###### Abstract  
Objective: Calibrated horizontal measurements (e.g., mm) from endoscopic procedures could be utilized for advancement of evidence-based practice and personalized medicine. However, the size of an object in endoscopic images is not readily calibrated and depends on multiple factors, including the distance between the endoscope and the target surface. Additionally, acquired images may have significant non-linear distortion that would further complicate calibrated measurements. This study used a recently developed in vivo laser-projection fiberoptic laryngoscope and proposes a method for calibrated spatial measurements. Method: A set of circular grids was recorded at multiple working distances. A statistical model was trained that would map from pixel length of the object, the working distance, and the spatial location of the target object into its mm length. Result: A detailed analysis of the performance of the proposed method is presented. The analyses have shown that the accuracy of the proposed method does not depend on the working distance and length of the target object. The estimated average magnitude of error was 0.27 mm, which is three times lower than the existing alternative. Conclusion: The presented method can achieve sub-millimeter accuracy in horizontal measurement. Significance: Evidence-based practice and personalized medicine could significantly benefit from the proposed method. Implications of the findings for other endoscopic procedures are also discussed. horizontal calibrated measurements; image distortion; laser calibration; flexible endoscopy; laser projection endoscope; high-speed videoendoscopy; instrumental voice assessment Article  
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of the vocal fold vibration using high-speed videoendoscopy (HSV) could be an ideal candidate for constructing computational models [1; 2; 3]. Such patient-specific models could advance personalized medicine in the fields of laryngology and speech-language pathology. Additionally, previous works have suggested vibratory characteristics and kinematic measures from laryngeal videos can be utilized for direct evaluation of treatment outcome [4; 5]. Reduction in post-treatment lesion size could be another objective measure for direct evaluation of a treatment outcome. Realization of such direct evaluation criteria could provide the required scientific evidence behind the efficacy of different interventions, and answer a recent call for evidence-based practice in the clinical assessment of voice [6; 7]. Performing spatially calibrated measurements could provide significant benefit for these applications. Accurate mapping of laryngeal diseases [8], studying the developmental aspects of vocal folds [9], the sex differences in the vocal fold morphology [10; 11], and the effects of singing on the morphology of the vocal folds [12; 13], are just few other examples among many possible applications that would significantly benefit from calibrated spatial measurements. Unfortunately, conventional images cannot be used for calibrated spatial measurements. Specifically, multiple factors may act as confounding variables for the length of an object in its image. To make a distinction between the actual length of an object and its length on the image, they will respectively be called mm length and pixel length for the rest of this paper. Everybody has experienced the inverse relationship between the pixel length of an object and its distance from the camera. Furthermore, the imaging system may exhibit specific non-linear distortions such that the pixel length of an object would depend on its spatial location [14]. Due to such factors, we cannot readily measure the mm length of an object from its pixel length. However, the existence of some _auxiliary_ information on the images could alleviate this. The _auxiliary_ information could come from some properly designed fiducial markers that are projected to the field of view (FOV).