Color negatives at the demise of silver halides

The directional arrangement of the illumination plays an important role on image contrast and sharpness of silver-based photographic film. This paper explores how the directional arrangement of light (directed or diffuse) affects cinematographic colors. The experimental results show that a consistent color difference can be observed between the image projected on screen (directed illumination) and the image acquired with a scanner (diffuse illumination), no matter how well the scanner is calibrated. This fact has to be properly considered in film scanning and color correction; otherwise early color films can be notably distorted during the digitization process. Introduction For many decades the public exhibition of motion pictures took place exclusively in cinema theaters. Projection on cinema screens has always been the supreme display of film productions, even when ‘home cinema’ became the most common modality of film consumption. The extreme image magnification of movie projection requires a very high luminous flux on the screen. To increase the luminous flux, in traditional film projectors the light source is located in the focus of a parabolic mirror, which directs the light towards the condenser. The condenser consists of a set of lenses that focus the light further and direct it to the main lens assembly, which images the photographic emulsion on the reflecting cinema screen. Directed illumination is the term that will be used here to describe how a film is illuminated in a projector (Fig. 1-a). To date, almost all cinema theaters around the globe have converted to digital projection, and traditional film projectors are only used by a restricted circle of film enthusiasts. Therefore, the easy access to the content of film reels relies only on their digital replica, which is supposed to create a digital visualization that matches the cinematographic images created by ‘old-style’ film projectors [1]. The vast majority of film scanners create digital replicas of cinematographic images illuminating the film with a light diffuser (e.g. opal glass, or integrating sphere), which provides a diffuse illumination (Fig. 1-b). This type of illumination has the advantage of easily obtaining a uniform illumination across the film gate, and reduces the appearances of blemishes, such as dust and scratches. The directed illumination of film projectors and, on the other hand, the diffuse illumination of film scanners are exact opposites as for the arrangement of ray directions. In directed illumination each point of the film receives light from only one direction, while in diffuse illumination each point of the film receives light from all directions. Some film types illuminated in these two opposite manners create quite different images (e.g. Fig. 1-c and 1-d), and this difference can lead to digital replicas with bad color reproduction. Figure 1. Below: Schemes of directed (a) and diffuse (b) illuminations. Above: Corresponding images of a photographic film (c and d respectively). The film is a print of “Das Cabinet des Dr. Caligari” (R. Wiene, 1920) that underwent metallic toning. This paper highlights the importance of the type of illumination in film digitization, exploring how the directional arrangement of light affects cinematographic colors, providing a contribution to the development of new approaches for the digitization and restoration of film colors. Scientific background The directional arrangement of the rays illuminating a silver-based photographic image plays a fundamental role in its sharpness and contrast [2]. Figure 1 depicts the two different types of illumination at the bottom, and shows the corresponding acquired images at the top. The illustration 1-a depicts the setup in which the film is illuminated by means of a condenser, which provides aligned light rays (directed illumination). Contrarily, in illustration 1-b the film is illuminated by means of a diffuser, which provides scattered light rays (diffuse illumination). The resulting images of a metal-based photographic film adopting the two types of illumination have different sharpness and contrast. In directed illumination the image (Fig. 1-c) is much ‘crisper’ and scratches are emphasized; in diffuse illumination the image (Fig. 1-d) appears ‘softer’, with smoothed details and lower contrast. This discrepancy has been known over a long period of time in black-and-white still-photography, observing the differences between the prints created with condenser or diffuser enlargers. The phenomenon became known as ‘Callier effect’, named after the scientist who defined the Q-factor, i.e. the ratio between the 188 © 2017 Society for Imaging Science and Technology optical densities of a photographic film measured in directed and diffuse illuminations [3]. The Callier effect is determined by the scattering phenomena at the silver particles, which are known to be wavelength-dependent [4]. As a consequence, the images created with directed and diffused illuminations not only differ in sharpness and contrast, but they also have different colors (as the images in Fig. 1). To date, no study has been published on the spectral dependence of the Callier effect (probably due to the fact that this color shift has no influence in the printing of black-and-white photographs). The present work fills this gap by investigating experimentally the relation between the Q-factor and the wavelength of light. The results of this study are particularly interesting for several types of early color films. The images on “modern” color film (i.e. chromogenic monopack) are constituted of dyes that scarcely scatter light; therefore these images do not significantly change with the type of illumination. On the other hand, in case of photographic color images comprised of scattering particles (usually metallic silver) [5], for instance additive screen processes, such as Autochrome and Dufaycolor, or applied colors, such as hand and stencil coloring, as well as tinted and toned films, light is intensely scattered and a strong Callier effect is produced. If this phenomenon is not properly considered, the appearance of applied colors, which were used in early cinema for metaphorical associations and to articulate the narrative structure, risks to be altered significantly by the digitization. In fact, the different illumination manner determines an intrinsic color mismatch, which is not related to the capability of the film scanner to accurately measure colors. Measuring setup The experimental method used here was to acquire two multispectral transmittance images of a photographic film using directed and diffuse illuminations. The measuring setup is depicted in Figure 2. The light was provided by a plasma lamp, coupled with a liquid light guide and a collimating adapter, generating a parallel broadband light beam. An aspheric condenser lens focused the light into a linear variable interference band-pass filter, where the center wavelength of the passed band shifts linearly across its length. Two other aspheric lenses focused the spectrally selected diverging beam coming out of the filter (FWHM = 20 nm) at the principal plane of the camera objective. The imaging system consisted of a 65 mm f/2.8 macro lens and a 16 Megapixel full-frame CCD monochrome camera. Each multispectral image was created combining 31 images between 400 and 700 nm, with a spectral step of 10 nm. The calculation of film transmittances was done with a flat-field correction, referring to the blank images without film and dark images. A light diffuser could be inserted just before the film to be imaged (see Fig. 2), switching from directed to diffuse illumination. Figure 2. The measuring setup adopted for the acquisition of the multispectral images Spectral dependence of the Q-factor The Q-factor (Q) is the curve obtained by plotting the ratio between the optical densities of a photographic film measured in directed (OD∥) and diffuse (OD∦) illuminations as a function of