5.1 The bispectral or two-monochromator method is the definitive method for the determination of the general radiation-transfer properties of fluorescent specimens (2). In this method, the measuring instrument is equipped with two separate monochromators. The first, the irradiation monochromator, irradiates the specimen with monochromatic light. The second, the viewing monochromator, analyzes the radiation leaving the specimen. A two-dimensional array of bispectral photometric values is obtained by setting the irradiation monochromator at a series of fixed wavelengths (μ) in the ultraviolet and visible range, and for each μ, using the viewing monochromator to record readings for each wavelength (?) in the visible range. The resulting array, once properly corrected, is known as the Donaldson matrix, and the value of each element (μ,?) of this array is here described as the Donaldson radiance factor (D(μ,?)). The Donaldson radiance factor is an instrument- and illuminant-independent photometric property of the specimen, and can be used to calculate its color for any desired illuminant and observer. The advantage of this method is that it provides a comprehensive characterization of the specimen's radiation-transfer properties, without the inaccuracies associated with source simulation and various methods of approximation.