I have been studying the design of a spectrometer using a transmission diffraction grating. I stumbled upon this link, https://ibsen.com/technology/spectrometer-design-guide/. It explains a Transmission grating-based spectrometer. Additionally, at the end of the page, it mentions that "transmission gratings are generally used in the Littrow configuration". However, on further doing research on the internet I am unable to find details on transmission grating being used in a Littrow configuration. Resources point out to reflection gratings.
So, my question is: Is a Littrow configuration spectrometer uses transmission or reflection grating or it can use both.
I am trying to understand the location of specific wavelengths on the screen/detector after passing through the focusing convex lens when transmission grating is used. For reference, see the image below.