How the Lightgate B5 system works

A Lightgate digital hologram comprises of an array of tiny diffractive pixels, each of which contains a plane-wave diffraction grating i.e. the interference fringes recorded in the pixel are straight and parallel to each other. When illuminated, these diffractive pixels redirect light to predetermined viewing locations in space. The light diffracted from many tens of thousands of such pixels combines to form a complete holographic image.

The Lightgate printer creates diffractive pixels by converging two laser beams to a point. Where they cross over an interference pattern or plane wave diffraction grating is created and this is recorded on a photosensitive plate. Once illuminated each pixel in the hologram diffracts light at a specific angle and in a direction that was determined when the hologram was made. The angle and direction that a pixel's grating diffracts light is specified by two factors:

1. The grating orientation.

The diffraction grating can be recorded in any orientation from 0 to 180 º. This is achieved by rotating the two laser beams.

2. The grating spacing.

Different grating spacings or spatial frequencies i.e. the distance between the individual interference fringes or the number of lines per mm can also be recorded. This is achieved by changing the angle between the two laser beams.

These two factors, the grating orientation and the grating spacing, determine precisely the angle and direction that the light will diffract from each pixel.

Changing the grating orientation

By changing the grating orientation the diffracted light can be made to skew horizontally left or right with respect to the vertical axis. Pixels with different grating orientations will therefore 'light up' from different angles and create the animated effect seen in 'kinetic' type digital holograms.

Changing the grating spacing or spatial frequency.

By changing the grating spacing the diffracted light can be made to skew vertically up or down with respect to the horizontal axis. As with all 'rainbow' holograms the angle that the light diffracts in the vertical direction determines the apparent colour of the hologram i.e. the portion of the spectrum that the viewer sees when viewing the hologram from a particular position. The vertical diffraction angle is primarily determined by the grating spacing of the hologram or, in the case of Lightgate digital holograms, the grating spacing of each diffractive pixel. The Lightgate's ability to vary the grating spacing of each pixel enables the production of multi-colour and full colour digital holograms. By selecting particular grating spacings each pixel can be assigned a different colour of the spectrum. It is important to remember however that, like all 'rainbow' holograms, the colours are relative not actual. For example when one pixel appears red another can appear blue. When the hologram is tilted or the illumination beam angle changes however the colour of the pixels will change and shift through the colours of the spectrum.

How the grating characteristics are specified

The Lightgate's computerised motion control system automatically adjusts the two focussed laser beams so as to produce the desired grating orientation and spacing for each pixel. It does this 'on the fly', pixel by pixel, as the hologram is being recorded. The Lightgate control software therefore needs to know the desired grating orientation and spacing for each of pixel in the hologram. The Lightgate printer utilises two different techniques to specify these values. These techniques can be used separately or combined to produce sophisticated holograms.

1) The use of one or several computer images.

a) A single grey scale image
b) Multiple grey scale image
c) Full colour images

2) The use of pre-installed custom computer software algorithms.

1) The use of one or several bit-mapped computer images.

A single grey scale image

A single grey scale computer image can be used to specify the grating orientation and spacing values for a single colour hologram. Each pixel in the computer image is used to define a corresponding diffractive pixel in the hologram. Traditional 'kinetic' digital holograms are made by simply rotating two laser beams to change the grating orientation for each pixel. Such holograms are usually 'single colour' and it is therefore not necessary to vary the grating spacing for any of the pixels. To make such a hologram, the grating orientation for each pixel is determined by the grey level, from 0 to 255, of the corresponding pixel in the computer image. Pixels in the computer image that are grey level 128 (50% grey) create diffractive pixels that diffract the light vertically. White pixels create diffractive pixels that skew the diffracted light to the left and black pixels skew the diffracted light to the right.

Whichever type of computer image is loaded into the Lightgate Control program it is first converted into a 24-bit RGB colour image that comprises of three colour channels, one for the red separation, one for the green separation and one for the blue separation of the image. The grey scale value that determines the grating orientation is taken from one of the three RGB colour channels (the blue channel is used by default). Below is a typical grey scale computer image together with a representation of the single colour 'kinetic' type digital hologram that can be made from it.

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Multiple grey scale images

Grating spacings can be specified by using more than one grey scale computer image. A multiple colour hologram, with many discrete spectral colours, can be made by using several computer images, one for each colour in the final hologram. Care is taken not to overlap areas of differently colour pixels by masking the images from each other using the colour black - grey level 0. The Lightgate printer does not expose pixels that have a grey level of 0. Below are two grey scale computer images that, if printed in register, would make the two-colour 'kinetic' type digital hologram depicted on the right.

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Full colour images

A further technique is to specify both the grating orientation and the grating spacing at the same time using a full colour computer image. In this case an RGB, full colour, digital hologram is created. This is achieved by either triple exposing each pixel or by spatially separating the red, green and blue pixels in the hologram.

The Lightgate printer currently offers three RGB, full colour, recording methods.

Triple exposed	Linear spliced	Chequerboard spliced

Triple exposed RGB, as the name suggests, creates full colour holograms by triple exposing each pixel. The angle between the two laser beams and hence the grating spacing is changed for each of the three exposures to produce a single pixel that diffracts red, green and blue light.

Linear spliced RGB creates full colour holograms by recording the red, green and blue pixels of an image in horizontal stripes.

Chequerboard spliced RGB creates full colour holograms by recording the red, green and blue pixels of an image in a chequerboard pattern. These two latter RGB features enable the Lightgate user to produce full colour holograms in the same time that it would normally take to record a single colour hologram.

For all three options, the grating orientation is determined as usual by the grey level value of each pixel in the computer image however, in this case, the colour channel from which the grey level value is taken depends upon which colour is being recorded. For example, if a red exposure is being made the grey level value of the pixel's red channel is used to determine the pixel's grating orientation. This technique produces a full colour 'kinetic' type digital hologram using only a single computer image.

Full colour, multi-channel or three-dimensional holograms can also be made by combining these RGB, full colour recording techniques with the algorithmic techniques described below.

2) The use of pre-installed custom computer software algorithms.

The techniques described above utilise one or more computer images to determine the grating orientation and the grating spacing for each pixel in a Lightgate digital hologram. The Lightgate printer however can also make extremely sophisticated digital holograms by utilising pre-installed software algorithms to determine the grating orientation and grating spacing. In other words the Lightgate's control program itself can determine the grating orientation and/or grating spacing for each pixel in the digital hologram using mathematical formulae.

The simplest example of this is a technique is commonly called a 'sparkle' or 'random angle' effect. This technique creates the glitter-like effect often seen in 'kinetic' digital holograms. In this instance the grating orientation for each pixel is determined randomly by the computer software and not by the computer image. Another example is the three-dimensional digital hologram. In this case the software accurately calculates the grating orientation and grating spacing for each pixel so that the diffracted light from every pixel in the hologram converges to form discrete stereographic viewing zones some distance in front of the hologram. As mentioned above, this technique can be combined with the RGB, full colour, techniques described above to make full colour, three-dimensional holograms. For algorithmic techniques the grey level of each pixel is used to determine the intensity or brightness of the corresponding diffractive pixel. This is achieved by modulating the exposure time given to each diffractive pixel. In this way 'photographic' or 'shaded image' digital holograms can be produced. Other types of algorithmically produced digital holograms or holographic effects include laser projected or 'hidden' images, curved holograms, multi-channel effects and digital holographic optical elements.

A 3D hologram with a random sparkle boarder