A method of updating stored calibration terms (1406, 812, 816) in an infrared imaging device, the method comprising the following steps: receiving image frames of a scene captured by the infrared imaging device; detecting (1504, 1600) a condition to initiate a virtual-shutter non-uniformity correction procedure to obtain NUC terms; obtaining the NUC terms by the virtual-shutter NUC procedure; converting (1516, 1808) the obtained NUC terms into updated calibration terms; comparing (1518, 1810) the updated calibration terms with calibration terms (1406, 812, 816) stored in a non-volatile memory (196, 1404) to determine whether to use the updated calibration terms or keep the stored calibration terms; and in response to determining that the updated calibration terms are to be used, writing (1520, 1812) the updated calibration terms into the non-volatile memory (196, 1404) to replace the stored calibration terms (1406, 812, 816) wherein the step of detecting (1504, 1600) the condition to initiate comprises: detecting (1602, 1604, 1606) that a host device comprising the infrared imaging device is holstered, and/or a field-of-view of the infrared imaging device is substantially blocked; and in response to detecting that the host device is holstered, and/or the field-of-view is substantially blocked, deciding to initiate the virtual-shutter NUC procedure to obtain the NUC terms without providing temperature controlled uniform black bodies, shutters or targets; and wherein the step of obtaining the NUC terms comprises: receiving one or more image frames of a substantially uniform scene provided by an external object acting as a virtual shutter; and processing the one or more image frames of the substantially uniform scene to generate the NUC terms. receiving image frames of a scene captured by the infrared imaging device; detecting (1504, 1600) a condition to initiate a virtual-shutter non-uniformity correction procedure to obtain NUC terms; obtaining the NUC terms by the virtual-shutter NUC procedure; converting (1516, 1808) the obtained NUC terms into updated calibration terms; comparing (1518, 1810) the updated calibration terms with calibration terms (1406, 812, 816) stored in a non-volatile memory (196, 1404) to determine whether to use the updated calibration terms or keep the stored calibration terms; and in response to determining that the updated calibration terms are to be used, writing (1520, 1812) the updated calibration terms into the non-volatile memory (196, 1404) to replace the stored calibration terms (1406, 812, 816) wherein the step of detecting (1504, 1600) the condition to initiate comprises: detecting (1602, 1604, 1606) that a host device comprising the infrared imaging device is holstered, and/or a field-of-view of the infrared imaging device is substantially blocked; and in response to detecting that the host device is holstered, and/or the field-of-view is substantially blocked, deciding to initiate the virtual-shutter NUC procedure to obtain the NUC terms without providing temperature controlled uniform black bodies, shutters or targets; and detecting (1602, 1604, 1606) that a host device comprising the infrared imaging device is holstered, and/or a field-of-view of the infrared imaging device is substantially blocked; and in response to detecting that the host device is holstered, and/or the field-of-view is substantially blocked, deciding to initiate the virtual-shutter NUC procedure to obtain the NUC terms without providing temperature controlled uniform black bodies, shutters or targets; and wherein the step of obtaining the NUC terms comprises: receiving one or more image frames of a substantially uniform scene provided by an external object acting as a virtual shutter; and processing the one or more image frames of the substantially uniform scene to generate the NUC terms. receiving one or more image frames of a substantially uniform scene provided by an external object acting as a virtual shutter; and processing the one or more image frames of the substantially uniform scene to generate the NUC terms.