My profession revolved around the printing machines you are asking about. I was technical manager for 7 giant photofinishing plants in the Southeastern United states. Each plant was sized to handle, on a daily basis the peak volume generated by the holiday season. My labs were sized to develop and print 20,000 rolls of color film a day. I worked for Eckerd Drugs, a chain of 2,000 drug stores. Photofinishing was big business in that era.

The rolls of film arrived at the lab in individual envelopes. The pre-printed envelopes were filled out by the customer with name, address and instructions as to number of prints per frame and what size prints wanted. The envelopes were opened, contents inspected, and sorted into bins by film type. Each roll, accompanied by its envelope was passed to a room in total darkness. Each bag and roll received an adhesive label with the same unique number. This twin check tied the roll to its envelope.

The twin checked rolls were spliced together making one long roll, hundreds of feet long. This long roll of spliced film was threaded onto a machine that transported the long roll into a series of vats filled with chemicals. The chemicals were maintained at a specific concentration and at a specific temperature. The duration time each roll remained in each chemical vat was precisely timed. The film exited the film as a roll of dry developed film.

The long roll of developed film was moved to the printing room. Dozens of automated printers and their operator worked under safelight conditions. These printers were loaded with a long roll of light sensitive photo paper. The printer transport mechanism advanced the long roll of film, frame by frame to a printing gate. Each frame is viewed by the printer operator and an exposure analyzer built into the printing machine. Based on experience, he/she can take no action or override the automated exposure assessment.

The printing machine’s shutter opened, an enlarged image of the film is projected on the light sensitive printing paper. The lamp-house contained automated colored filters that custom modify the color of the exposing light. Thus the exposure time and color of the exposing light applies corrections that counter camera exposure errors and lighting conditions at the time of film exposure. The rear of the print paper receives a printed number and letter code. The code corresponds to corrections applied to the color and timing of the exposure. They will be used if the finished print must be reprinted to improve its color balance.

The now exposed long roll of paper moves to an automated paper developing machine. The paper is transported through a series of chemical vats. The paper is developed and dried.
Next the developed paper, developed film, and envelope converge at an assembly station. An operator feeds paper and film into cutting machines. The film is cut into short strips and each is inserted into protective transparent sleeves. The operator inspects each print for off color. Prints that can be manually reprinted for improvement are culled. If no action is taken the film and prints are inserted into the respective envelopes. The finished package is returned to the drugstore.

Let me add, from about the mid 1950’s to the end of the 20th century, photofinishing machines evolved with higher exposure determination accuracy. Operators were able to see a computer generated image that pre-visualized the finished print. This allowed the operator to override and apply manual corrections. The goal always was a high quality product with little need to reprint a frame.

Today’s digital printing stands on the shoulders of a century of evolving exposure determination technology. It was my occupation, I loved every minute.