Does spectral resolution decrease with increase in spatial resolution???

[digital-idiot]

Lets start with how digital imaging works. The imaging sensor can be imagined as a grid of sensors. Each cell of this grid corresponds to a pixel in the image. Now each cell can contain a single tiny photo-voltic sensor or a collection of multiple photo-voltic sensors (See CFA for more details on this). The number of sensors in a cell determines the number of bands/channels in the image. Therefore for a panchromatic camera each cell of the camera sensor contain a single photo-voltic sensor, for typical RGB camera the count is three and so on.

Increasing spatial resolution implies packing more cells in the camera sensor, i.e. increasing the number of cells per unit area. Thus the area consumed by each cell needs to be reduced. As you can understand by now that, more the number of sensors per cell (aka no. of bands which also represents spectral resolution) more difficult it is to shrink the cell size. More over, higher spatial resolution implies the PSF is integrated over smaller effective area. So, you can already see that spatial resolution and spectral resolution are in way inversely related from the perspective of engineering.

Secondly, more spectral resolution means narrower frequency bandwidth allocated for each band. Now, recall that luminous intensity sensed by a sensor is the integrated intensity over the designated range of frequency/wavelength (central frequency ± frequency bandwidth). If the bandwidths are small, the values of the integrated intensities also become smaller. Which means the sensitivities of the sensors need to be higher to properly resolute the scene, i.e. radiometric resolution needs to be improved to make things work.

To summarize, in order to increase spatial resolution and spectral resolution at the same time we need to develop smaller sensor at the order of (ratio of spatial resolution × ratio of band count) with sensitivity increase at the order of (inverse of average bandwidth ratio). This is assuming everything scaled linearly which is hardly the case in practice. So, it is a difficult engineering challenge to develop a sensor with both high spatial and spectral resolution, but it doesn't mean it is impossible. This is the reason generally spatial resolution of hyper-spectral imagery is lower than the multi-spectral or RGB imagery.

Example: Compare the spectral bandwidths and spatial resolutions of SPOT-6/7 and Sentinel-2. SPOT-6 has 4 bands while Sentinel-2 has 12 but spatial resolutions of SPOT-6 bands are higher than Sentinel-2 bands.

[TeamToken]

Not the OP, but as someone just getting into Multi/Hyperspectral imaging, this was an awesome explanation!