Spectral curves / bandwidth

The holograms for TL2 were shot at 640 nm, 532 nm and 473 nm

We allow for a modest amount of shrinkage as this improves the rendition, so the current replay central wavelengths are approximately 625 nm, 525 nm and 460nm. 

The spectral curve is shown on the following graph 

The red and green are at minimum 50% efficient, though 60% has been reached. The blue trails in efficiency at between 30 to 35% and we are working to improve this. 

The nominal bandwidths FWHM are approximately: 

Colour                                               Bandwidth

Red                                                    25 - 30 nm

Green                                                 20 - 25 nm

Blue                                                    15 - 20 nm

Angle of view

The angle of view is 18 to 20 degrees total though the more accurate colour is in a narrower area

Initial viewing through the waveguide without a display source

The waveguide can be checked for functionality by holding the left hand side coupler up to the right eye. 

The right hand coupler can then access views (say 20 m away) arriving from behind the head, as long as the hand holding the coupler or the head itself do not obscure the line of sight. 

See right for illustration

Augmented reality wearable displays with the holographic waveguide

The simplest form of AR using the waveguide optics requires a microdisplay, a collimating lens and a frame to hold them together so that the microdisplay is imaged through the waveguide to the eye. The microdisplay will need a connection to the real time imaging source – for example a smartphone.

The microdisplay would normally be at the side of the head pointing forwards to the input holographic coupler on the waveguide. The collimating lens would be between the microdisplay and the waveguide. The other end of the waveguide would be positioned so that the output coupler would be directly in front of the eye.

With optimum alignment of all the elements the microdisplay image should be visible in the transparent output coupler and at a comfortable focus for the eye. This allows the environment to be viewed at the same time as the received imaging from the microdisplay. 

The optimum angle at which the micro display and lens assembly couple to the waveguide is typically slightly off 90 degrees laterally. This offset is best found by actual experiment and is only a matter of a few degrees, typically 3 degrees from the normal. 

Diagram illustrating the setup of an augmented reality device using the TL2 optic

Colour gradation issues

Due to the diffraction characteristic of the holographic technology there is some tendency for the image to show a discreet lateral colour bias i.e red to the left, green in the centre and blue to the right.

We are looking at a number of ways to solve this issue including:

1. Software enabled modification to boost opposite colour bias on the image to offset that caused by diffraction 
2. Linear variable edge filters on the backlight 
3. Stereo pair use – by using two sets of TL2, one for each eye 

What we have found is in our 3D version the colour spectrum is balanced by utilising the opposite sense for right and left eye and this counterbalances the colour gradation so that the 3D image appears in good colour over the entire field of vision.

4. Digital input using a fibre optic

The method shown below is a possible way to modify imaging and reduce the lateral colour bias.

In this regime we would utilise a spatial light modulator combined with a holographic optic and a filter to optimise aspects of the image.

Here the image is injected via optic fibre and the hologram is for enabling the angles required for the SLM to work effectively. The SLM would be fast switching to deal with the RGB illumination sequentially as it can only deal with one colour per computed generated fringe pattern.