image becomes dark wen capturing using webcam

I have following code to take photo from a external cam

vid1 = videoinput('winvideo',1,'YUY2_320x240')
%     vid2 = videoinput('winvideo',2,'YUY2_320x240')
    % Set video input object properties for this application.
    set(vid1,'TriggerRepeat',Inf);
%     set(vid2,'TriggerRepeat',Inf);
    vid1.FrameGrabInterval = 1;
%     vid2.FrameGrabInterval = 1;
    set(vid1,'ReturnedColorSpace','rgb');
%     set(vid2,'ReturnedColorSpace','rgb');
    set(vid1,'FramesPerTrigger', 1000);
%     set(vid2,'FramesPerTrigger', 1000);
    temp1=zeros(240,320,3);
    temp2=zeros(240,320,3);
    % Create a figure window.
    h1=imshow(uint8(temp1));
    h2=imshow(uint8(temp2));
    pause(0.01)
    hold on;
    '---------'
    % Start acquiring frames.
    start(vid1)
%     start(vid2)
%     while(vid1.FramesAcquired<=10) % Stop after 100 frames
        tic
        data1 = getdata(vid1,1);
        imshow(data1)
%         data2 = getdata(vid2,1);
%         set(h1,'Cdata',data1);
%         set(h2,'Cdata',data2);
        flushdata(vid1);
%         flushdata(vid2);
        toc

%     end
    stop(vid1)
%     stop(vid2)
    delete(vid1)

 wen i capture the image becomes dark in colour like black  ,but wen i use preview(vid),the image is clear but wen captured it becomes very dark like black in colour,kindly help