In this thesis, an existing non-contact dermatoscope will be further developed on the basis of knowledge and experience, and established as a new prototype for dermatoscopy at the Hannover Institute of Optical Technologies (HOT).In this system, the light generated by a white LED is collimated and polarized by a lens system, and generates a homogeneous light spot at a distance of 60cm. By cross polarization, the light reflected directly onto the skin surface can be suppressed, so that only light reflected in deeper skin layers can pass through the analyzer, and contributes to the image information. Due to the difficult handling of the original device, the further developed (advanced) system was compactified and automated, taking into account the basic principle of non-contact dermatoscopy. The illumination unit used in the original non-contact dermatoscope was replaced with a newly constructed reflector in order to improve the brightness, and the homogeneity of the light spot in the target area. These two reflectors were measured with a near field goniophotometer to characterize the illuminance distribution. The conducted tests included the definition of an ideal setup of the lens system, both in practice, and in optical systems simulations by using Zemax. It could be shown that the reflectors improve the illuminance, and generates a homogeneous light spot in the target area, which homogeneously illuminates the image area of the camera. Furthermore, this system has been completely automated by providing automatic focus as well as adjustment of one of the polarizers (analyzer) used. For this purpose, a automatic focus lens was integrated on the existing objective and a mid range infrared distance sensor was installed into the system. By various tests, such as the determination of the resolution with the modulation transfer function, the new camera system was characterized. Based on this tests, the highest possible resolution was determined and the work area could be defined. In this work area structures of 30 μm can be resolved sharply. In addition to the automation of the focus, a stepper motor has been installed to control the analyzer. A program was written in LabVIEW, which controls all components, automates the image acquisition, and provides the possibility of image processing (blood contrast enhancement). Subsequently, the entire system has been mounted on a variably adjustable swivel arm, in order to improve the handling for the dermatologist.
Table of Contents
1 Introduction
1.1 State of the art
1.1.1 Dermatoscopy devices by HEINE
1.1.2 Dermatoscopy devices by FotoFinder
1.2 Contact dermatoscopy
1.3 The original prototype
1.4 Research question and aims
2 Theory
2.1 Design and functionality of a white-light-emitting diode
2.1.1 The functionality of an LED
2.1.2 Energy and momentum conservation at a pn-junction
2.1.3 The functionality and realization of a phosphor-based WLED
2.2 Focus tunable lenses
2.2.1 Functionality of the focus tunable lens
2.3 Basics of image formation and image quality
2.3.1 RGB-camera
2.3.2 Criteria for assessing image quality
2.3.3 Process for determining the image quality
3 Characterization of the CCD-camera with objective and focus tunable lens
3.1 Connection of the focus lens with the objective
3.2 Resolution
3.2.1 Determination of the subjectively perceivable resolution
3.2.2 Determination of the modulation transfer function to characterize the objective resolution
3.3 Depth of field
4 Characterization of the simulated and constructed reflector
4.1 The simulation with Zemax
4.1.1 The simulated reflector
4.1.2 Design and construction of the simulated reflector
4.2 Near field goniophotometer
4.2.1 Measurement with the near field goniophotometer
5 The program
5.1 The programming language LabVIEW
5.2 Program structure
5.2.1 The program flow
6 The further developed dermatoscopy device
6.1 Specifications of all commercially available components of the further developed prototype
6.1.1 Specification of the CBT-90 white LED
6.1.2 Specification of the camera Flea3 2.8 MP Color GigE Vision
6.1.3 Specification of the (automated) focus tunable lens EL-16-40-TC
6.1.4 Specification of the mid range distance sensor DT35
6.2 Design and operation of the further developed dermatoscopy device
7 Summary and outlook
Research Objectives and Topics
This thesis focuses on the further development and automation of a non-contact dermatoscopy device, originally established at the Hannover Institute of Optical Technologies. The primary goal is to enhance the device's handling for dermatologists while maintaining a resolution capable of resolving 30 μm structures, by integrating custom-designed reflectors for improved illumination and automated focus control.
- Design and optimization of a compact illumination unit using freeform optics and Zemax simulations.
- Automation of the device through the integration of a focus tunable lens and an infrared distance sensor.
- Implementation of a LabVIEW-based control program for automated image acquisition and post-processing (blood contrast enhancement).
- Mechanical redesign of the prototype to enable mounting on an adjustable swivel arm for improved patient comfort and ease of use.
- Characterization of the imaging system using modulation transfer function (MTF) analysis and near-field goniophotometry.
Excerpt from the Book
1.3 The original prototype
The original prototype (see fig. 1.3) uses a 2250 lm CBT-90 white LED by Luminus, Inc. (Sunnyvale, USA) as a illumination source (CBT90). The undirected radiation is captured and directed by a reflector type TYRA-S by Ledil Oy (Salo, Finland), which reduces the losses due to the wide radiation angle of the LED. The subsequent lens system consists of three lenses by Thorlabs Inc. (New Jersey, USA) (from right to left):
• Focal length 100 mm (S/N: LA1050-A)
• Focal length 60 mm (S/N: LB1723-A)
• Focal length 60 mm (S/N: LA1401-A)
These three lenses direct and limit the divergence and form the necessary collimation, such that at 60 cm distance a homogeneously illuminated image field is created. After thepropagating through lens system, the light passes through a polarizer, and reaches the target. The reflected light falls onto the CCD-camera type Flea3 2.8 MP Color GigE Vision by FLIR Integrated Imaging Solutions Inc. (Richmond, Canada) through an analyzer and the objective MVL75M1 by Thorlabs Inc. (New Jersey, USA). The smallest structures relevant to dermatoscopic diagnostics have a diameter of 30 μm - 180 μm (Meinhardt-Wollweber et al., 2017). In combination with the objective and the CCD-camera, structures of 19.7 μm size can still be resolved, hence, no information is lost due to poor resolution. Another challenge in dermoscopic imaging is the suppression of disturbing (unwanted) skin surface reflections. Due to the high intensity of light reflected directly at the surface, information from deeper layers is superimposed and lost. To avoid this, the so-called cross-polarization was installed in the pototypes.
Summary of Chapters
1 Introduction: Provides an overview of current dermatoscopy methods, identifies the limitations of contact-based devices, and introduces the original non-contact prototype as the basis for development.
2 Theory: Covers the physical principles of light-emitting diodes (LEDs), focus tunable liquid lenses, and the fundamentals of digital image acquisition, including CCD sensors and image quality assessment methods like MTF.
3 Characterization of the CCD-camera with objective and focus tunable lens: Details the integration of the focus tunable lens into the objective system and analyzes the resulting optical resolution and depth of field.
4 Characterization of the simulated and constructed reflector: Documents the simulation process using Zemax to design a compact reflector, its CNC milling production, and performance validation via goniophotometry.
5 The program: Explains the development of the LabVIEW-based control system, detailing the producer/consumer architecture used for automating camera control, image acquisition, and blood contrast enhancement.
6 The further developed dermatoscopy device: Presents the final prototype specifications, including all hardware components, the design of the integrated illumination unit, and its operation on an adjustable swivel arm.
7 Summary and outlook: Reviews the improvements made in the system's compactness, handling, and automation, and discusses potential future research directions like multispectral analysis.
Keywords
Dermatoscopy, Non-contact, White LED, Focus tunable lens, Zemax, Modulation transfer function, CCD-camera, Image processing, LabVIEW, Illumination unit, Cross-polarization, Skin cancer detection, Optical design, Automation, Reflector optimization.
Frequently Asked Questions
What is the primary objective of this thesis?
The primary objective is to further develop an existing non-contact dermatoscopy prototype by automating its focus and improving its illumination unit to enhance handling, reduce investigation time, and maintain high diagnostic resolution.
What are the central themes of the work?
Key themes include optical system design, automated image acquisition and control, digital image quality assessment using modulation transfer functions, and the practical implementation of ergonomic engineering in medical diagnostics.
What research question does the thesis address?
The core research question involves how to make a non-contact dermatoscopy system more compact, ergonomic, and automated while ensuring that diagnostic structures as small as 30 μm can still be accurately resolved.
What scientific methodology is utilized?
The methodology combines optical ray-tracing simulations (Zemax), experimental measurement of illuminance distribution (near-field goniophotometer), resolution characterization (USAF test chart and MTF), and software engineering (LabVIEW) for systems automation.
What does the main part of the work cover?
The main part covers the theoretical foundation of imaging and LEDs, the technical design and characterization of the custom reflector optics, the integration of an automated focus lens, and the development of a LabVIEW-based program for image management and processing.
Which keywords define this work?
Essential keywords include Dermatoscopy, non-contact imaging, focus tunable lens, Zemax, modulation transfer function, and LabVIEW-based automation.
Why is cross-polarization used in the prototype?
Cross-polarization is used to suppress disturbing reflections from the skin surface, allowing light reflected from deeper skin layers to be captured, which is crucial for clear dermatoscopic imaging.
What are the benefits of the custom-designed reflector?
The custom reflector provides a more compact illumination unit compared to the original bulky lens cage system, improves light collimation, and ensures a homogeneous light spot, which is essential for uniform skin illumination.
How does the automated focus lens function?
The focus tunable lens utilizes optical fluids and a polymer membrane. Piezo elements deform this membrane to change the lens's curvature, allowing the focal length to be adjusted rapidly based on distance data from an infrared sensor.
How does the LabVIEW program enhance blood contrast?
The program implements an algorithm that subtracts the red color channel from the green channel and differentiates by their sum, amplifying the visual contrast of hemoglobin to highlight vascular structures in the skin.
- Quote paper
- Thomas Hildebrandt (Author), 2018, Design, simulation, and construction of an illumination unit for non-contact dermatoscopy, Munich, GRIN Verlag, https://www.grin.com/document/442621
