We examine how the luminance distributions of overlaid surfaces affect the perception of transparency of neutral density filters. Pairs of neutral density filters were generated overlying variegated backgrounds of varying luminance distributions, and observers adjusted a single parameter of one filter until the pair appeared equally transparent. Physically identical filters appeared equally transparent on similar backgrounds, but did not appear equally transparent when backgrounds differed in luminance or contrast. Reducing luminance or contrast of the background decreased perceived transparency of the overlaying filter by a multiplicative factor. Observers matched perceived transparency of physically dissimilar filters by applying a linear trade-off between reflectivity and inner transmittance. In a second experiment, filters had their spatial structure altered in order to abolish the perception of transparency and appeared as patterned opaque disks, and observers equated perceived contrast of the two overlaid areas. Match settings gave results similar to the previous experiment, indicating that, in general, perceived transparency corresponds closely to the perceived contrast of the overlaid region.