Measurement technology for lasers in medical applications
For any application, a focused laser beam should live up to previously defined specifications. But for medical technology, precise adherence to the parameters is of even greater importance. Both for regulatory and ethical reasons, the laser beam must be carefully tested throughout the entire value chain: from the development of the laser source to its application on the patient and/or production of medical products. Scrimping on the purchase price of the measuring device can quickly result in hidden costs that turn out to be far higher than any initial savings.
Measurement technology for lasers is basically divided into two areas. On the one hand, we have the pure measurement of a laser beam’s power or energy with the help of sensors. But on the other: Camera- or slit-based measurement methods make it possible to display the laser’s beam profile as well. The application at hand should be the deciding factor for which parameters need to be determined and which technology should be used. An essential feature for comparing different measuring instruments is their accuracy. A distinction is made here between absolute accuracy, which indicates the deviation from the true value, and repeatability, which compares values determined under the same conditions in successive measurements.
Power measurement – absolute vs. repeatable accuracy
Let's first take a look at energy and power measurement. Laser power is still an expensive commodity, even though the prices per W have fallen significantly in recent years. When a new laser is developed, the aim is to achieve the specified power as accurately as possible. The power should neither be too low, nor should there be too many reserves built in, to avoid unnecessarily increasing the price of the laser source. This fact can only be checked with measuring instruments that deliver results that are simultaneously precise and repeatable. Here, the prices make a clear difference, in terms of both absolute accuracy and reproducibility. The absolute accuracy of a power sensor depends essentially on its sensor technology, on the optical qualities (e.g. homogeneity and spectral behavior) of the coating of the absorber, and how well the calibration is matched to it. The ideal coating would be spectrally flat and would provide the same values at each wavelength; theoretically, calibrating the sensor to one wavelength would then suffice. In practice, however, the absorption behavior of the coating is different at different wavelengths. So, in order to achieve high absolute accuracy, the sensor needs to be calibrated to several wavelengths. This is how MKS Instruments attains absolute accuracies of ± 3—5% for its Ophir sensors (depending on sensor type), based on a NIST or PTB certified standard. Even this value can be improved upon by up to 1% with an individual OEM calibration that takes into account the user’s exact operating conditions. Based on the measurement results of precisely calibrated Ophir sensors, manufacturers can optimally specify the laser power and reduce the safety margins to a minimum.
Figure 1: Ophir offers a wide range of sensors and display devices for power and energy measurement (source: MKS Inst., Ophir).
An important but rarely discussed quality criterion is the sensor’s repeatability from one measurement to the next, or the comparability of several sensors of the same type. Here, too, the higher the measuring device’s tolerances, the more safety buffers the manufacturer of lasers or laser systems must build in – at great cost. With high-quality Ophir sensors, the standard deviations are well below 1%, when used in the same measuring environment in the per mil range. Particularly for companies that, for example, use several sensors in parallel in a production process, the differences in accuracy show themselves very quickly. At first glance, cheaper solutions often entail high follow-up costs. Consider for example the high-resolution thermal Ophir Sensor 3A: It has a 9.5mm aperture and is suitable for measurements ranging from 10μW to 3W and from 20μJ to 2J. The market comparison in the power range up to 3W shows that the acquisition costs of this sensor are above the average market price. And not without reason: Indeed, the Ophir 3A measures the laser beam with one sensor and the environmental influences in the housing with another concealed sensor. The instrument thus achieves a repeat accuracy of almost 100%. This quickly pays off for the user: For example, with ultra-short pulse lasers such as those used in ophthalmology, the series scattering from one laser to the next can be nearly eliminated.
Figure 2 and 3: The beam profile – in 2D (bottom) or 3D (top) – shows at first glance whether the beam deviates from the given definitions (source: MKS Inst., Ophir).
Analyzing beam profiles – clearly and precisely
Numerous applications in medical technology require more than just knowing the overall power or energy of a laser beam. Many more parameters play a role, such as power distribution, power density or focal shift. Measurement is performed by camera-based technologies using conventional CCD or CMOS cameras that are intelligently combined with optical components and powerful software. Here, too, the measurements show clear differences in quality, in terms of both absolute measurement results and reproducibility. In addition to the type of camera chosen by the instrument’s manufacturer, the software algorithms and optical components implemented exert a major influence on this.
A high-quality, camera-based measuring instrument even allows the measurement of tunable lasers, which can be adjusted from UV to IR – and still delivers reliable results. This, in turn, results in direct cost savings for the user, as only one measurement setup is needed for wide spectral ranges. The camera resolution should also be taken into account: If a laser system requires special beam shaping, the manufacturer must know the energy distribution within the beam. For example, if a homogeneous energy distribution in the laser beam is required for an application in, say, dermatology, the camera resolution must be correspondingly high. Otherwise, the power peaks are simply not displayed.
Measuring laser beams in medical applications is beyond question: It is absolutely essential to measure power, energy and/or beam profile along the entire value chain, starting with the laser source manufacturer through to the end user of the laser system, whether in medical production or patient settings. It is certainly worth one’s time to be very exacting when selecting a measuring instrument. Quantitative criteria must be taken into account, including the power and energy range to be measured, the aperture size in relation to the beam size and the operating temperature of the sensor. However, qualitative aspects must also be considered, as they can impact the quality of the end product and/or the success of the therapy on the patient. If one tries to save money on the quality of the measuring devices, this can either mean forfeiting any optimization potential or require a significantly higher effort in terms of time and cost to achieve the same laser beam quality.
Fig. 4 and 5: Measuring laser beams in medical applications is beyond question
Article originally published in German in MedEngineering 6/2019