Impact of atmospheric scintillation on detection probabilities for pointtarget and small targets at large ranges are studied in the infrared wavelength region. Useful parameters are developed that describes the size of the source and the receiver. Introducing Gaussian weights of the source and sensor aperture functions, analytical functions can be obtained for the rangeweighted variance. Four bands are being considered in the wavelength region 1.5 till 5.2 µm together with Cn2 measured from a scintillometer. Impact of weather parameters on the level of turbulence is also discussed. The results can be used in the analyses of detection probabilities of small targets.