Electrical and optical characteristics of the transparent magnetic recording layer in the Advanced Photo System

We report optical and recording properties typical of the transparent magnetic recording layer in the recently commercialized Advanced Photo System. In this system, a photographically transparent recording layer is provided on film base by a thin, dilute, reverse side coating containing well dispersed, cobalt surface-treated γ-Fe₂O₃ particles with high specific surface area and high coercivity. This layer also contains well-dispersed, sub-micron α-alumina particles for head cleaning and film durability purposes. Although both of these types of particles scatter and absorb some incident light, this is reduced by keeping the median particle size less than the wavelength of light and employing low, but effective, particle concentrations. The transmission of visible wavelengths by the magnetic layer is at least 70%. Under the restriction that the magnetic recording layer is about 1.2 micron thick and the volume fraction of magnetic and abrasive particles are each of the order 0.01, the optical density of the layer is linear with each particle concentration (mass/volume) or each particle laydown (mass/area) and the magnetic recording output signal amplitude is linear with the magnetic particle laydown