A protective enclosure for a thermal imaging device to be used in a hazardous environment, the thermal imaging device having a pupil plane and a lens, wherein the protective enclosure comprises: a housing having a proximal end, a distal end and a sidewall coupling the proximal end to the distal end; a window assembly located near the distal end of the housing, the window assembly comprising: a window providing passage for infrared radiation to the thermal imaging device within the enclosure; and a protective grid located on one side of the window at the distal end of the housing wherein the protective grid is exposed on one side to the environment and is exposed on an opposite side to the window, the protective grid comprising a plurality of intersecting bars, having a pitch and a bar width, characterised in that a) the cross-section of the bars is either round or rectangular and oriented normal to the grid plane, and the grid comprises an irregular pattern with the pitch increasing and/or the bar width decreasing from a center of the grid to a periphery thereof; or b) the grid comprises a regular pattern having a constant pitch and bar width from the center to the periphery, and each of the plurality of intersecting bars comprises a cross-section oriented at an angle such that a centerline through the cross-section is oriented toward a point on the pupil plane; such that a ratio of blocked beam to adjacent unblocked beam projected on the pupil plane remains constant thereby providing a constant average grid attenuation which may be compensated for with one transmissivity coefficient. the thermal imaging device having a pupil plane and a lens, wherein the protective enclosure comprises: a housing having a proximal end, a distal end and a sidewall coupling the proximal end to the distal end; a window assembly located near the distal end of the housing, the window assembly comprising: a window providing passage for infrared radiation to the thermal imaging device within the enclosure; and a protective grid located on one side of the window at the distal end of the housing wherein the protective grid is exposed on one side to the environment and is exposed on an opposite side to the window, the protective grid comprising a plurality of intersecting bars, having a pitch and a bar width, characterised in that a) the cross-section of the bars is either round or rectangular and oriented normal to the grid plane, and the grid comprises an irregular pattern with the pitch increasing and/or the bar width decreasing from a center of the grid to a periphery thereof; or b) the grid comprises a regular pattern having a constant pitch and bar width from the center to the periphery, and each of the plurality of intersecting bars comprises a cross-section oriented at an angle such that a centerline through the cross-section is oriented toward a point on the pupil plane; such that a ratio of blocked beam to adjacent unblocked beam projected on the pupil plane remains constant thereby providing a constant average grid attenuation which may be compensated for with one transmissivity coefficient. a housing having a proximal end, a distal end and a sidewall coupling the proximal end to the distal end; a window assembly located near the distal end of the housing, the window assembly comprising: a window providing passage for infrared radiation to the thermal imaging device within the enclosure; and a protective grid located on one side of the window at the distal end of the housing wherein the protective grid is exposed on one side to the environment and is exposed on an opposite side to the window, the protective grid comprising a plurality of intersecting bars, having a pitch and a bar width, characterised in that a) the cross-section of the bars is either round or rectangular and oriented normal to the grid plane, and the grid comprises an irregular pattern with the pitch increasing and/or the bar width decreasing from a center of the grid to a periphery thereof; or b) the grid comprises a regular pattern having a constant pitch and bar width from the center to the periphery, and each of the plurality of intersecting bars comprises a cross-section oriented at an angle such that a centerline through the cross-section is oriented toward a point on the pupil plane; a window providing passage for infrared radiation to the thermal imaging device within the enclosure; and a protective grid located on one side of the window at the distal end of the housing wherein the protective grid is exposed on one side to the environment and is exposed on an opposite side to the window, the protective grid comprising a plurality of intersecting bars, having a pitch and a bar width, characterised in that a) the cross-section of the bars is either round or rectangular and oriented normal to the grid plane, and the grid comprises an irregular pattern with the pitch increasing and/or the bar width decreasing from a center of the grid to a periphery thereof; or b) the grid comprises a regular pattern having a constant pitch and bar width from the center to the periphery, and each of the plurality of intersecting bars comprises a cross-section oriented at an angle such that a centerline through the cross-section is oriented toward a point on the pupil plane; such that a ratio of blocked beam to adjacent unblocked beam projected on the pupil plane remains constant thereby providing a constant average grid attenuation which may be compensated for with one transmissivity coefficient.