$$
% abbreviations
\gdef\inf{\infty}
\gdef\w{\omega}
\gdef\W{\Omega}
%---
% roman variables
\gdef\d{\mathrm{d}} % differential, dx
\gdef\o{\mathrm{o}} % outgoing, ie w_o
\gdef\i{\mathrm{i}} % incoming, ie w_i
\gdef\p{\mathrm{p}} % point p
%---
% differentials
\gdef\dx{\d x}
\gdef\dy{\d y}
\gdef\dw{\d \w}
\gdef\dW{\d \W}
% ---
% trigonometry
\gdef\costheta{\cos \theta}
\gdef\costhetai{\cos \theta_\i}
\gdef\costhetao{\cos \theta_\o}
\gdef\cosphi{\cos \phi}
\gdef\sintheta{\sin \theta}
\gdef\sinphi{\sin \phi}
\gdef\abscostheta{\lvert \costheta \rvert}
\gdef\abscosthetai{\lvert \costhetai \rvert}
\gdef\abscosthetao{\lvert \costhetao \rvert}
% ---
% differential areas
\gdef\dAperp{\d A^\perp}
\gdef\dA{\d A}
% ---
% radiometric quantities
\gdef\Li{L_\i}
\gdef\Lo{L_\o}
% ---
% differential radiometric quantities
\gdef\dE{\d E}
\gdef\dLo{\d \Lo}
% ---
% differential angles
\gdef\dtheta{\d\theta}
\gdef\dTheta{\d\Theta}
\gdef\dphi{\d\phi}
\gdef\dPhi{\d\Phi}
% ---
% solid angles
\gdef\wi{\w_\i}
\gdef\wo{\w_\o}
% ---
% differential solid angles
\gdef\dwi{\d\wi}
\gdef\dwo{\d\wo}
\gdef\domega{\d \w}
\gdef\dOmega{\d \W}
\gdef\domegaperp{\d\w^\perp}
$$
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