# Measurement of Contrast and Spatial Resolution for the Photothermal Imaging Method
## 1 Introduction  
Recently, the photothermal imaging method has been studied as an industrial tomographic tool. It acquires morphological information from the inside of an object by means of the photothermal effect of a temporally modulated heat source [1; 2; 3; 4]. The photothermal effect is a physical phenomenon that occurs when a material absorbs light energy and converts it to thermal energy [5; 6]. As shown in Figure 1a, when a material is heated by a temporally modulated pump beam, the temperature of both the air layer on the material surface together and the material itself increases as heat is conducted from the material to the air. In addition, the refractive index of the air layer changes with temperature because of the density change. It is possible to achieve a modulated temperature change through the refraction of the probe beam that passes through the air layer. The physical phenomena that result from the photothermal effect are based on the thermal properties of the target material to be measured. Hence, the phase delay between the period of the refraction of the probe beam and that of the heat source (light energy) must be analyzed in order to acquire information on the thermal properties of the specimen [7; 8; 9; 10; 11]. This method can be applied to imaging. As illustrated in Figure 1b, when the surface of a material is scanned by the pump beam and probe beam, the phase delay changes as the thermal properties are changed by the distribution of internal materials.  
This photothermal imaging method has been studied for more than ten years by [PERSON], [PERSON], and [PERSON] [12; 13; 14], since [PERSON] and [PERSON] published a study on how to acquire morphological information on a subsurface internal structure from the phase delay [4]. The related studies to the photothermal imaging method have been focused on the possibility of detecting subsurface structures using photothermal signals (phase delay). Previous studies have shown that the photothermal imaging method is experimentally feasible. However, there have been attempts to study the quality of images acquired from the photothermal imaging method. Therefore, the limitation of measurement in photothermal imaging has not been investigated or reported.  
The applicability of the developed imaging method depends on the image quality for the image of the internal structure acquired from the imaging method. Image quality is represented by spatial resolution, contrast, sharpness, and other metrics, and the scope of imaging method applicability depends on these factors [15; 16; 17].  
Image quality varies with the type of imaging method, specifically owing to the configuration of the components of the relevant equipment, the characteristics of the object to be measured, and the measuring system operator's skills. Analyzing how such factors affect image quality not only requires comprehensive and complicated tasks, but is also of great importance in evaluating a given imaging method [13; 14; 15].  
The image quality analysis techniques and processes are well established. However, the quality of images acquired from the photothermal imaging method has not been quantitatively analyzed. Moreover, no one has investigated whether the image quality analysis techniques can be applied to photothermal imaging. The present study was carried out to analyze image quality by focusing on the contrast and spatial resolution of an imaging method that utilizes the photothermal effect. In addition, the factors that influence the acquired images from the photothermal imaging method are investigated, expecting to provide experimental measurement conditions for the photothermal imaging method.