Precise measurement of internal transmittance of high absorbing filter glass

Optical filter glass plays an important role in many different applications like industrial equipment or health care. In some applications an exact knowledge of the transmittance of an optical filter glass is essential. However, in most cases the transmittance of a high absorbing filter cannot be measured. Thus, an exact value of the internal transmittance at a reference thickness is needed in order to calculate the transmittance for a much thicker filter glass sample. The calculation of the internal transmittance for optical glasses is well established. In the case of high transmittance the internal transmittance can be calculated using transmittance data originated from two different sample thicknesses. Filter glass does not fulfill this condition and the amount of absorption has an influence on the calculation of internal transmittance. SCHOTT presents a precise calculation method which takes into account the high absorption of filter glass based on a proposed special measurement technique. Typical measurement uncertainties are discussed and typical examples are presented. Finally recommendations for suitable sample thicknesses are given.