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11 Image Displays
11.1 Advantages and challenges of soft-copy reading 软拷贝阅读的优势和挑战
11.2 Impact of the human visual system on display design 人类视觉系统对展示设计的影响

• 11.2.1 Contrast sensitivity and spatial resolution 对比敏感度和空间分辨率
• 11.2.2 Contrast and dynamic range 对比度和动态范围
• 11.2.3 Color vision 色觉

最近，我正在研究与医学图像相关的书籍。建议您阅读上述标题。这是一本由西门子医疗系统许多医学专家联合编写的医学教科书，从人体最基本的成像原理到MRI，CT所有介绍了所有基本结构。

本专栏主要是阅读本书、翻译、解读和做一些阅读笔记。
如果专业医务人员需要阅读这本书，请支持正版和原版。这篇文章会有偏差，仅供爱好者学习参考。

Somewhat more that one hundred years after the invention of the cathode ray tube(CRT) by Karl Ferdinand Braun in the year 1897, electronic image displays are a standard tool of primary diagnostic interpretation in the field of radiology. The increasing use of information technology, including the networking of radiology departments to provide remote and comprehensive access to critical information, has helped to bring about a surge in diagnostic productivity. This development has been accompanied by an opportunity to reduce healthcare costs. In an electronic and networked environment, the electronic display device represents the important interface between the image acquisition system and the medical user. What is lost during the acquisition or image reconstruction process can naturally not be recovered at the moment of display.The relevant information available in the image data need not be lost on account of the display process.
在卡尔·费迪南德·布劳恩于1897年发明了阴极射线管(CRT)一百多年后，电子图像显示器成为放射学领域主要诊断和解释的标准工具。信息技术的应用越来越多，包括为关键信息提供远程和全面访问，这有助于提高诊断生产率。随着这一发展，有机会降低医疗成本。在电子和网络环境中，电子显示设备代表图像采集系统和医疗用户之间的重要接口。在采集或图像重建过程中丢失的东西在显示失的东西。图像数据中可用的相关信息不需要因显示过程而丢失。

The digitization of imaging modalities began with the advent of computed tomography in the 1970s. At present, projection radiography – the modality that presumably meets the highest information content demands per image – is undergoing a transition to digital detectors. This development has lead to a similar digitization in the area of image display. Digital imaging has the advantage of enabling one to independently optimize image acquisition systems and output media (i.e. the display) so as to meet the requirements of the radiological diagnostic process.
成像模式的数字化始于20世纪70年代计算机断层扫描的出现。目前，投影射线照相术——这种可能满足每幅图像最高信息含量需求的方式——正在向数字探测器过渡。这一发展导致了图像显示领域类似的数字化。数字成像的优点是能够独立优化图像采集系统和输出介质(即显示器)，以满足放射诊断过程的要求。

An image displayed on an electronic display device is referred to as soft copy while thefilm image is referred to as hard copy.
In the present chapter, we offer a brief review of the human visual system (HVS) as it relates to soft-copy reading before moving on to present an overview of the most commonly used display technologies, including cathode ray tubes (CRT) and liquid crystal displays (LCD). We then venture a glance at a number of potential future technologies before concluding the chapter with a technical characterization of monitors.
显示在电子显示设备上的图像称为软拷贝，而电影图像称为硬拷贝。本章简要回顾了人类视觉系统(HVS)，因为它与软复制阅读有关，然后介绍最常用的显示技术，包括阴极射线管(CRT)液晶显示器(LCD)。然后，在本章以显示器的技术特性结束之前，我们大胆浏览了一些潜在的未来技术。

• Digital image processing 数字图像处理
  Given that the data are provided in digital form by imaging acquisition systems, it is obvious that digital image processing methods can be applied directly to the images without first having to digitize them, as wold be the case for film radiographs.Examples of such processing include contrast enhancement by applying look-up tables and edge enhancement by filtering. The image display can be customized to various applications, depending on the circumstances involved. For instance, the display can be designed for a specific task, for a specific user, for a specific organ, or for specific ambient light conditions. Furthermore, the importance of electronic displays suited to the task of visualizing data from three-dimensional methods is likely to increase.
  鉴于图像采集系统以数字形式提供数据，显然数字图像处理方法可以直接应用于图像，而无需首先将其数字化，这类处理的例子包括通过应用查找表进行对比度增强和通过过滤进行边缘增强。根据所涉及的情况，图像显示可以根据不同的应用进行定制。例如，显示器可设计用于特定任务、特定用户、特定器官或特定环境光照条件。此外，适用于三维方法数据可视化任务的电子显示器的重要性可能会增加。

• Elimination of film problems 消除胶片问题
  Replacing film by soft-copy reading will also help to eliminate specific film problems. These include environmental problems associated with film chemistry, lost studies and incorrectly exposed films. Siegel [11.1] has reported that the rate of lost examinations dropped by more than a factor of 10 and the image retake rate dropped from 5% to around 0.8% following the introduction of picture archiving and communication systems (PACS).
  用软拷贝阅读代替胶片也有助于消除特定的胶片问题。这些问题包括与胶片化学相关的环境问题、丢失的研究和错误曝光的胶片。Siegel[11.1]报告说，检查丢失率下降了10倍多，图像重拍率从5%下降到0左右。在引入图片存档和通信系统（PACS）后，增长8%。

• Information technology 信息技术
  The replacement of film is only one aspect of the transition to the filmless hospital. A major additional aspect centers on the provision of access to crucial information via modern information technology. The simultaneous and instantaneous availability of images at different sites in a networked environment is one of the most important benefits. The healthcare process will include immediate access not only to images, but also to all relevant patient information.
  更换胶片只是向无胶片医院过渡的一个方面。另一个主要方面是通过现代信息技术提供对关键信息的访问。在网络环境中，不同站点上的图像同时和即时可用是最重要的好处之一。医疗过程不仅包括对图像的即时访问，还包括对所有相关患者信息的即时访问。

• User interface 用户界面
  The major differences in the handling of images on electronic displays of film radiographs will necessitate the creation of user interfaces for specific tasks. Strategies for the handling of images (e.g. their presentation and storage) will have to be developed. The transfer of images via networks will have to be fast enough to avoid time delays. Similarly, the reading process will have to be sufficiently quick for softcopy interpretation to have a clear value. All of these challenges will have to be addressed as health care professionals make the transition to primary soft-copy reading in radiology.
  在胶片射线照片的电子显示器上处理图像的主要差异将需要为特定任务创建用户界面。必须制定处理图像的策略（例如，图像的呈现和存储）。通过网络传输图像必须足够快，以避免时间延迟。同样，阅读过程必须足够快，以便软拷贝解释具有明确的价值。随着医疗保健专业人员向放射科初级软拷贝阅读过渡，所有这些挑战都必须得到解决。

• Image quality 图像质量
  The crucial prerequisite, however, for the acceptance of soft-copy diagnosis is image quality. Do electronic displays provide sufficient image quality to render all of the information necessary for a safe diagnosis? Which technical approaches should be applied to provide and secure the best possible image quality in clinical practice? To answer these questions, it will be helpful to first have a look at the human visual system (HVS).
  然而，接受软拷贝诊断的关键先决条件是图像质量。电子显示器是否提供足够的图像质量，以提供安全诊断所需的所有信息？在临床实践中，应采用哪些技术方法来提供和确保最佳的图像质量？要回答这些问题，首先看一下人类视觉系统（HVS）会很有帮助。

An overall optimization of the imaging chain presupposes a consideration of the human visual system because this system is a part of the entire system. In order to optimize reading performance, it will be necessary to understand how the HVS is influenced by specific reading circumstances and ambient light conditions (cf. chapter 1). The technical parameters of display devices will thus have to be matched to the characteristics of the HVS, which comprises the optical system of the eye and the information processing that takes place from the receptor cells in the retina to the visual cortex [11.2]. In what follows, we review some of the most relevant properties of the human visual system, including contrast sensitivity, spatial resolution and dynamic range [11.3].
成像链的整体优化以考虑人类视觉系统为前提，因为该系统是整个系统的一部分。为了优化阅读性能，有必要了解特定阅读环境和环境光照条件对HVS的影响（参见第1章）。因此，显示设备的技术参数必须与HVS的特性相匹配，HVS包括眼睛的光学系统和从视网膜中的受体细胞到视觉皮层的信息处理[11.2]。在下文中，我们回顾了人类视觉系统的一些最相关的特性，包括对比敏感度、空间分辨率和动态范围[11.3]。

Contrast sensitivity is defined by the smallest perceivable difference ΔL in the luminance of an object of luminance L in front of a background or surround luminance L0. It can be expressed as the inverse threshold contrast:
对比度灵敏度由背景或周围亮度L0前面亮度为L的物体亮度的最小可感知差值ΔL定义。它可以表示为反向阈值对比度：

Contrast sensitivity is usually determined via observer experiments involving certain test targets such as a square in the center of a screen in front of an area of constant luminance. It is the measure of the probability that a standard observer will be able to perceive this object. The task of experimentally determining contrast sensitivity is rather complicated because the result depends on the test pattern, its size, its shape and its spatial frequency content. Furthermore, contrast sensitivity depends on the particular background signal, the noise in the image itself and the observer.
对比度灵敏度通常通过观察者实验确定，该实验涉及某些测试目标，例如恒定亮度区域前屏幕中心的正方形。它是标准观察者能够感知这个物体的概率的度量。实验测定对比灵敏度的任务相当复杂，因为结果取决于测试图案、大小、形状和空间频率内容。此外，对比度灵敏度取决于特定的背景信号、图像本身的噪声和观察者。

The model introduced by Barten [11.4] seems to offer an appropriate description of contrast sensitivity as a function of the luminance and spatial frequency of a test pattern consisting of a square with sinusoidal modulation in front of a uniform background. Fig. 11.1 shows the calculated contrast sensitivity for this test pattern [11.5] of 2 degrees extension.
Barten[11.4]介绍的模型似乎提供了对比度灵敏度的适当描述，该对比度灵敏度是由均匀背景前正弦调制的正方形组成的测试图案的亮度和空间频率的函数。图11。1显示了该测试模式[11.5]的2度延伸的计算对比敏感度。

The curves are parameterized by background luminance values ranging from 0.1 cd/m2 to 1,000 cd/m2. The situation illustrated in fig. 11.1 represents the ideal case because it is valid for adapted observers who are not subject to threshold elevation on account of insufficient adaptation to a certain luminance, masking effects [11.6] or irritating extraneous light. The graph demonstrates that the contrast sensitivity of the HVS reaches its maximum for spatial frequencies between 3 cycles/degree and 6 cycles/degree. Assuming a viewing distance of 0.4 m, these values correspond to image object sizes of 1.2 mm and 0.6 mm, respectively.
曲线由范围为0的背景亮度值参数化。1 cd/m2至1000 cd/m2。图11所示的情况。1代表理想情况，因为它适用于适应的观察者，这些观察者由于对特定亮度、掩蔽效应[11.6]或刺激性外来光的适应不足而不受阈值升高的影响。该图表明，HVS的对比敏感度在3个周期/度和6个周期/度之间的空间频率下达到最大值。假设观察距离为0。4 m，这些值对应于1的图像对象大小。2毫米和0。分别为6毫米。

Contrast sensitivity for higher spatial frequencies rapidly decreases depending on luminance. The resolution limit is approached at roughly 30/degree, which corroborates the claim that the eye can resolve approximately 1 arc minute.
较高空间频率的对比度灵敏度根据亮度迅速降低。分辨率极限约为30/度，这证实了眼睛可以分辨大约1弧分钟的说法。

Contrast sensitivity for low spatial frequencies is independent of luminance in the case of medium to high luminance values. This is known as the Weber-Fechner law [11.7]. Contrast sensitivity decreases roughly according to L1/2 as the luminance is lowered below a few cd/m2. This dependency is known as the de Vries-Rose law [11.7]. This range marks the transition from photopic vision with cone receptors to scotopic vision with rods – where visual acuity is strongly impaired.
在中高亮度值的情况下，低空间频率的对比度灵敏度与亮度无关。这就是众所周知的韦伯-费希纳定律[11.7]。当亮度降低到几cd/m2以下时，对比度灵敏度根据L1/2大致降低。这种依赖关系被称为德弗里斯-罗斯定律[11.7]。这一范围标志着从带有锥体受体的明视视觉到带有视杆的暗视视觉的转变，在这种情况下，视力严重受损。

Contrast sensitivity can also be ascertained as a function of temporal frequency [11.6]. Contrast sensitivity for frequencies above a certain limit falls below the threshold of flicker perception. This critical flicker frequency amounts to approximately 70 Hz for commonly used luminance [11.8]. The frame rate of radiological monitors must exceed this limit in order to be flicker free.
对比敏感度也可以确定为时间频率的函数[11.6]。高于某个极限的频率的对比灵敏度低于闪烁感知阈值。对于常用亮度而言，该临界闪烁频率约为70 Hz[11.8]。辐射监测器的帧速率必须超过此限值才能无闪烁。

The HVS is capable of adapting to extremely different light situations, covering an absolute range in luminous intensity of several orders of magnitude. What is important for the diagnostic process, however, is the local dynamic range that can be perceived within a single radiograph [11.9]. Measuring the hyperpolarization potential V in vertebrate cone cells, Normann et al. [11.10, 11.11] showed that this photoreceptor response to a stimulus of luminance L can be expressed by the function where Vmax is a normalization constant and La approximates the adaptation luminance. In this equation, it is necessary to measure the luminance in units of cd/m2. These curves are plotted in fig. 11.2 for different adaptation luminance values.
HVS能够适应极端不同的光照情况，覆盖几个数量级的发光强度的绝对范围。然而，对于诊断过程来说，重要的是在一张X光片中可以看到的局部动态范围[11.9]。测量脊椎动物锥体细胞中的超极化电位V，Normann等人[11.10,11.11]表明，感光细胞对亮度L刺激的反应可以用函数表示，其中Vmax是标准化常数，La近似于适应亮度。在该方程式中，有必要以cd/m2为单位测量亮度。这些曲线如图11所示。2适用于不同的自适应亮度值。

Fig. 11.2 can be interpreted as follows: If the HVS is adapted to a certain luminance level, only a stimulus in the neighborhood of the adaptation level will cause the maximum response. Excitations either at the lower end or at the upper end of the adaptation range evoke a reduced response signal due to the smaller slope of the curves. Thus, contrast can be simultaneously perceived in a limited range of stimuli only.
图11.2可以解释为：如果HVS适应某一亮度水平，则只有适应水平附近的刺激才会引起最大响应。由于曲线斜率较小，自适应范围下端或上端的激励引起响应信号降低。因此，对比度只能在有限的刺激范围内同时感知。

Light scattering in the eye, expressed by the point-spread function [11.12, 11.13], and permanent involuntary eye movements known as microsaccades [11.11] lead to a further reduction in perceivable contrast.
由点扩散函数[11.12,11.13]表示的眼睛中的光散射，以及称为微痉挛的永久性不自主眼球运动[11.11]导致可感知对比度的进一步降低。

In summary, the perceivable contrast will be much smaller than expected in the ideal case. The phenomenon of adaptation to a complex image scene represents an essential consideration. Based on the model and experimental data underlying fig. 11.2, it is obvious that the contrast ratio (the ratio of maximum to minimum stimuli luminance that can be experienced in a single nonuniform image) will be hardly above 100. This contrast ratio value is smaller than the estimates of the best-case scenarios [11.5]. These indicate contrast ratio values of 200 to 300. However, the exact value naturally depends on the specific image and adaptation process.
总之，在理想情况下，可感知的对比度将远小于预期。适应复杂图像场景的现象是一个重要的考虑因素。基于图11所示的模型和实验数据。2.很明显，对比度（单个非均匀图像中可经历的最大与最小刺激亮度之比）几乎不会超过100。该对比度值小于最佳情况下的估计值[11.5]。这些表示对比度值为200到300。然而，确切的值自然取决于特定的图像和适应过程。

The dynamic range (i.e. the number of perceivable gray levels) is closely related to contrast ratio. The dynamic range is largely determined by the characteristic display curve that transforms the digital driving level or digital code value into luminance on the screen. The dynamic range can be maximized via a so-called perceptual linearization of the gray scale in order to provide equal luminance differences for equal input driving level differences over the full intensity range. However, this linearization process also depends on the image, image noise, the observer, the image’s surroundings and ambient conditions. Although several methods of perceptual linearization have been discussed [11.14, 11.15, 11.16], the DICOM [11.17] grayscale function can be expected to become the accepted standard. However, it should be pointed out that the objective of such a standard is only to establish a common image appearance on various display devices and not to optimize image quality.
动态范围（即可感知灰度的数量）与对比度密切相关。动态范围在很大程度上取决于将数字驱动电平或数字代码值转换为屏幕亮度的特性显示曲线。动态范围可以通过所谓的灰度感知线性化来最大化，以便在整个亮度范围内为相等的输入驱动电平差提供相等的亮度差。然而，这种线性化过程还取决于图像、图像噪声、观察者、图像的环境和环境条件。虽然已经讨论了几种感知线性化方法[11.14,11.15,11.16]，但DICOM[11.17]灰度函数有望成为公认的标准。然而，应该指出的是，该标准的目标只是在各种显示设备上建立通用的图像外观，而不是优化图像质量。

Although digital radiographs are generally black-and-white (B/W) images, it is also important to consider issues relating to color. B/W medical images can also be displayed on color monitors. Furthermore, some imaging modalities generate color images (e.g. ultrasound and nuclear medicine) and additional physiological and functional information can be overlayed in color on B/W images.
虽然数字射线照相一般是黑白的（B/W）图像，但考虑与颜色有关的问题也是重要的。黑白医学图像也可以显示在彩色监视器上。此外，一些成像方式产生彩色图像（如超声波和核医学），附加的生理和功能信息可以彩色叠加在B/W图像上。

Color vision is provided by the cone cells, a phenomenon that also offers the highest degree of spatial resolution. The eye is sensitive to light at wavelengths between 400 nm and 700 nm, with maximum sensitivity at 555 nm. The perceived image will therefore depend on the type of the display (i.e. on the type of the CRT phosphor or the color of the LCD backlight).
彩色视觉是由锥形细胞提供的，这种现象也提供了最高程度的空间分辨率。眼睛对波长在400纳米到700纳米之间的光敏感，最大敏感度为555纳米。因此，感知到的图像将取决于显示器的类型（即CRT荧光粉的类型或LCD背光的颜色）。

A blue-tinted backlight or a bluish phosphor (e.g. P45) has been found to be advanta-
geous for perceived sharpness. This phenomenon can be explained by the higher sen-
sitivity of the rod cells that control pupil size in the presence of green-blue light
[11.18]. A slightly blue background leads to a smaller pupil diameter, and therefore
higher acuity.
蓝色背光或蓝色磷光体（例如P45）被发现有利于感知清晰度。这种现象可以用控制绿蓝光下瞳孔大小的杆状细胞的更高灵敏度来解释[11.18]。稍蓝的背景会使瞳孔直径变小，从而提高视力。

As shown in fig. 11.3, the electronic display technologies currently in use for direct viewing include cathode ray tubes (CRT) and flat-panel displays (FPD). Not included in this overview are projection view displays and head-mounted devices.
如图11所示。3、目前用于直视的电子显示技术包括阴极射线管（CRT）和平板显示器（FPD）。本概述中不包括投影视图显示器和头戴式设备。

The classical device is the cathode ray tube (CRT). Unlike most consumer television sets, which are based on color tubes, high-fidelity CRTs (i.e. monitors offering high luminance and high spatial and contrast resolution) are based on black-and-white (monochrome) tubes. However, the high price of monochrome monitors has led to an increase in the use of color CRTs to display B/W medical images. This trend has also been fueled by the widespread use of the Internet to make images available on networked office computers.
经典的器件是阴极射线管（CRT）。与大多数基于彩色管的消费类电视机不同，高保真CRT（即提供高亮度、高空间分辨率和对比度分辨率的显示器）基于黑白（单色）管。然而，单色显示器的高价格导致彩色CRT用于显示黑白医学图像的使用增加。互联网的广泛使用也助长了这一趋势，使图像可以在联网的办公电脑上使用。

Liquid crystals are materials that exist in a partially liquid and partially solid physical state. When sandwiched between two glass plates, their optical properties can be altered by applying an electric field. This effect enables one to modulate the light that traverses the structure.
液晶是以部分液体和部分固体物理状态存在的材料。当夹在两块玻璃板之间时，它们的光学性质可以通过施加电场来改变。这种效果使人能够调节穿过结构的光线。

Having presented various display technologies, we turn in the present section to a discussion of the most important design specifications for ensuring monitor image quality.
在介绍了各种显示技术之后，我们将在本节讨论确保监视器图像质量的最重要的设计规范。

The standard-sized film radiograph is 14"×17" (35 cm×43 cm). The screens of common 21" monitors are slightly smaller, namely 30 cm×40 cm. Given that bigger CRT displays are heavier and are less energy efficient, the 21" screen size is considered to be a good compromise between desirable image size and other considerations. However, this could change with the introduction of space-saving flat-panel displays. The limiting factor today is the cost of a large-area LCD.
标准尺寸的胶片射线照片为14英寸×17英寸（35厘米×43厘米）。普通21英寸显示器的屏幕稍小，即30厘米×40厘米。鉴于更大的CRT显示器更重且能效更低，21英寸的屏幕尺寸被认为是理想图像尺寸和其他考虑因素之间的良好折衷。然而，随着节省空间的平板显示器的引入，这种情况可能会改变。今天的限制因素是大面积LCD的成本。

The assumption that contrast sensitivity (fig. 11.1) is also relevant to medical image viewing and diagnosis enables us to draw conclusions about what one can reasonably expect from a monitor with regard to spatial resolution. If the resolution limit of the HVS is considered to be 30/degree, the minimum object size Δx that can be perceived from a viewing distance of 0.5 m is 0.145 mm. This applies to the just discernable object for a standard observer. The corresponding number of TV lines is about 2,600 for an image height of 0.38 m, which is typical for a 20" portrait CRT or an 18" LCD. On the other hand, the monitors that offer the highest number of addressable pixels available today have 2,560×2,048 pixels, a quantity that fairly well matches what the HVS can perceive.
对比敏感度（图11.1）也与医学图像查看和诊断相关的假设使我们能够得出结论，即在空间分辨率方面，可以从监视器中合理期望得到什么。如果HVS的分辨率极限被认为是30/度，则从0的观察距离可以看到的最小物体尺寸Δx。5米等于0。145毫米。这适用于标准观察者的可辨别对象。图像高度为0时，相应的电视线数量约为2600条。38米，这是典型的20英寸纵向CRT或18英寸LCD。另一方面，目前提供最多可寻址像素的显示器有2560×2048像素，这一数量与HVS可以感知的相当匹配。

All of this assumes that the user is optimally adapted and that the monitor’s resolution, defined by the modulation transfer function, corresponds approximately to one display pixel or, in the case of a CRT, to one TV line. The latter is not a trivial requirement because the beam spot size on the screen, and thus the MTF, is different in horizontal and in vertical directions. It depends on the electron optics, on the phosphor spreading of the light generated by the impinging electron beam, on halation effects in the glass faceplate and additionally (in the horizontal direction) on the frequency response of the electronic circuitry [11.22].
所有这些都假设用户是最佳适应的，并且由调制传递函数定义的监视器分辨率大致对应于一个显示像素，或者在CRT的情况下，对应于一条电视线。后者不是一个简单的要求，因为屏幕上的光束光斑大小以及MTF在水平方向和垂直方向上是不同的。它取决于电子光学、撞击电子束产生的光的磷光体扩散、玻璃面板中的光晕效应以及电子电路的频率响应[11.22]。

Typical image formats used for medical displays are listed in table 11.1. The first column in this table gives the international format name, the second column lists the addressable pixels in each direction and the third column shows the total number of pixels.
表11列出了用于医疗显示器的典型图像格式。1.此表中的第一列给出国际格式名称，第二列列出每个方向上的可寻址像素，第三列显示像素总数。

Given that the beam spot is broader for higher beam currents [11.22], it is necessary to limit the maximum luminance of a CRT display to maintain spatial resolution. The maximum luminance (Lmax) of almost all currently available medical CRT monitors is between 200 and 600 cd/m2, depending on the transmissivity of the glass faceplate and on the luminous efficiency of the phosphor. To the extent that the highest spatial resolution is required for the diagnostic task at hand (e.g. for the primary diagnosis of digital radiographs), practical experience with common medical monitors indicates that the luminance should not be set to values higher than about 300 cd/m2.
考虑到束流越大，光斑越宽[11.22]，有必要限制CRT显示器的最大亮度，以保持空间分辨率。根据玻璃面板的透射率和磷光体的发光效率，目前几乎所有可用的医用CRT显示器的最大亮度（Lmax）都在200到600 cd/m2之间。在一定程度上，当前诊断任务需要最高的空间分辨率（例如，对于数字射线照片的初级诊断），普通医疗监视器的实际经验表明，亮度不应设置为高于约300 cd/m2的值。

The situation is different for LCDs. Here, the maximum luminance depends on the backlight and on the transmission coefficients of the various layers (cf. section 11.3.2).
液晶显示器的情况则不同。这里，最大亮度取决于背光和各层的透射系数（参见第11.3.2节）。

It follows from the above considerations that the lowest luminance (Lmin) should not be too low. One approaches the scotopic region (where vision is gradually impaired, threshold contrast increases, and detectability is reduced compared to the photopic region) at around 1 cd/m2.
根据上述考虑，最低亮度（Lmin）不应太低。一个接近暗视区（与明视区相比，在暗视区视力逐渐受损，阈值对比度增加，可检测性降低），约为1 cd/m2。

In the case of CRTs, there are several reasons that prevent Lmin from becoming very low in practical circumstances. Applying realistic images, light traversing the faceplate is reflected at the interfaces and scattered inside the system, consisting of glass, phosphor and aluminum layer. Light originating from bright parts of the image is thus scattered into adjacent dark parts (fig. 11.5). A similar effect occurs with respect to the backscattering of electrons inside the tube [11.29]. These effects lead to the phenomena of veiling glare and halation haze [11.30] and tend to increase the actual Lmin, depending on the image involved. The minimum luminance Lmin is furthermore increased by ambient light reflected from the boundaries of the faceplate and scattered [11.31]. Reducing these effects through the use of darkened glass and anti-reflection coating unfortunately reduces the transmissivity of the glass faceplate. The use of higher beam currents is therefore required to generate the same luminance. This, however, increases the engineering challenge and electron-optical design task of keeping the beam spot size small. Fortunately, modern state-of-the-art CRT monitors are equipped with these faceplates and high-performance electron tubes. Nevertheless, it is necessary to keep the ambient light to an appropriately low level in order to achieve a high contrast ratio. The tradeoff between resolution and maximum luminance for state-of-the-art CRT monitors is illustrated in fig. 11.10.

在CRT的情况下，有几个原因可以防止Lmin在实际情况下变得非常低。应用真实图像，穿过面板的光线在界面处反射，并在系统内部散射，系统由玻璃、磷光体和铝层组成。因此，来自图像明亮部分的光散射到相邻的黑暗部分（图11.5）。管内电子的后向散射也会产生类似的效应[11.29]。这些效应会导致遮掩眩光和光晕混浊的现象[11.30]，并倾向于增加实际Lmin，具体取决于所涉及的图像。最小亮度Lmin通过从面板边界反射并散射的环境光进一步增加[11.31]。不幸的是，通过使用深色玻璃和防反射涂层来减少这些影响会降低玻璃面板的透射率。因此，需要使用更高的光束电流来产生相同的亮度。然而，这增加了保持束斑尺寸小的工程挑战和电子光学设计任务。幸运的是，现代最先进的CRT显示器配备了这些面板和高性能电子管。然而，为了获得高对比度，有必要将环境光保持在适当的低水平。最先进的CRT监视器的分辨率和最大亮度之间的折衷如图11.10所示。

The increase in electron scatter due to the shadow mask makes the situation regarding glare even worse for color CRTs.
由于荫罩增加了电子散射，使得彩色CRT的眩光情况更加糟糕。

In the case of LCDs no electrons are scattered and less internal light scattering occurs. LCDs thus offer the advantage of low veiling glare. Here, minimum luminance is determined by the non-zero transmission in the off-state of a liquid crystal cell and by the reflected ambient light, as no anti-reflection coating is applied in most cases.
在液晶显示器的情况下，电子不会散射，内部光散射也会减少。因此，液晶显示器具有低遮蔽眩光的优势。这里，最小亮度由液晶盒在关闭状态下的非零透射和反射的环境光确定，因为在大多数情况下没有应用防反射涂层。

Measurements show that the contrast ratio Lmax/Lmin in realistic situations is usually lower than 300 and lower than what can be obtained under ideal conditions [11.31]. While the contrast ratio for LCDs is often specified as 600:1, this value can only be achieved in very dark rooms and for viewing directions that are perpendicular to the screen surface. Nevertheless, a contrast ratio of 200 to 300 well matches the contrast ratio that the HVS is able to process (cf. section 11.2.2). Regarding an X-ray film radiograph hanging in front of a light box, only those image parts whose optical density is between 0.4 and 2.4 can be recommended for diagnosis [11.5, 11.32]. Given the logarithmic relation between optical density and luminance, this corresponds exactly to a luminance contrast ratio of 100. The range of recommended optical densities for less demanding applications [11.32] is between 0.5 and 2.1 (corresponding to a luminance contrast ratio of 40).
测量表明，现实情况下的对比度Lmax/Lmin通常低于300，低于理想条件下的对比度[11.31]。虽然LCD的对比度通常指定为600:1，但该值只能在非常黑暗的房间和垂直于屏幕表面的观看方向上实现。然而，200到300的对比度与HVS能够处理的对比度相匹配（参见第11.2.2节）。对于悬挂在灯箱前面的X射线胶片射线照片，只有光密度在0之间的图像部分。4和2。4可推荐用于诊断[11.5,11.32]。鉴于光密度和亮度之间的对数关系，这正好对应于100的亮度对比度。对于要求较低的应用[11.32]而言，推荐的光密度范围介于0。5和2。1（对应于40的亮度对比度）。

The term dynamic range refers to the number of just noticeable gray levels. Assuming ideal conditions, the number of discernible gray levels in the luminance range under consideration should be larger than 500. It is unlikely in most realistic viewing situations – in which the threshold of the HVS is elevated by noise, surround luminance and ambient light – that smaller luminance differences than those corresponding to 256 gray levels will be perceivable in a complex radiological image [11.33].
术语“动态范围”指的是刚好可见的灰度级别的数量。假设在理想条件下，所考虑亮度范围内可辨别灰度级的数量应大于500。在大多数真实的观察情况下——在这种情况下，HVS的阈值因噪声、周围亮度和环境光而升高——在复杂的放射图像中不太可能看到比256灰度对应的亮度差更小的亮度差[11.33]。

The characteristic curve of a CRT monitor is essentially given by the voltage-to-current
relationship of the tube’s electron gun. While this relationship is highly nonlinear, it
is similar to a perceptually linearized display curve. However, one cannot expect the
intrinsic monitor curve to yield exact perceptual linearization. It is thus necessary to
maximize the dynamic range by choosing a proper display curve to achieve perceptual
linearization.
阴极射线管显示器的特性曲线基本上由电子枪的电压-电流关系给出。虽然这种关系是高度非线性的，但它类似于感知线性化的显示曲线。然而，我们不能期望固有的监视器曲线产生精确的感知线性化。因此，有必要通过选择适当的显示曲线来最大化动态范围，以实现感知线性化。

In digital systems, this can be done by applying a look-up table (LUT) to the digital driving levels. The application of a LUT may reduce the number of gray levels. Here, care has to be taken to avoid losing gray levels and thus generating artifacts. The application of LUTs, as with any other image processing steps, has to take place in the component with the highest available bit depth in order to avoid a loss of dynamic range.
在数字系统中，这可以通过对数字驱动电平应用查找表（LUT）来实现。LUT的应用可以减少灰度级的数量。在这里，必须注意避免丢失灰度，从而产生伪影。与任何其他图像处理步骤一样，LUT的应用必须在具有最高可用位深度的组件中进行，以避免动态范围的损失。

The image look has to be optimized to the specific task and user so as to provide optimal softcopy diagnostic capability. In general, this can be regarded as a kind of dynamic range maximization in the luminance range under consideration, and may also include spatial frequency processing.
图像外观必须针对特定任务和用户进行优化，以提供最佳软拷贝诊断能力。通常，这可以被视为所考虑的亮度范围中的一种动态范围最大化，并且还可以包括空间频率处理。

The electronic circuitry for the DAC and video amplifier in a CRT is the main contributor to temporal noise. The major source of spatial noise comes in the form of phosphor grain noise. Roehrig et al. [11.34] have shown that temporal display noise plays a role only in the case of very low luminance and that spatial noise dominates in the case of higher luminance values.
CRT中DAC和视频放大器的电子电路是产生时间噪声的主要因素。空间噪声的主要来源是磷光体颗粒噪声。Roehrig等人[11.34]已经证明，时间显示噪声仅在亮度非常低的情况下起作用，而空间噪声在亮度值较高的情况下占主导地位。

The phosphor grain noise power spectrum (NPS) can be estimated on the basis of measurements. Its impact on image quality has been assessed by the use of simulations and observer experiments [11.35]. Image display noise is of great interest and importance because it can mask small details, and thus elevate the detection threshold. The use of a low-noise phosphor (e.g. P45) is thus recommended when it comes to designing high-resolution monitors. Doing so helps one to avoid compromising image quality and to achieve the expected high spatial resolution [11.35].
磷光体颗粒噪声功率谱（NPS）可根据测量值进行估算。它对图像质量的影响已经通过模拟和观察者实验进行了评估[11.35]。图像显示噪声是一个非常有趣和重要的问题，因为它可以掩盖小细节，从而提高检测阈值。因此，在设计高分辨率显示器时，建议使用低噪声荧光粉（如P45）。这样做有助于避免损害图像质量，并达到预期的高空间分辨率[11.35]。

LCD noise can also be generated by driving electronics, luminance variations in the liquid crystal cell caused by variations of the electric field and cell thickness. The pixel substructure represents the most obvious contribution to spatial noise in an LCD (fig. 11.8). The noise behavior of modern flat-panel displays is still in need of clinical investigation.
液晶显示器的噪声也可以通过驱动电子器件产生，由电场和电池厚度变化引起的液晶电池的亮度变化。像素子结构代表了LCD中空间噪声的最明显贡献（图11.8）。现代平板显示器的噪声特性仍需临床研究。

In order to secure optimized image quality, it is necessary to consider the display unit in the context of the entire imaging chain. The display is only one component in the system and is required to provide the user with all of the information acquired by the imaging system. When making the transition from acquisition space to display space, image size, magnification ratio, spatial resolution and contrast can all be modified.
为了保证优化的图像质量，需要在整个成像链的上下文中考虑显示单元。显示器只是系统中的一个部件，需要向用户提供成像系统获取的所有信息。从采集空间过渡到显示空间时，图像大小、放大率、空间分辨率和对比度都可以修改。

Image size and magnification can usually be manipulated by software. These processes are referred to as zooming and panning. It is important for the display software to offer one the option of displaying the image (or perhaps a subsection of it) at full resolution, i.e. in a manner such that one pixel of the acquisition matrix corresponds to one pixel of the display matrix. Furthermore, in the interest of gaining an overall image impression, it should also be possible to display the whole image on a single screen (perhaps at reduced spatial resolution). The mathematical methods underlying these processes center on interpolation algorithms.
图像大小和放大率通常可以由软件控制。这些过程称为缩放和平移。对于显示软件来说，提供一个以全分辨率显示图像（或图像的一部分）的选项是很重要的，即。E以使得获取矩阵的一个像素对应于显示矩阵的一个像素的方式。此外，为了获得整体图像印象，还应该能够在单个屏幕上显示整个图像（可能以降低的空间分辨率）。这些过程的数学方法以插值算法为中心。

Image processing is not only applied to the acquired image (raw data processing), but also to the displayed image. For instance, in order to compensate for limited spatial resolution in the display, one can deploy a form of spatial frequency processing such as edge enhancement.
图像处理不仅应用于获取的图像（原始数据处理），而且还应用于显示的图像。例如，为了补偿显示器中有限的空间分辨率，可以部署一种形式的空间频率处理，例如边缘增强。

The contrast of the displayed ima