Sodium vapor process

I have Googled 'why did Disney use yellow screen' which then lead to: How does Mary fly in Mary poppins?" The answer was pretty clear to read down below - 

I then Google imaged "sodium vapor process" which I got a few great images involving Mary Poppins too. These diagrams tells itself it is pretty easy to notice what this technique does.

 Image above shows a fantastic example of how it is explained by images. 

I found this from a book on Google - "Techniques of special effects of cinematography": 

This explains from sketches to filming with the sodium vapor process technique to the final screening:

The website:

http://books.google.co.uk/books?id=4qF5AgAAQBAJ&pg=PT384&lpg=PT384&dq=why+does+disney+use+yellow+screen&source=bl&ots=QFaLZ1gapG&sig=B-qCN4qDlbHyBna2IceAxoKYwnM&hl=en&sa=X&ei=D64RVJD8CNHaaoXrgPAO&ved=0CFwQ6AEwCQ#v=onepage&q&f=false