An open path gas detector comprising: an optical element (126; 320; 520) operable to receive radiation from an objective optical element (116; 315; 515) and separate the radiation into a first radiation portion (325; 525) and a second radiation portion (345; 545); a first beam-shaping optical element (140; 335; 535) operable to receive the first radiation portion (325; 525); a filter (135; 350; 550) operable to receive and filter the second radiation portion (345; 545) to produce a second filtered radiation portion; and a second beam-shaping optical element (142; 355; 555) operable to receive the second filtered radiation portion, characterised in that the open path gas detector further comprises: a first photodiode (128a; 340; 540) comprising a first active sensing region operable to receive the first radiation portion (325; 525) from the first beam-shaping optical element (140; 335; 535); and a second photodiode (128b; 360; 560) comprising a second active sensing region operable to receive the second filtered radiation portion from the second beam-shaping optical element (142; 355; 555); wherein the first beam-shaping optical element (140; 335; 535) and the second beam-shaping optical element (143; 355; 555) are configured to provide a substantially uniform illumination intensity profile across an entire first active sensing region of the first photodiode (128a; 340; 540) and the active sensing region of the second photodiode (128b; 360; 560). an optical element (126; 320; 520) operable to receive radiation from an objective optical element (116; 315; 515) and separate the radiation into a first radiation portion (325; 525) and a second radiation portion (345; 545); a first beam-shaping optical element (140; 335; 535) operable to receive the first radiation portion (325; 525); a filter (135; 350; 550) operable to receive and filter the second radiation portion (345; 545) to produce a second filtered radiation portion; and a second beam-shaping optical element (142; 355; 555) operable to receive the second filtered radiation portion, characterised in that the open path gas detector further comprises: a first photodiode (128a; 340; 540) comprising a first active sensing region operable to receive the first radiation portion (325; 525) from the first beam-shaping optical element (140; 335; 535); and a second photodiode (128b; 360; 560) comprising a second active sensing region operable to receive the second filtered radiation portion from the second beam-shaping optical element (142; 355; 555); wherein the first beam-shaping optical element (140; 335; 535) and the second beam-shaping optical element (143; 355; 555) are configured to provide a substantially uniform illumination intensity profile across an entire first active sensing region of the first photodiode (128a; 340; 540) and the active sensing region of the second photodiode (128b; 360; 560). a first photodiode (128a; 340; 540) comprising a first active sensing region operable to receive the first radiation portion (325; 525) from the first beam-shaping optical element (140; 335; 535); and a second photodiode (128b; 360; 560) comprising a second active sensing region operable to receive the second filtered radiation portion from the second beam-shaping optical element (142; 355; 555); wherein the first beam-shaping optical element (140; 335; 535) and the second beam-shaping optical element (143; 355; 555) are configured to provide a substantially uniform illumination intensity profile across an entire first active sensing region of the first photodiode (128a; 340; 540) and the active sensing region of the second photodiode (128b; 360; 560).