Abstract: This paper proposes a calculation method of infrared detector illuminance non-uniformity when the detector is directly exposed to an extended-area blackbody. The method is compared with solid-angle nonuniformity calculation method, establishing a framework for evaluating illuminance non-uniformity of infrared detector assemblies. The calculation results demonstrate that with a fixed exit pupil diameter, the difference between the two methods is reduced as the height between exit pupil and focal plane decreases. For a 320×256 detector with 30μm pixel pitch, when the exit pupil diameter varies from 5mm to 25mm with a pupil-to-focal-plane height of 20mm~25mm, the difference between the two methods is not more than 1%. The difference between the two methods exceeds 5% when the height is 5mm. Notably, exit pupil diameter exhibits greater influence on detector non-uniformity under direct extended-area blackbody irradiation conditions.