Dear Molli,

Rubylith and Becquerel terms are opened a new question. Can we develop ordinary modern film with that method.
I will open a new thread and post the link as I promised. Thank you for the tip.

Rubylith is red colored transparent film.

Becquerel daguerreotypes are developed using orange or red light over a period of a few hours. Often the film or plate holder used during exposure is covered by a red or orange film such as Rubylith. This is then exposed to the sun or under an incandescent lamp for several hours. With a well prepared and exposed plate the image can start to be seen within the first 15 minutes of development.