Digitization of color negative films with LEDs lighting systems and DSLR cameras: a signal processing analysis

A photographic film carries the intrinsic information of an image by means of the 2D distribution of its optical density. In the framework of the strategies for cultural heritage conservation, one of the most important techniques is the digitization of color negatives, which is realized by means of the creation of digital images. It has to be pointed out that, unlike the case of many other cultural materials (such as prints, paintings, etc.), the color of a negative film is not directly related to the color that this medium intends to reproduce. Due to this fact, the digital reproduction of a negative film, rather than pursuing the optimal color accuracy, aims to maximize the information recorded. The ideal light source for the image acquisition of a 3-layer photographic film consists of three monochromatic lights whose wavelengths match the peaks of absorption of the three dyes of the film. When a digital image is acquired using a RGB device, such as a digital camera, as the emission bands of the source are narrow, the more expected to lower the correlation between the different channels and, consequently, the acquired color information should increase. For this reasons, the utilization of LEDs for the digitization of the color negatives has become widespread due to their relatively narrow emission bands. Nowadays, a very typical setup involves the usage of a LED lighting system and a DSLR camera.This paper evaluates quantitatively the performances the acquisition systems when different illumination system are adopted. In particular, the xenon lighting systems is compared to two systems based on LED lights demonstrating that LEDs lighting systems are more effective choice. This paper also suggests improvements to achieve better results.