CRT and laser-based/////; recorders progressively expose each pixel onto film by electronically controlling the position and intensity of a CRT beam or by mechanically deflecting R, G, B laser beams. Recorders (and scanners) that deflect in both the X-axis and Y-axis use traditional registered pin film movements. Other laser recorders deflect in the X-axis only and rely on rolling the film smoothly in the Y-axis to record the film frame area. Once the mechanical stability problems are resolved, an advantage of laser-beam recorders is that they have sufficient light output to expose higher resolution lab intermediate film stocks. Film exposure times in existing film recorders vary from under ten seconds to several minutes per frame depending on the device and resolution.
It is important to address several issues before filming a CGI shot: how the computer will translate the calculated pixels into color exposure values and how the spectral emission characteristics of the cathode ray tube (CRT) or RGB laser beams will match the film sensitivity curves. The computer can define color values according to a system of hue, luminance and saturation, or according to a system of Red, Green and Blue values. In either case, three sets of numbers describe the color of each pixel in the final image. Color calibration, which is the relationship between the calculated color space and the actual film exposure, is achieved through the use of a color look-up table (CLUT), and other matrix transform color corrections.
The CLUT is a graph of film density plotted against calculated color exposure. The technician doing color calibration derives the CLUT from carefully plotted curves determined through densitometry of the exposed negative. Using the CLUT the technician matches the emission energy of the CRT or laser, combined with high-efficiency RGB filters, to provide exposure in the straight-line portion of the film exposure curve. The computer accomplishes this by translating color space numbers into the RGB exposure values determined from the color look-up table. It is possible, through the use of the CLUT, to precisely control film image contrast. It is often useful to use logarithmic representation for the pixel values. Logarithmic pixel values translate easily to logarithmic film density during calibration of scanning and recording devices.
One problem that is typical for high-resolution CRTs is the creation of an unwanted halo by internal glass reflections in the CRT faceplate. The halo affects the image in the form of an unwanted exposure surrounding the highlight areas. Techniques to reduce this problem include the addition of a neutral-density panel bonded to the surface of the CRT, the tinting of the CRT faceplate, and the bonding of a thick clear panel to the CRT faceplate.
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