Spectral Probing Multi-Spectral Imaging by Optimized Wide Band Illumination

We present a novel active imaging approach that uses optimized wide band filtered illumination to obtain multispectral reflectance information. Our optimization algorithm utilizes light source and camera spectral information in order to maximize the signal strength and the robustness to noise. Through the use of active wide band illumination, our system can obtain material reflectance information in the presence of moderate (indoor) unknown ambient illumination. Our method is very simple and does not require special equipment. It can be used by professional and amateur photographers alike to obtain material properties and to synthesize captured scenes under arbitrary illumination. 1 Problem, Contribution and Limitations We seek a very simple approach to multi-spectral imaging to be used by amateur and professional photographers in everyday environments without special equipment. Multi-spectral reflectance data allows better color constant rendering and spectral re-lighting under arbitrary illumination, which is important for applications such as recording and archiving for cultural e-Heritage and digital museums[15]. We address the following problems: (i) Filter Selection Optimal filter selection for digital photography is a difficult problem, even for trichromatic imaging [9]. In this paper, we selected 16 of 208 possible filters. (ii) Ambient Illumination analyzing the spectral properties of the radiance emitting from an object using a passive multi-spectral camera with ∗This work was done while Cui Chi was a visiting student at Microsoft Research Asia. wide field of view [17, 18] does not usually reveal the actual reflectance properties of the material as the irradiance at each object point is generally unknown1. Moving the object to a dark room or scanning it using a spectrometer that measures a single point (while shielding it from stray light) is time consuming and may not always be practical. In this paper we approach these problems by probing the scene using active illumination. The main contributions of this paper are: (i) we provide a truly simple and affordable2 approach to multi-spectral imaging using only a camera, a light source (slide projector or a flash) and a set of theatrical color filters. (ii) we propose an algorithm for selecting an optimized set of filters from a large set of wide band filters. (iii) we show that by modulating the illumination we can greatly reduce the effect of ambient light, including spatially varying ambient illumination. The main limitation of our active method, is that it cannot be used to photograph distant objects, such as landscape, and cannot be used in direct sunlight. However, our filter selection method is still applicable to these circumstances. Related work Multi-spectral imaging is an extremely wide field of research ranging from material classification [3] and color measurement [20] to terrestrial imaging [22, 21] and astronomical imaging [6]. In this paper we only focus on aspects of multi-spectral imaging that are related to computer vision and image based rendering. In particular, we 1This would require complete knowledge about the light sources, geometry of the scene and BRDF of the object. 2The entire set of filters used in this paper costs less than USD$1. in ria -0 02 64 80 4, v er si on 1 18 M ar 2 00 8 Author manuscript, published in "Proceedings of the First International Workshop on Photometric Analysis For Computer Vision PACV 2007, Rio de Janeiro : Brazil (2007)"