An improved tristimulus sample cup of the type that generally includes an optically transparent bottom portion, a side wall portion and an open mouth. Such cups have a cavity for holding the sample, the cavity being defined by the bottom and side portions of the cup. The side wall portion has a surface which is exposed to the cavity. The improvement of the present invention relates to the optical properties of this surface of the side wall portion of the cup which is exposed to the cavity, i.e., that it have a diffuse reflectance at 580 nanometers of preferably more than eighty percent. In addition, if the sample cup is equipped with a plunger positioned in the mouth of the cup, the plunger having a surface exposed to the cavity, then the plunger surface exposed to the cavity should also have a diffuse reflectance at 580 nanometers of preferably more than eighty percent. A highly preferred cup of the present invention is made with a flat optical glass bottom, sides of white TEFLON . brand PTFE and a plunger of white TEFLON brand PTFE. The primary benefit of the present invention is improved color sensitivity and precision when a somewhat translucent, lightly colored granular or pelletized sample is analyzed.
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