Medical laboratory instrument principles, structure and maintenance

Author: Chief Editor: Liu Fengjun
Publisher:
Publishing Date: 2002-01-01
Features:
Content Summary This book primarily introduces the principles, structures, and maintenance of common medical testing instruments. The entire book is divided into 12 chapters, covering topics such as biological microscopes, photometric colorimeters, spectrophotometers, automatic biochemical analyzers, urine analyzers, pH meters and titrators, potassium-sodium ion analyzers, blood gas analyzers, blood cell counters, separation and analysis instruments, maintenance of medical testing instruments, and the management and metrology of medical testing instruments. It focuses on commonly used, large-scale, complex, and intelligent testing instruments. The book primarily explains the principles of instruments and systematically introduces the working principles of various medical testing instruments. To meet practical needs, it comprehensively covers various common testing instruments and, in the selection of typical instruments, prioritizes models that are widely equipped and well-used in hospitals with intermediate or higher levels. Additionally, the book emphasizes maintenance methods and the analysis of typical faults, combining general and special faults with general and systematic analyses to elevate instrument fault maintenance to a theoretical level. To popularize metrology knowledge of medical instruments and meet the practical needs of maintenance personnel, the book also includes content related to the metrology of medical testing instruments. This book is written as a textbook for clinical medical engineering (maintenance) programs in higher education institutions and can also serve as a reference for medical instrument operators, maintenance personnel, and management staff.
Excerpt:
Place the object AB slightly outside the object-side focal point F1 in front of the objective lens. Light emitted from the object AB is magnified by the objective lens O1 to form an inverted real image A'B' near the focal point of the eyepiece O2. Further magnified by the eyepiece O2, we obtain an inverted virtual image A”B” at the observer's distance of distinct vision.
II. The Optical System of the Microscope
The optical system is the most important part of a microscope, primarily consisting of four components: the objective lens, the eyepiece, the condenser, and the reflector. Strictly speaking, it also includes the light source, filters, slides, and cover slips.
(1) Objective Lens: The objective lens is the core component of a microscope, directly determining or influencing its imaging quality and optical performance. It is often the "heart" of the microscope.
1. Types of Objective Lenses Objective lenses can be classified based on their magnification powers:
(1) Low-power Objective Lens: Typically composed of a doublet lens, with a magnification of 3–6× and a numerical aperture (NA) of 0.04–0.15 (Figure 1-2a).
(2) Medium-power Objective Lens: Composed of two doublet lenses (known as List objective lenses), with a magnification of 5–25× and an NA of 0.15–0.40 (Figure 1-2b).
(3) High-power Objective Lens: Composed of a front lens and two doublet lenses (known as Amici objective lenses), with a magnification of 25–65× and an NA of 0.35–0.85 (Figure 1-2c).
(4) Immersion (High-power) Objective Lens: Its structure is more complex, with a plano-convex lens added between the front lens and the middle lens of the high-power objective lens. The front lens must be immersed in oil or water when in use, with a magnification of 90–100× and an NA of 1.2–1.5 (Figure 1-2d). It must be noted that immersion lenses are designed with specific immersion oils, which are provided with the instrument (distilled water excepted) and cannot be arbitrarily selected. After passing through the lens, the object is expected to form a clear image similar in shape and color to the original. However, due to various factors, the image's shape and color may differ from the ideal image. This difference is called aberration. The presence of aberration directly affects the clarity of the image or the similarity of the object image. Based on the causes and conditions of aberration, it is mainly divided into spherical aberration, chromatic aberration, astigmatism, coma, field curvature, and distortion. According to the correction of aberration, objective lenses can also be classified into achromatic lenses, apochromatic lenses, and plan lenses.
(1) Achromatic Lens: This type of lens corrects the axial color difference and spherical aberration at the axis and eliminates the sine difference at the paraxial point but cannot eliminate secondary spectrum aberration. It has a relatively large secondary spectrum and field curvature, making it unsuitable for important research work and microscopic photography.
(2) Apochromatic Lens: Some lenses in this type are made of flint glass, which provides better correction of secondary spectrum aberration, resulting in superior image quality. However, its magnification chromatic aberration cannot be fully corrected. When the magnification chromatic aberration exceeds 1%, it must be compensated with an eyepiece. Its field curvature remains relatively large. This type of lens has a complex structure, uses rare materials, and is difficult to manufacture, making it expensive. However, due to its excellent imaging quality, it has a larger numerical aperture at the same magnification, requires no stringent light source requirements, and is typically used in research-grade universal microscopes.
(3) Plan Lens: By adding a thick plano-convex lens to the system, the curvature of the image plane can be corrected, creating a plan lens. Compared to ordinary lenses, its main advantage is a significantly larger field of view, with a linear field of view on the image plane of up to 25mm, and an increased working distance. Plan lenses are often used for wide-field observation and microscopic photography.
2. Identifying Objective Lenses The characteristic parameters of the objective lens are usually engraved on the outer surface of the lens housing (Figure 1-3). In the figure, (a) is a 10× objective lens with an NA of 0.25, a mechanical tube length of 160mm, and a cover slip thickness of 0.17mm; (b) is a 100× objective lens with an NA of 1.25, an infinite mechanical tube length, and a cover slip thickness of 0.17mm, with "oil" indicating an immersion lens. Specific characters are also used on the lens housing to indicate optical properties such as plan, achromatic, or apochromatic performance. Thus, users can identify the performance of the lens by examining the housing.
(2) Eyepiece: The eyepiece of a microscope is essentially a magnifying glass used to observe the image magnified by the objective lens (the intermediate image). The structure of the eyepiece is simpler than that of the objective lens, typically composed of 2–6 lenses arranged in two or three groups. The upper end of the tube, which contacts the eye, is called the ocular lens, while the lower end, close to the field of view, is called the field lens or convergence lens, both of which serve to magnify. A field stop is set at the object-side focal plane of the eyepiece to limit the object-side field of view. The real image magnified by the objective lens is imaged on this stop surface, where an eyepiece micrometer or eyepiece pointer can be placed. The eyepiece belongs to a small numerical aperture, large field of view system, where axial aberration can be ignored, and off-axis aberration should be corrected. Common eyepieces used in microscopes include Huygens eyepieces, Ramsden eyepieces, compensating eyepieces, plan eyepieces, plan-compensating eyepieces, wide-angle eyepieces, and photographic negative eyepieces.