Influence of the laser spot size on the image quality of a CR system
Steve Jelfs, NDT Specialist, DÜRR NDT
Computed Radiography (CR) is a digital imaging technology where phosphor imaging plates (IP’s) take the place of conventional radiographic film. Once the IP’s are exposed to radiation, the CR scanner uses a focused laser to read the information from the imaging plate and transfer it to software that renders an image on a computer.
Computed Radiography was first introduced as a new technology in the mid-1970’s. Further development continued through the 1980’s and CR was embraced by the medical market during that decade.
CR scanners were crude by today’s standards and typically only had the ability to produce low-resolution images. This was due to several factors – the design of the scanner itself, the laser spot size and the technology of the imaging plates which had relatively large grains of phosphor.
As better technology was developed, CR transitioned into the industrial realm and started replacing film radiography in some limited applications. However, code-quality images were not possible until both improved scanner technology and more capable imaging plates became available.
Laser Spot Size
Many early industrial CR scanners had a laser focused to give a spot size of 50 microns at the imaging plate. This liberates an optimum amount of signal from the exposed plate and thus produces a high signal-to-noise ratio (SNR) and good contrast, but limits the basic spatial resolution (SRB) of the scanner and thus the size of indications that can be imaged.
Among the CR scanner developments to be introduced in the 2000’s was a smaller laser spot size. In conjunction with higher-resolution imaging plates this allowed smaller indications to be detected and thus weld-quality images to be produced. However, the smaller laser spot size reduces the signal liberated from the imaging plate, limiting image contrast and signal-to-noise ratio (SNR), and thus the utility of the scanner for certain applications.
Although laser spot size directly influences the Basic Spatial Resolution and Contrast of an image, it is not the only aspect of the system that can affect the final image quality.
High Resolution Requirements
Scan resolution or image resolution are measures of the size of the scanner’s samples or steps and the size of each pixel in the final image file. Although this affects final image quality, it is not fundamental to the system’s ability to image small indications. Basic Spatial Resolution (SRB) is a definitive measure of the whole imaging system’s ability to resolve fine details.
In order to achieve a higher SRB and thus allow imaging of smaller indications, a smaller laser spot is required. There are systems on the market that have small laser spot sizes specifically designed to support the highest possible resolution.
In order to achieve the highest Basic Spatial Resolution, both the scan resolution and the scanner’s laser spot must be correctly selected (as well as the imaging plate itself, of course). For example, scanning at an image resolution of 20 microns/pixel with a laser spot size of 50 microns is pointless – it results in a large image file but does not provide the required detail because of the large laser spot size. Conversely, scanning at a 100 microns/pixel resolution with a 12.5 micron laser spot also misses the mark, resulting in a low resolution and low contrast image.
If the laser spot is small enough to support it, selecting a high scan resolution will result in a high Basic Spatial Resolution that can be measured as part of the system’s classification to standards such as EN 17484-1:2005, ISO 16371-1:2011 or ASTM E2446-05 (2010).
Currently, there are only two CR scanners on the market that have a laser spot small enough to offer the highest Basic Spatial Resolution. Both of these scanners have been BAM certified to have a 30 micron/pixel SRB and both qualify for system class IP 1/30 or IP Special/30 according to the above standards.
High Contrast Requirements
For some applications image contrast may be more desirable than pure image resolution. In these cases, having a larger laser spot is beneficial since it gathers more information from the imaging plate by stimulating a larger area of the phosphor. In informational radiography applications such as erosion/corrosion or wall-loss evaluation, image resolution is not as important as shot time and efficiency. Having a larger laser spot in the scanner can reduce the dose required on the plate and thus speed up the whole process.
Typical CR scanners have a laser spot size that is fixed by the design of the system’s optical hardware. Thus the user is forced to make a decision between contrast and resolution when they purchase the system, and this cannot subsequently be changed.
If the user has a high-resolution system and wins an informational RT job, then it is likely that the scanner system cannot provide the most effective or efficient results. Conversely, and possibly worse, if the user only has a high-contrast system, they cannot expect to compete for high-resolution work because the system is simply incapable of performing to the level required.
As a solution to this problem, most CR manufacturers produce a system which compromises by fixing the laser spot size at an intermediate size (typically 25 or 30 microns). While this is acceptable for many applications, it cannot address high-resolution requirements as are typically found in aerospace or military applications, nor can it provide the efficiency and low-dose response which can make informational radiology tasks simple and profitable.
The ideal and most obvious solution to this problem is to have a CR scanner with an adjustable laser spot size – one which can easily address the entire range of applications that a typical user might encounter. It is therefore surprising that only one CR scanner manufacturer has expended the engineering effort to design such a system.
Revolutionary new CR scanner technology
The CR scanner designed to overcome the above limitations offers a laser with a variable spot size that can be adjusted to match the needs of the current application. With three laser focus settings – 50, 25 and 12.5 microns – this scanner is capable of addressing both high-resolution and high-contrast needs, as well as offering a mid-range setting that is applicable to many general-purpose radiography applications.
Inside the laser tube, an iris diaphragm adjusts the laser beam diameter via a simple software setting in the same way that other scanner control parameters are adjusted. Since the perfect laser spot can be selected in this way for each object and application type, the optimum results for any radiography requirement can easily be achieved. This convenience and flexibility means that one single piece of hardware is all that is needed to address multiple radiography applications, reducing the user’s investment and expanding the range of jobs that a single crew with a single piece of equipment can accommodate. Ultimately this can improve not only image quality and thus the customer’s satisfaction, but also profitability.
When the laser spot is set to 12.5 microns, the scanner can easily address the highest resolution applications, and as mentioned above is certified by BAM to have the highest Basic Spatial Resolution in the industry – 30 microns.
Additionally, simply selecting a larger laser spot size in the software automatically adjusts the diaphragm inside the scanner and produces a 25 micron or 50 micron laser spot, giving the higher contrast required for general purpose applications or for imaging thicker specimens using isotopes.
The graphic above shows the areas of application that are addressed by different laser spot sizes, and it can clearly be seen that having an adjustable laser vastly increases the utility of the scanner.
According to the ISO 17636-2 standard, there is a defined signal-to-noise ratio (SNR) required to test various material thicknesses to either Class A or Class B accuracy. Given that the SNR varies with laser spot size and dose on the imaging plate (and thus exposure time), it is evident that having that ability to adjust the laser spot itself allows the user to more easily optimize the inspection parameters while meeting the requirements of the standard.
Indeed, with a fixed laser spot size it may not even be possible to reach the SNR required by industry standards for every application. The flexibility of the CR system, and thus the user’s ability to successfully address and win multiple jobs, is compromised.
The ongoing advances in CR technology have resulted in a system that is unique in its ability to address multiple applications in an optimal manner. This allows the user to closely match scanner performance to industry standards and real-world needs, resulting in an efficient and cost-effective solution unparalleled in the CR world.
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