Transmittance spectra in the visible range of potato starch granules in autochrome plates

Our test autochromes have various origins, different ageing states, and most probably, different manufacturers. This variability makes it valuable to perform spectrophotometric point measurements of their colour primaries at full spectral resolution (ca. 2 nm). For this purpose, the selected autochrome plates were analysed using a custom-built point-based micro-spectrophotometer. The measurement system (shown in Fig. 3) consisted of a Quartz Tungsten–Halogen lamp, two condensing lenses, a 40 microscope objective (Leica HC PL Fluotar), a fibre optic spectrophotometer (Ocean Optics USB2000+XR1-ES) with a measurement aperture of 400 µm, an XYZ linear stage, and a white foamboard. The microscope objective projects a magnified image of the illuminated autochrome plate onto the foamboard, through which the fibre optic is inserted. By adjusting the position of the plate using the linear stage, individual colour granules can be measured for spectral radiance and transmittance, with the corresponding bare light radiance serving as a reference.