Top 5 Optical Design Parameters for Chip on Tip Endoscopes
Introduction
Defining optical design specifications for custom chip on tip endoscopes can be a daunting task. In optics, the critical specifications are literally invisible, and it is common that parameters critical to the success of the device are overlooked. Furthermore, it is now understood that the current ISO standards for measuring some of these parameters have known limitations and sources of error. The error is greatest for close focus endoscopes, see Wang et al. Vol. 8, No. 3, 1 Mar 2017, BIOMEDICAL OPTICS EXPRESS 1441 for more details.
Here is a list of the most important optical parameters to use to define and design a custom chip on tip endoscope:
1. Field of View 2. Direction of view 3. Depth of field 4. F/# 5. Optical resolution
In this article, we explain what each of these parameters mean, list common specifications, and recommend the correct way to measure them.
Optical design parameters for endoscopes
1. Field of View
The Field of view (FoV) of a chip on tip endoscope defines the size of the object viewable at the extent of the displayed image. The FoV is defined in angular (typically degrees) rather than linear dimensions. This is because the endoscope is used across a board range of working distances (see depth of field below) and defining the FoV in angle vs linear dimension removes the dependence on a fixed working distance. To what image location the FoV is defined and measured to is where confusion typically arises. The ISO standard defines the FoV as the "largest visible" angle. For endoscopes that have a square or rectangular display size, this means the FoV is measured to the diagonal or corners of the image. For endoscopes with a circular FoV defined by a mask (mechanical or digital), this refers to the diameter of the circle. In some applications the horizontal or vertical FoV is important. It is acceptable to define FoV to these dimensions as long it is clearly identified in the specification.
In defining the FoV of a chip on tip endoscope it is important to specify: 1. The FoV in angle 2. The image feature defining the FoV (Image diagonal vs Diameter)
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2. Direction of view
Direction of view refers to the view angle of the endoscope relative to the axis of the device. Typically, if an endoscope has a non-zero direction of view, the distal or front face will be at the same angle as the optical direction of view. Direction of view is defined in angle relative to the axis or long dimension of the device.
3. F/#
F/# (pronounced "F-number") is the same parameter on a chip on tip endoscope as a digital camera. Formally in optics, this parameter is defined as the ratio of the effective focal length to diameter of the entrance pupil. It can be viewed as defining the cone of light collected by the endoscope from the object. As in digital cameras, F/# impacts the amount of light captured by the endoscope, the depth of field, and sets an upper limit on the achievable optical resolution. This parameter is typically the hardest to specify on a chip on tip endoscope, but needs to be defined properly to balance the overall imaging performance.
4. Optical resolution
In digital imaging systems, optical resolution can be easily confused with the pixel resolution. Both of these parameters are important to understand and balance in the system design. In a digital imaging system, optical resolution is defined as the resolution limited by the lens, while sensor resolution is defined by the pixel pitch and arrays size of the image sensor. The system resolution is the final image resolution as a result of the lens and the image sensor. In the case of an endoscope, the optical resolution must be specified in conjunction with the pixel size, image sensor array size, and F/# of the system.
Optical resolution is best quantified for endoscopes using a modulation transfer function (MTF). This is a continuous function that describes how well a lens focuses objects of different sizes or frequencies to the image sensor.
For chip on tip endoscopes where the primary use is to display an image at a fixed size or zoom on a traditional monitor, the optical resolution should be defined and measured in a way that quantifies the optical performance contributing to the perceived image sharpness. This typically corresponds to an object size or frequency within the middle of the total range of sizes/frequencies captured by the lens. This target is defined as a particular MTF frequency, commonly in lp/mm units. The target frequency is determined by the display resolution and view distance of the monitor. For more on perceived image quality and how it relates to the display system, see this link at Imatest.com http://www.imatest.com/docs/sqf/
5. Depth of field
Depth of field (DoF) is a specification that is influenced by the lens F/#, quality of the lens, and lens focal distance. For example, an endoscope with a fixed F/# may have two very different DoFs simply by changing the focal distance from close to the endoscope to far away. The apparent DoF visible on the display is also influenced by the size of the object. For example, a 5mm object viewed at 30mm from the endoscope may be at the limit of resolution of the system, while the same 5mm
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object viewed 3mm from the endoscope will appear larger and in focus. Thus to quantify DoF, it is best to define it in terms that are independent of object size. This is best done by defining the DoV as sharpness in image space.
See these links for additional information: http://www.lighthouseoptics.com/tutorial/measuring-endoscope-field-of-view/
http://www.lighthouseoptics.com/tutorial/endobench-endoscope-image-quality-tester/
Measurement methods
1. Field of View
ISO 8600-3: 1997 prescribes a method for measuring the Field of View (FoV) of endoscopes using a target placed at a specific distance from the distal window of the endoscope. Wang et al. Vol. 8, No. 3, 1 Mar 2017, BIOMEDICAL OPTICS EXPRESS 1441 goes in to great detail describing the limitations of this approach, showing how significant errors are present in the values obtained through this method especially for close focus endoscopes. Briefly, the limitations are associated with the way lenses work and the actual location of the entrance pupil within the lens.
Lighthouse Imaging's Endobench product implements another method of measuring Endoscope FoV that circumvents this limitation.
Using a concentric ring target, the FoV is quantified by determining the delta d value along the scopes optical axis where the outer and inner circle of the target are aligned to the diagonals of the image. This method has been shown to have lower error than the current ISO method.
Concentric ring target
r2 r1 FoV/2 Endoscope
r2 − r1 d 퐹표푉 = 2atan( ) ∆푑
2. Direction of view
Direction of view (DoV) is measured with respect to the axis of the endoscope. ISO 8600-3: 1997 describes a basic way to measure endoscope DoV. This is a simple method using a protractor aligned perpendicular to the endoscope axis. Lighthouse's Endobench includes a target system that
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first allows one to find the true optical axis of the endoscope, and second measure the DoV relative to the fixture using a rotary stage with encoders. If a system like an Endobench is not available, the ISO method is generally adequate.
DoV Endoscope Endoscope axis
3. F/#
Measuring F/# directly is challenging and requires specialized equipment. Lighthouse recommends confirming F/# through design inspection of the physical aperture stop prior to assembling the lens. For near diffraction limited systems (perfect lens systems), the F/# can be inferred through measurement of the MTF. The frequency in which the MTF goes to zero, or cut off frequency, can be used to calculate the F/# assuming diffraction limited lens performance.
4. Optical resolution
Optical resolution as defined by ISO 8600-5: 2005 is measured at the resolution limit of the endoscope. While this measurement is valuable, it typically does not quantify the perceived sharpness of the endoscope. This is because when viewing the image formed by the endoscope, our eyes are most sensitive to object sizes of frequencies in the mid-range of resolution, rather than the extreme. The standard does specify to convert a linear resolution, measured in line pairs per millimeter, to an angular resolution, line pairs/ degree. This conversion is required for ridgid endoscopes with an eye piece, and particularly valuable when comparing two different types of endoscopes, and quantifying depth of field.
The method specified by ISO does limitations for use as a means of representing perceived image sharpness as stated above. It also has limitations for chip on tip endoscopes where the results are to be correlated back to a design or specification.
Lighthouse uses and recommends slanted edge MTF for quantifying image sharpness for chip on tip endoscopes. Slanted edge MTF uses targets with rotated squares (as shown in the image below) to measure line spread function and MTF across the image. The advantage to slanted edge MTF over line pair based measurements is that the full MTF curve can be calculated from a single measurement. The MTF is also calibrated to the image sensor pixel size, which is typically well known by the image sensor vendor. Software packages like Imatest (http://www.imatest.com/) are well suited for processing slanted edge target images and allow for direct correlation to MTF design data. See this link for a detailed discussion on slanted edge MTF http://www.imatest.com/docs/#sharpnessiq
In order to capture the correct MTF data, the image processing state of the sensor must be known and set so that sharpening and other enhancements are turned off. Acquiring raw sensor data is preferable.
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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3
MTFat 50lp/mm 0.2 0.1 0 0 10 20 30 40 50 Working distance (mm)
Design DoF Measured DoF
5. Depth of field
Lighthouse uses the slanted edge MTF method described above to measure depth of field. In a chip on tip endoscope, the focus is typically at a fixed location. The Depth of field can be quantified by measuring the MTF at regular distances from the tip of the endoscope. A plot of MTF at the optimal frequency vs distance forms the depth of field function. If the optimal MTF frequency has been chosen correctly, values below 50% MTF will begin to appear blurry. Lighthouse defines depth of field as the range of working stances by which the MTF is greater than 50% at the optimal MTF frequency.
Common specifications of endoscopes
Parameter 2mm 4mm Arthroscope 10mm Laparoscope Cystoscope/Hysteroscope Field of View 90 degrees (Diagonal) 105 degrees (Diameter 70 degrees (Diagonal) of image circle) Direction of 0 degrees 30 degrees 0 degrees View F/# F/4 F/7 F/7 Optical MTF > 50% at 50 lp/mm MTF > 50% at 50 lp/mm MTF > 50% at 50 lp/mm Resolution Depth of Field 2 to 20mm, MTF > 50% at 50 3 to 30mm, MTF > 50% 15 to 50mm, MTF > 50% lp/mm at 50 lp/mm at 50 lp/mm
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Summary + conclusions
In summary, the image quality and performance of chip on tip endoscopes can be measured using the right equipment and methods. Since chip on tip endoscopes incorporate an image sensor, measuring the optical performance alone is challenging. Using the scope's image sensor configured in a raw state to quantify MTF provides the ability characterize how the scope will perform when in use. Measurement of chip on tip endoscope optical parameters are not well described in the current ISO standards.
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