# Design and Construction of an LED-Based Excitation Source for Lock-In Thermography
## 1 Introduction
In their work, they compare halogen lamps, LEDs, and VCSELs in terms of modulation bandwidth, spectrum, and irradiance. The work shows the clear advantage of LEDs and VCSELs with respect to frequency stability. The theoretical minimal irradiance, which is necessary to obtain a clear thermographic signal on various types of coated and uncoated metals, is compared on the basis of a set of technical assumptions. The authors conclude that the LED sources are mainly limited by their irradiance compared to VCSEL. While the disadvantages of halogen lamps are often noted, they offer some distinct advantages over other sources. Halogen lamps are powerful and inexpensive sources, which are easy to obtain. They can be easily integrated into a setup without the need of expensive periphery. Moreover, they can be safely operated, in particular in comparison with high-power lasers. Yet, the maximum modulation frequency as well as the dynamic range are limited by the high thermal inertia of halogen lamps. In comparison, they feature low spatial resolution and often exhibit inhomogeneous excitation. The focusing and modulation of halogen lamps via optics and shutters is often experimentally tedious. In addition, the emission of halogen lamps has a spectral overlap with the infrared detection range. Powerful halogen lamps often induce a high amount of ambient heating, which potentially causes temperature drifts during the experiment. On the other end of the price range are powerful laser systems such as DPSS (diode-pumped solid state lasers), gas laser, high-power laser diode bar modules, and VCSEL (vertical-cavity surface-emitting laser). For different laser types and modulation frequencies, various modulation schemes can be applied. This includes the modulation of the driving current of diode laser or diode pumped lasers, the employment of acousto-optic modulators, galvanometers, external shutters, or the use of Q-switched lasers. While this makes the usage of lasers very versatile and customizable, each combination of laser and modulation scheme comes with its own challenges. For example, modulation of a diode laser by modulating its pump current is comparatively easy. However, the nonlinear nature of the laser emission might lead to nonlinear behaviour of the optical output power in the non-equilibrium stage after changing the pump current. As a result, the temporal shape of the optical output power is distorted if not taken into account properly. Nevertheless, lasers offer high optical power with a small etendue, enabling precise shaping of their beams by optical elements. This could be used to generate specific excitation pattern or tightly focused spots. However, damage thresholds have to be taken into account, as high local excitation powers are not be suitable for all samples [14]. While the experimental challenges of using a laser system are one thing, the safety issues when working with high-power lasers are another. High optical powers and tightly confined beams make thorough enclosure of the experiments a necessity. A good compromise with respect to price-to-performance ratio is the use of LEDs. While LEDs are not as bright as lasers, the output power can be controlled in a wide range via the driving current. However, the use of a conventional LED-based luminaire is often not practical. This is usually due to the restrictions of the included driver-circuit as well as the limited radiant intensity. A work on LEDs for non-destructive testing by [PERSON] et al. [23] uses water-cooled phosphor-based LEDs for long-pulse excitation and lock-in thermography. The paper notes the advantages of LEDs in terms of price, technical complexity, and emission spectrum. However, the paper also notes problems with residual infrared emissions in the camera detection range and heat sink design. Certain monochromatic LEDs, for example in the infrared spectrum (with emission outside the camera detection sensitivity), have increased efficiency and are less temperature sensitive as no phosphor for light conversion is necessary.