The '''Minnaert function''' is a photometric function used to interpret astronomical observations<ref>{{cite journal | doi = 10.1016/S0019-1035(03)00075-7 | bibcode=2003Icar..163..150C | volume=163 | title=Probing Titan's lower atmosphere with acousto-optic tuning | journal=Icarus | year=2003 | pages=150–163| last1=Chanover | first1=N.J. | last2=Anderson | first2=C.M. | last3=McKay | first3=C.P. | last4=Rannou | first4=P. | last5=Glenar | first5=D.A. | last6=Hillman | first6=J.J. | last7=Blass | first7=W.E. | issue=1 }}</ref><ref>{{cite journal | doi = 10.1016/j.icarus.2006.05.006 | bibcode=2006Icar..184..401S | volume=184 | title=Martian phase function: Modeling the visible to near-infrared surface photometric function using HST-WFPC2 data | journal=Icarus | year=2006 | pages=401–423| last1=Soderblom | first1=J. | last2=Belliii | first2=J. | last3=Hubbard | first3=M. | last4=Wolff | first4=M. | issue=2 }}</ref> and remote sensing data for the Earth.<ref>{{cite journal | doi = 10.1080/01431160500104194 | volume=26 | title=The use of the Minnaert correction for land-cover classification in mountainous terrain | journal=International Journal of Remote Sensing | year=2005 | pages=3831–3851| last1=Blesius | first1=L. | last2=Weirich | first2=F. | issue=17 | bibcode=2005IJRS...26.3831B | s2cid=129750287 }}</ref> It was named after the astronomer Marcel Minnaert. This function expresses the radiance factor (RADF) as a function the phase angle (<math>\alpha</math>), the photometric latitude (<math>\varphi</math>) and the photometric longitude (<math>\lambda</math>).
:<math> \text{RADF} = \frac{I}{F} = \pi~A_M~\mu_0^k~\mu^{k-1} </math> where <math>A_M</math> is the Minnaert albedo, <math>k</math> is an empirical parameter, <math>I</math> is the scattered radiance in the direction <math>(\alpha,\varphi,\lambda)</math>, <math>\pi F</math> is the incident radiance, and :<math> \mu_0 = \cos\varphi~\cos(\alpha-\lambda) ~;~~ \mu = \cos\varphi~\cos\lambda ~. </math> The phase angle is the angle between the light source and the observer with the object as the center.
The assumptions made are: * the surface is illuminated by a distant point source. * the surface is isotropic and flat.
Minnaert's contribution is the introduction of the parameter <math>k</math>, having a value between 0 and 1,<ref>{{Cite journal|url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1941ApJ....93..403M&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf|bibcode=1941ApJ....93..403M|title=The reciprocity principle in lunar photometry|last1=Minnaert|first1=M.|journal=The Astrophysical Journal|year=1941|volume=93|page=403|doi=10.1086/144279}}</ref> originally for a better interpretation of observations of the Moon. In remote sensing the use of this function is referred to as ''Minnaert topographic correction'', a necessity when interpreting images of rough terrain.
== References == <references/>
Category:Observational astronomy Category:Photometric systems Category:Equations of astronomy