Abstract: An test cell is adapted for both making and testing samples. The cell includes a bottom plate and a top plate having concentric apertures defining a central test cavity. A post attached to the bottom plate closes off the bottom of the test cavity. A slide closes off the top of the test cavity. The top of the post is spaced slightly from the underside of the slide to define a test cavity of substantially uniform thickness. The test cavity is filled with phosphor suspended in uncured resin, closed, and the resin is cured. Once cured, the sample is stable, although delicate, and can be re-measured several times with reproducible results. The measurement takes place in the cell, using a thin film of oil for wetting surfaces.