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This question already has an answer here:

The panchromatic band on Landsat 8 OLI sensor (band 8) covers a narrower portion of the spectrum than the corresponding band in Landsat 7 ETM+: 0.50 - 0.68 micrometers (L8 OLI) in contrast to 0.52 - 0.90 micrometers (L7 ETM+) (see complete tables here). Most remarkably, L8 OLI band 8 does not cover the near infrared, thus precluding pan sharpening in that portion of the spectrum and making up a significant difference with the L7 ETM+ panchromatic band. Why?

In this interesting blog post, the author mentions the issue without being able to find the logic behind it, and citing the explanation given by Irons et al (2012) “The OLI panchromatic band, band 8, is also narrower relative to the ETM + panchromatic band to create greater contrast between vegetated areas and surfaces without vegetation in panchromatic images" as not clear or insufficient, and hinting to possible design glitches.

I am also left wondering about this and could not find a decent explanation anywhere. Does anyone in this forum know why?

Irons, J.R. et al. (2012) The next Landsat satellite: The Landsat Data Continuity Mission. Remote Sens. Envion.,122, 11-21. doi:10.1016/j.rse.2011.08.026

marked as duplicate by Andre Silva, tinlyx, whyzar, aldo_tapia, xunilk Jan 19 '18 at 23:59

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Forgive me if this is too basic an answer: panchromatic and infrared are mutually exclusive.

Panchromatic means all visible light, which is generally considered to range 0.4μm to 0.7μm in wavelength.

Near (or reflected) infrared energy is generally considered to range 0.7μm to 0.9μm in wavelength, just beyond visible.

See Infrared vs. Panchromatic - Mt. Reynolds, for example.

One might ask instead why the panchromatic band on Landsat 7 included near infrared.

Designers of remote sensing scanners have specific spectral windows in mind for each band. Someone else more au fait or more au courant (much more knowledgeable than me) should provide detailed reasons.