How to Make a Holographic Optical Element Film

By Chris Chinnock, Insight Media

Holographic Optical Element (HOE) films are an emerging class of optical device that can be used to bend, shape, or direct light. Simple HOE structures like grating are used in waveguides to capture, direct and expand images for Augmented Reality (AR) devices. HOEs can be used to create true static holographic images or to enable Head-up Displays (HUDs) in automobiles.

One way to create a hologram is to recreate the light from a physical object in a holographic film. This method, sometimes called analog, requires an object laser to illuminate the object along with a reference laser. The laser light reflected from the object has its phase changed by the contours of that object, and when interfered with the reference beam, creates a series of fringes. These fringes which can now be recorded in the volume of a photo-sensitive polymer film (photopolymer) create the hologram. When this hologram is subsequently illuminated with the same angled reference laser (or LED) beam, the fringes alter the phase of the light and recreate an image of the object in true three dimensions.

For the automotive HUD application, a company called Ceres Holographic is supplying a HOE film that acts like a projection screen for an LED projector. While this may sound like a simple optical function to record, it is way more complicated. For example, the light from the LED projector arrives at the HOE at different angles and distances from the projector. This projected light must be redirected into a fairly narrow driver eyebox with good uniformity and color fidelity. It must also maintain high resolution, be bright, but also highly transparent to external light.

Analog hologram recording methods don’t work well for creating the HOE for this application as there is no physical (diffuser) device that can create this functionality accurately. As a result, Ceres has developed what they call pixel-wise HOE mastering. Here, the idea is to create a large HOE projection film via a series of smaller holograms. These so-called pixel-wise holograms are only 250×250 microns with each pixel-wise hologram calculated for its position in the overall HOE film. A step and repeat process is then implemented to complete the full-size HOE.

Each pixel-size hologram, is a series of fringes or interference patterns recorded in the photopolymer film. Fringes for red, green, and blue LED light are calculated and blended together as an RGB fringe pattern. This pattern is then written to a spatial light modulator, which is a special type of Liquid Crystal on Silicon device, and simultaneously illuminated with red, green, and blue laser light. This is the “object beam” light that now interferes with the reference beam to expose the photopolymer in the 250×250 micron window. Ceres Holographic likes to call this holographic mastering process software-controlled printing.

Ceres has now established both a hologram mastering process and a roll-to-roll holographic replication capability which uses the masters generated from their Gen 2 mastering machines. In this way, it can produce large windshield-sized films for lamination into windshields.

Ceres has primarily been developing automotive HUDs that feature a simple compact LED projector and a diffuser HOE which is embedded into the windshield. This HOE allows the projected image to be visible to the driver at the windshield surface.

In the AR-HUD application, HOEs with lensing optical power can project an image out in front of the car at a virtual distance and with large field-of-view. Making masters like this using the same digital printed approach can be done, and is in Ceres roadmap to develop the digitally controlled printer technology to do this.

However, the use of the end lens/mirror HOEs in an automotive windshield remains challenging. The problem, which Ceres CEO Andy Travers says nobody has yet solved, is the creation of unwanted rainbows visible to the driver when sunlight or streetlamps are diffracted by the windshield lens type HOE.  While there might be some solutions to reduce the visible rainbows in the driver’s eyebox, like windshield coatings, these are expensive and not likely to be viewed favorably by the auto makers. Other approaches include putting the lensing HOE inside or on top of the HUD projector box.

While development on this continues, Ceres is really focused on making diffuser-based HOEs which don’t exhibit the rainbow problem but deliver a vibrant and very wide field of view Transparent Display HUD.

share this post