You need not to use loop for this:
c0=2.997*10.^8;
h=6.625*10.^-34;
k=1.38*10.^-23;
n=1;
T=307;
filter=linspace(1,1,1000);
fun=@(lambda)(2*pi.*h.*(c0.^2))./((lambda.^5).*(exp((h.*c0)./(k.*T.*lambda))-1));
k=linspace(1*10.^-6,20*10.^-6,1000);
plank_vec=fun(k);
If you want to use loop:
plank_vec = zeros(1,length(k)) ;
for i = 1:length(k) ;
plank_vec(i)=fun(k(i));
end
