Optical instruments are affected during their use by a number of different environmental parameters which they are required to resist without significant reduction in performance and to remain within defined specifications. The type and severity of these parameters depend on the conditions of use of the instrument (for example, in the laboratory or workshop) and on its geographical location. Individual parameters cause a variety of different and overlapping effects on instrument performance. The manufacturer attempts to ensure, and the user naturally expects, that instruments will resist the likely rigours of their environment throughout their life. This expectation can be assessed by exposure of the instrument to a range of simulated environmental parameters under controlled laboratory conditions. The severity of these conditions is often increased to obtain meaningful results in a relatively short period of time. In order to allow assessment and comparison of the response of optical instruments to appropriate environmental conditions, the standards of the ISO 9022 series contain details of a number of laboratory tests which reliably simulate a variety of different environments. The tests are based largely on IEC standards, modified where necessary to take into account features special to optical instruments. This part of ISO 9022 specifies the methods relating to the inspections of optical instruments including any additional assemblies from other fields (for example mechanical, chemical, and electronic devices), under equivalent conditions, for their ability to resist the influence of low pressure combined with cold, including the potential condensation and freezing of moisture, ambient temperature, and dry or damp heat. This part of ISO 9022 is applicable to optical instruments including any additional assemblies from other fields, designed for operation and/or transport in high mountainous areas or on board aircraft or missiles. The purpose of the testing is to investigate to what extent optical, climatic, mechanical, chemical, and electrical (including electrostatic) performance characteristics of the specimen are affected by combined low pressure and low, ambient, or high temperature. Furthermore, the additional effects of moisture condensing and freezing on the instrument or components can be determined. Examples are instruments which are installed or externally mounted on aircraft or missiles or transported inside aircraft or flying objects not providing any pressure equalization. This standard draws together and streamlines the previous Parts 5, 18 and 21 of ISO 9022 into a single Part 23 of the ISO 9022 standard series. In the course of that, the combined tests which could not be established in practice and have, therefore, become obsolete have been deleted. The committee responsible for this standard is NA 027-01-02 AA "Grundnormen der Optik" ("Fundamental standards for optics") of the Standards Committee Optics and Precision Mechanics (NAFuO) at DIN.