[racket] Is there any way to get the bit depth of a bitmap?

Hi,I am recently working in the book Digital Image Processing with
racket. I want to know is there any way to get the bit depth or color
depth of a input bitmap in Racket? I found the get-depth method in
racket/draw,bitmap%, but it only return 1 and 32, that's not what i
really want.I input a bitmap whose bit depth is 8 but i got 32.
following is the code:

> (send (read-bitmap "Fig0305(a)(DFT_no_log).tif")
get-depth)
32

Is there anything I am wrong? Thank you!