How can I get borders and edges for transparent objects

%I am trying to get borders for transparent plastic object. but because of uneven light fail to % get it.My code is as follows. Please help

BW = imread('D:\Documents and Settings\sandeep\My Documents\MATLAB\black backgrnd.tif'); BW = rgb2gray(BW); BW = imadjust(BW,[.5 .7],[.3 .9],1.5); thresholdValue = 90;

BW = BW > thresholdValue;
[B,L,N,A] = bwboundaries(BW);
figure; imshow(BW); hold on;
for k=1:length(B),
  if(~sum(A(k,:)))
     boundary = B{k};
     plot(boundary(:,2),...
         boundary(:,1),'r','LineWidth',2);
     for l=find(A(:,k))'
         boundary = B{l};
         plot(boundary(:,2),...
             boundary(:,1),'g','LineWidth',2);
     end
  end
end

figure(2), imshow(BW);