Image fusion techniques are widely used to integrate a lower spatial resolution multispectral image with a higher spatial resolution panchromatic image, such as Thematic Mapper (TM) multispectral band and SPOT Panchromatic images. However, the existing techniques either cannot avoid distorting the image spectral properties or involve complicated and time-consuming frequency decomposition and re-construction processing. A simple spectral preserve fusion technique: the Smoothing Filter-based Intensity Modulation (SFIM) has thus been developed based on a simpli ed solar radiation and land surface re ection model. By using a ratio between a higher resolution image and its low pass ltered (with a smoothing lter) image, spatial details can be modulated to a co-registered lower resolution multispectral image without altering its spectral properties and contrast. The technique can be applied to improve spatial resolution for either colour composites or individual bands. The delity to spectral property and the spatial textural quality of SFIM are convincingly demonstrated by an image fusion experiment using TM and SPOT Panchromatic images of south-east Spain. The visual evaluation and statistical analysis compared with HSI and Brovey transform techniques con rmed that SFIM is a superior fusion technique for improving spatial detail of multispectral images with their spectral properties reliably preserved. IntroductionFor optical sensor systems, image spatial resolution and spectral resolution are contradictory factors. For a given signal to noise ratio, a higher spectral resolution (narrower spectral band) is often achieved at the cost of a lower spatial resolution. Image fusion techniques are therefore useful for integrating a high spectral resolution image with a high spatial resolution image, such as Thematic Mapper (TM) (six spectral bands with 30 m resolution) and SPOT Panchromatic (SPOT Pan; panchromatic band with 10 m resolution), to produce a fused image with high spectral and spatial resolutions. For simplicity in this paper, the term 'resolution' is used as spatial resolution unless it is stated otherwise.A well-known fusion technique for RGB (red, green and blue) colour composite images is based on the hue, saturation and intensity (HSI) transform (Haydan et al. 1982 ). By replacing the intensity component of a lower resolution colour composite image (e.g. a TM colour composite) with a co-registered higher resolution image (e.g. a SPOT Pan), the technique produces a colour composite with an improved spatial

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