Problem about float texture!

How to construct a float texture using normal byt iamge data?
I used following code:

 glBindTexture( GL_TEXTURE_2D , floatTexture ) ;

 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

glTexImage2D(GL_TEXTURE_2D, 0,
    GL_FLOAT_RGB32_NV, 128, 128, 0, GL_RGB, GL_UNSIGNED_BYTE,

textureImage->data);

where textureImage->data is normal byte image data. But, when I rende
somthing with this floatTexture, all is black! Any onw has idea about that?

Dongsheng