A method of determining the transmittance of a transparent article (250) includes the steps of obtaining a measurement of a first intensity of electromagnetic radiation reflected or emitted by reference surface (80) with an intensity measuring device (400), positioning the transparent article (250) over the reference surface (80), obtaining a measurement of a second intensity of electromagnetic radiation transmitted through the transparent article (250) that is reflected or emitted by a region (110) of the reference surface (80) that is covered by the transparent article (250) with the intensity measuring device (400); and calculating the transmittance using the measurements of the first intensity and the second intensity.