A spectral radiation gas detector has at least one lenslet with a circular blazed grating for diffraction of radiation to a focal plane. A detector is located at the focal plane receiving radiation passing through the at least one lenslet for detection at a predetermined diffraction order. A plurality of order filters are associated with the at least one lenslet to pass radiation at wavelengths corresponding to the predetermined diffraction order, each filter blocking a selected set of higher orders. A controller is adapted to compare intensity at pixels in the detector associated with each of the plurality of order filters and further adapted to determine a change in intensity exceeding a threshold.