1.1 This test method describes an apparatus and procedure that is suitable for measuring the transmissivity of large, thick, or curved transparent parts including parts already installed. This test method is limited to transparencies that are relatively neutral with respect to wavelength (not highly colored). 1.2 Since the transmissivity (transmission coefficient) is a ratio of two luminance values, it has no units. The units of luminance recorded in the intermediate steps of this test method are not critical; any recognized units of luminance (for example, foot-lamberts or candelas per square metre) are acceptable for use, as long as use is consistent. 1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.