how to get the raw 16 bit luminance data buffer from tiff object ?

[zy4944796]

Good day sir:
My trouble is about tiff format that be transformed by the function tiff.toCanvas.
The type of my tiff image is 16 bit grayscale, which the color format is luminance.
I defined a tiff object from a buffer like this :
var tiff = new Tiff({buffer: result.target.result});
var canvasTiff = tiff.toCanvas();
When I called tiff.toCanvas() function , I found the type of the tiff data which should be 16 bit grayscale was transformed to 8bit *4 RGBA type.
I don't know how to turn it back to 16 bit luminance;

Can you help me to get the raw image data buffer from the tiff object?