Transparency and imaginary colors

Unlike the Metelli monochrome transparencies, when overlays and their backgrounds have chromatic content, the inferred surface colors may not always be physically realizable, and are in some sense "imaginary." In these cases, the inferred chromatic transmittance or reflectance of the overlay lies outside the RGB spectral boundaries. Using the classical Metelli configuration, we demonstrate this illusion and briefly explore some of its attributes. Some observer differences in perceiving transparencies are also highlighted. These results show that the perception of transparency is much more complex than conventionally envisioned.