Holocurtains: Programming Light Curtains via Binary Holography

How it works

For each rolling-shutter row, a projector selectively illuminates different locations on the plane imaged by that row. By changing the pattern that is illuminated for each row, a light curtain of arbitrary shape can be formed. For example, we can create a bunny-shaped light curtain by illuminating the intersection of each rolling-shutter row with a bunny mesh.

The projector source used for such an application needs to be (1) light efficient, (2) fast, and (3) programmable. Existing approaches utilized mirror-based systems to scan a laser line across the scene - light efficient and fast, but not highly programmable. Because these setups can only create line patterns, the resulting light curtains are constrained to ruled surfaces.

In our work, we utilize a digital micromirror device (DMD) in a holographic projector. By leveraging computer-generated binary holography, such a setup is light efficient, fast (up to 10 kHz), and programmable. As a result, our system can form light curtains of arbitrary shape, which we show in the rest of this webpage. We believe that this system may be applicable for other structured-light applications beyond light curtains, like 3D scanning or light transport probing.

Light curtains of arbitrary shape

Such a system can form light curtains of arbitrary shape that traditional setups cannot create. For example, our system can image a light curtain with both vertical and horizontal components (left), as well as any more complex shape (right).

Such a system may be useful in robotics. Here, we form a form-fitting light curtain about 5cm off the surface of a mannequin. A robot can use this curtain to determine whether it is in a safe operating range (left). A feeding robot could also use this curtain as a cue in assisted feeding for where to position food (right).

Instead of proximity information, our system can also be used to selectively measure regions of 3d space. As a result, our setup can be useful in situations where privacy needs to be preserved. In this example, we selectively image just the surface of the teapot. The confidential document is not visible in the images captured by the light curtain camera (right).

Optical disturbance detection

We can capture a tight light curtain over the surfaces in a scene, and then compute difference images on the resulting signal to create an optical disturbance map (right). Compared to a difference image from a normal camera setup (middle), such an approach registers changes in scene geometry much more densely and robustly.

Light Curtain Multiplexing

Because a DMD has no such smoothness limitations like that of a galvo scanning mirror, our system can sequentially display vastly different patterns at fast rates. We leverage this property to multiplex multiple light curtains onto different rows in a single rolling shutter frame.

One way this multiplexing may be useful is in the context of the optical disturbance detection as described above. By multiplexing curtains of different thicknesses, we can determine the magnitude of a particular disturbance. Consider the case of a thin curtain (middle) and a thick curtain (right). If both curtains detect a large change in signal, the disturbance must have been large. However, if only the thin curtain detects a change in signal, the disturbance must have been small.

An Optical Touch Interface

Such a light curtain system can also be used to generate a 3D touch interface from any desired object. By forming a light curtain just a few centimeters above some target surface, we can register when that surface is interacted with.