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A GIANT FLIP-BOOK IN A SUBWAY The advertising displays in tunnels are a series of static images, put up on the tunnel’s wall, that appear to move as a train rolls by. Riders looking out of the window get to see what seems like a silent movie clip. Here’s how it works As the train enters the tunnel, the box senses the speed, acceleration and deacceleration of the train Microprocessors control the illumination of each box on which ad is mounted and is lit for a fraction of a second as the train zips by The boxes are lit exactly when a passenger window is perfectly aligned to it The display design varies with each tunnel, based on factors like distance from the window, train speed, average viewing distance, etc

To create the feel of a moving clip, which could be an advertising spot or even a social message, images printed on special paper are fixed on the tunnel walls. Successive images are placed in a manner that they tell a story when viewed from a moving train, just like strip books that one used to get before cricket matches in the past.

To add light, the images are mounted on steel and aluminium boxes - 1.2 m-1.4 m in length and 25-40 cm in thickness - that are lit from the rear. The wiring used is fire retardant and low smoke. It is important to synchronise the placement of the box (image) with the speed of the train, as the human mind registers successive images at a set pace to be able to comprehend them like a sequence.

"It has taken DMRC and the private company, C2E Technology Labs, a good two years to complete research and development of the concept. There is a lot of physics and mathematics involved in determining the positions of the boxes and their number to make sure that when the train moves, they appear like a movie. To ensure persistence of vision, the images will be interspersed with black shutters,'' explained Anuj Dayal, DMRC's chief spokesperson. The specifications and placement of boxes was determined after looking at tunnel conditions, train speed, distance of the box from the train window and the average passenger viewing distance. On Saturday's trial, a three-second clip was developed by placing 30 boxes on the tunnel walls.

Officials drew a comparison with the New York metro where 200 boxes are put for a 30-second display, which would only allow a display of 15 seconds on Delhi Metro. This is because Delhi Metro runs at a speed of 80 kmph in the tunnel stretch, which is double the speed of its New York counterpart.