment is to be 
taken in an illustrative, as opposed to a limiting sense.