几十年来无损检测（NDT）行业, film has been the Radiographic imaging method of choice. Its evolution and industry competition has driven film into a high quality-level with many options that cover virtually anything requiring radiography. Slow film is used for fine resolution options while faster speeds cover larger or less critical materials. Today, we have many new competitors attempting to dethrone film as the industry king. Many of these options share a single result: digital imaging.

The transition from traditional to digital images has been a gradual one with very small leaps and bounds. We’ve all, likely, been presented with it in one form or another, but recent NDT industry changes have put digital squarely in many of our future business plans. Projections of better quality along with promises of a much higher production are sparking the interest of service providers and fabricators alike. Before we understand the new technology we’re up against, let’s understand what digital imaging really is.

What is Digital Imaging for NDT?

如果我们要采用数字技术，去掉手把手的部分，我们就剩下数字成像了。计算机射线照相（CR）、数字探测器阵列（DDA）、线性二极管阵列（LDA）和其他“射线照相技术”导致数字成像。“成像介质”是辐射剂量的接收器，它赋予每种成像设备自己的“个性”。

While digital imaging is a constant to these processes, the images produced have their own characteristics based on practical applications. A CR image tends to have a high resolution, though can also suffer from noise. A DDA image, inversely, will tend to have a lower resolution, but should excel at noise reduction. Alternative applications may also have an effect.

信号、噪声和射线照相

管道上的计算机射线照相与钢铸件上的CR具有不同的噪声级。这就引出了我们的前两个主题：信号和噪声。信号是传输到成像介质的主要辐射束。然而，噪声是到达同一成像介质的任何额外能量的组合。光、散射辐射、电子噪声和光晕只是噪声的几个来源。任何技术都将根据设备的特性具有信号和噪声特性。理想情况下，100%信号的零噪声将是我们的目标，但由于噪声是由信号驱动的，反之亦然，这一目标不仅不切实际，而且不可能实现。不管我们的好意如何，所有的射线照相术都有噪音。一个成熟的过程应该给出一个最佳的信噪比，而不是完全消除它。

无损检测数字成像的最佳分辨率

Another factor we should understand, at least a little bit: resolution. Consider the television market as our example. Television screens are composed of a grid of tiny pixels making up the picture. Once high-definition TV’s took the market, we started discussions on resolution. 720P, 1080P, and now 4K are all what we would consider a total resolution, or the total of pixels in one line or one panel. In a 1080P television, the vertical lines of the screen are 1080 pixels long. There are then 1920 of those vertical lines, meaning that we have a resolution of 1920 x 1080, or 2.07 million pixels, in the panel.

如果我们考虑一台分辨率完全相同，但物理尺寸更大的电视，像素本身就必须更大。它每行仍有1080个像素，其中1920个像素在屏幕上，但每个像素需要覆盖更多的物理区域，以构成更大的屏幕尺寸。这迫使业界采用“像素间距”格式，而不是总像素。

像素间距描述了从一个像素的中心到下一个像素的中心的测量，消除了总像素计数作为我们的度量。这使分辨率标准化，而不需要图像大小，从而对不同大小的图像进行简单的分辨率比较。间距以微米或毫米的1/1000为单位，普通像素间距为50-200微米。理想情况下，您希望图像中尽可能多的像素。然而，过小的音高会产生更多的噪音，所以最小的并不总是最好的。一个强大的技术人员应该知道如何协商一个可接受的解决方案。

4096 Shades of Grey: Determining Radiographic Bit-Depth

We’ve established that pixels are laid out in a two-dimensional grid but haven’t discussed how to fill them. That’s where bit-depth comes into play. Each tiny pixel in an image has a ‘grey value’ represented by a number. The potential range of that number, known as the ‘dynamic range’, depends on the bit-depth of the image.

One bit is a piece of information containing either a 1 or 0 representing ‘on’ or ‘off’. If we take a second bit and string the two together, we can get the on/off positions, as well as two more in between. The two bits (2 bit-depth) could represent 00 or 11 but could also be 01 or 10. If we added another bit (3 bit-depth), the options could be 000, 001, 010, 100, 011, 110, 101, or 111. The amount of options doubles each time we add one bit.

Now, let us consider these shades of grey from white to black rather than an on/off scenario. We’ll also be discussing this at a much higher bit-depth. A bit-depth of 12 would give us 2¹²options or 4096 shades of grey while 16 bits would give us 2¹⁶or 65,536 shades of grey. A set of all zeros would result in no exposure, or white, and a set of all ones would result in complete saturation, or black. The remaining options in between would be shades of grey.

如果我们要对一个厚度变化很大的铸件进行射线照相，16位会给我们一个范围，把它们全部放在一个图像上。厚度变化最小的焊缝在12位时会更好，因为高范围对该应用没有好处。位元深度越高，辐射暴露越大，噪声越大，分辨率越高。许多应用程序将看不到更高范围的好处，所以12位通常是足够的。需要大量细节的射线照相可能需要14-16位设置，而不考虑额外的噪声。这是技术人员需要做出的决定。

Digital Imaging is the Future of NDT

显然，数字图像中有许多选项和功能，这可能是我们大多数人读过的最常见的图片。然而，简单的事实是：数字成像是我们的未来，我们需要为此做好准备。

Film will be obsolete eventually and an ounce of prevention is worth a pound of cure. The earlier the industry gets ahead of the curve, the more time and money our clients will save later. Moreover,正确培训无损检测技术人员可以开发sound program in the long run.

Computed Radiography Webinar

有兴趣了解更多关于无损检测和数字成像的未来吗？点击下面的链接，与作者Wesley Soape一起注册一个网络研讨会。