* rnd@hih.org.gr : phone +30 210 8211 587 : www.hih.org.gr

Holofos: an optimized LED illumination system for color reflection holograms display

Andreas Sarakinos*a, Nikos Zervosa, Alkiviadis Lembessisa

aHellenic Institute of Holography (HiH), 28 Dionysou Str., 15234 Chalandri Attikis, Greece

ABSTRACT

True color reflection holograms can be successfully recorded by exposing panchromatic holographic plates to 3 or more LASER beams of suitable wavelengths. Traditional halogen spotlight illumination of color holograms relying on reflection holograms’ Bragg diffraction sampling capabilities has many drawbacks. This kind of illumination, especially for a broadband hologram, results in heightened levels of chromatic dispersion and blurring of image points far from the hologram’s surface. On the other hand, by intensity mixing of selected narrow band LEDs with peak wavelengths matched to those used during recording, high quality reproduction of deep color holograms can be achieved. In this paper we will present the Holofos LED RGB and RGBW color hologram illumination devices. These devices have a wide color gamut achieved by precision, digitally controlled, RGB intensity mixing at pre-selected wavelengths. Dichroic and refractive optics combine the RGB or RGBW LEDs’ beams into quasi point-source output beam of uniform color cross section. A quantitave spectro-radiometric characterization of the Holofos devices and resolution tests results using a series of test holograms will also be presented.

Keywords:

1. INTRODUCTION Holography is a form of optical information storage method that can record a three-dimensional object or scene on a two-dimensional recording medium and playback the original object or scene to the unaided eyes as a three dimensional image. The general principles are described by Leith et al1 and holography theory and its applications are well described in the literature2. In 1962, Dr. Y.N. Denisyuk from Russia combined holography with 1908 Nobel Laureate Gabriel Lippmann's work in natural color photography. Denisyuk's technique produced a reflection hologram which, for the first time, could be viewed under ordinary incandescent bulb light3. By using 3 or more laser lines, one can record color Denisyuk holograms of artifacts4 .

For decades display reflection holograms have been illuminated either by theatrical lights or halogen spots. This kind of illumination has many drawbacks including -among other- image blurring, high power consumption and thermal degradation of the illuminated hologram. On the other hand, an RGB-LED illuminant is characterized by narrow per color bandwidths and small source size, while power consumption and thermal effects are reduced. Consumer available RGB LEDs fixtures suffer, from holography’s view, of non-axial placement of the RGB LEDs and poor spatial mixing of the discrete beams.

Holofos is our LEDs based illuminating device that can supply to a hologram the necessary narrow bandwidth in red, green and blue light with peaks near the recording wavelengths. At the same time the small footprint of the Holofos LEDs chips allows placing the lighting fixture at a closer distance to a hologram while retaining image sharpness and enhanced depth. Electronic control of the LEDs driving currents permits the accurate intensity mixing of the emitted RGB bands in order to fine tune the colors of a hologram. We fabricated two such devices: the Holofos III that provides coaxial mixing of single chip LEDs beams and the Holofos IV that relies on a power multi-chip RGBW-LED.

2. COLOR REFLECTION HOLOGRAMS DISPLAY The choice of proper illumination sources for the reconstruction of color reflection holograms is of great importance. Assuming that there are no distortions caused by the developing and drying processes, a perfect reconstruction can be achieved if a color reflection hologram is illuminated by the same lasers and at the same geometry used during recording. In all other cases the finite dimensions and the spectral characteristics of the illuminating source introduce various levels of image blur, color shifting and scatter noise. When using a tungsten halogen lamp only a small part of

the emitted light contributes to the image intensity while the rest and larger part is scattered lowering the image’s contrast. A good approximation to the perfect reconstruction can be achieved using LEDs with dominant wavelengths that match the lasers used during recording, small chip sizes and narrow bandwidths.

2.1 Source size blur

The finite angular size of an illuminating source produces an image blurring that is equal to the angle that the source subtends at the hologram5, 6. If the source subtends an angle θr and the distance of an image point from the hologram’s plane is D, let δ be the lateral blurring of the image point, then the angular blurring is

rDδθ = (1)

and

rDδ θ= (2)

So the lateral blurring due to the finite source size increases with θr and the distance of the image point from the hologram’s plane. For a fixed image point at distance D the blurring δ will vary inversely to the source’s distance from the hologram’s surface and proportionally to the source’s extent. The small spatial extent of LEDs chips assists in minimizing the source size blur.

2.2 Chromatic blur

Illumination of a hologram with a very small source of finite spectral bandwidth Δλ, results in a chromatic blur Δχ for an image point located at a distance D from the hologram’s plane5, 6:

sinD i λχ θλ∆

∆ = (3)

where λ the wavelength of the recording beam and θi the angle of the reconstruction beam relative to the hologram. Equation (3) implies that for a broadband reflection hologram under tungsten halogen illumination, the chromatic blur will be substantial for image points far from the hologram’s surface. Illuminating such a hologram with a narrow-band LED will lessen the chromatic blur.

2.3 Wavelength matching

The human eye has photoreceptors (called cone cells) for photopic vision, with sensitivity peaks in short (420–440 nm), middle (530–540 nm), and long (560–580 nm) wavelengths. Thus, in principle, three parameters describe a color sensation. These tristimulus values of a color can be conceptualized as the coordinates of three primary colors in a tri-chromatic additive color space like the CIE XYZ.

Two light sources made up of different mixtures of various wavelengths may appear to be the same color; this effect is called metamerism. Such light sources have the same apparent color to an observer when they produce the same tristimulus values, no matter what the spectral power distributions of the sources are. In this aspect researchers in color holography have aimed at choosing three suitable wavelengths for recording a true color hologram which would cover a sufficient area of the CIE chromaticity diagram.

Thornton7 found that the reflectivity of an object at three well-specified wavelength bands (near 450, 540 and 610nm) has a disproportionately high color rendering index. Using these wavelengths, a wide range of highly saturated colors can be produced. Hubel and Ward8 in 1989 recorded color reflection holograms using 647, 528 and 458nm lasers with a wide color gamut. In 1991 Hubel and Solymar9 found another good practical choice of wavelengths in using a combination of 442, 532 and 633nm. Bjelkhagen and Mirlis10, Peercy and Hesselink11 and Kubota et all12 claim that more than three wavelengths are needed for true color rendition.

In order to illuminate a color reflection hologram with a RGB-LED device, wavelengths, that can be matched to the dominant wavelengths of currently available LEDs summarized in Table 1, have to be selected for the recording.

Such recording wavelengths can be 457, 532 and 638nm which we currently use in our Z3RGB transportable camera for single beam reflection holograms recordings.

This combination of wavelengths gives a gamut (figure 1) that includes yellow, purple, dark blue and violet as the Hubel and Ward one, and overlaps the Wintringham data.

Figure 1. Recording wavelengths of the Z3RGB transportable camera for single beam reflection holograms

Table 1. Wavelengths of currently available LEDs for color reflection hologram illumination.

Color Dominant Wavelength (nm)

Royal Blue 450-465

Blue 465-485

Green 520-535

Amber 585-595

Red-Orange 610-620

Red 620-640

Deep Red 650-670

3. THE HOLOFOS ILLUMINATING DEVICES The Holofos III is an illuminating device that uses dichroic combiners to produce a coaxially mixed RGB beam (figure 2). The device is equipped with single chip red, green and blue LEDs with independent precision current control for each color. In this way, the color gamut achievable by the selected LEDs can be covered by varying the driving currents. The light emitted by each LED is collected by a small lens and steered through two dichroic combiners that mix the three beams into a coaxial exit beam.

The Holofos IV is fitted with a multi chip RGBW LED with independent precision current control per channel and two biconvex lenses for beam shaping (figure 3).

Figure 2. Holofos III, head, color mixing controller, schematic.

Figure 3. Holofos IV, head, color mixing controller, schematic.

3.1 Holofos spectrum characteristics

As we have already stated the spectral characteristics and size of an illuminating source are of great importance to the optimization of the reconstruction of a color reflection hologram. Narrow band emission near the recording wavelengths ensures a high degree of color reproduction, increased resolution and better contrast as there are less spectrum components to contribute to scatter noise. In the case of a broadband color reflection hologram, LED illumination can assist to a better image and increased depth. By variable intensity mixing of the RGB LEDs beams, fine adjustment of the white point can be achieved after the recording and developing steps.

Using a Jeti Specbos 1211 spectro-radiometer we took radiometric measurements of the light reflected from a pure white “x-rite”, white balance target by illuminating it with the Holofos units and tungsten halogen spot. The target was illuminated at 45 degrees from a distance of one meter, the Specbos taking readings at zero degrees to the target surface (figure 4).

Figure 4. Left: Jeti Specbos 1211 spectro-radiometer. Center: Taking measurements. Right: Layout for measurements.

We were able to easily tune the Holofos devices very close to the CIE white point by varying the LEDs driving currents. Snapshots of the measurements taken with the Specbos spectro-radiometer are presented in Figures 5, 6, 7, 8. The CIE points and the radiance distributions are presented side by side for each measurement.

Figure 5. Pure white target, Holofos III RGB illumination. Spectrum and corresponding CIE point close to CIE white point.

Figure 6. Pure white target, Holofos IV RGB illumination. Spectrum and corresponding CIE point close to CIE white point.

Figure 7. Pure white target, Holofos IV, only white channel on. Spectrum and corresponding CIE point.

As can be established by these spectro-radiometric measurements the Holofos devices have dominant wavelengths within the ranges presented in Table 1. The white channel of the Holofos IV can be used to fill in the “gaps” of color shifting due to development and drying procedures of a hologram.

The points cloud on the CIE diagram of figure 5 shows the ability to easily vary the white point temperature of the Holofos illumination by controlling the LEDs driving currents. The spectral distribution of both devices shows that narrow band reconstruction of a hologram can be achieved.

In figure 8 we present the corresponding measurement data for a typical tungsten halogen spotlight.

Figure 8. Pure white target, tungsten halogen spotlight. Spectrum and corresponding CIE point.

It is obvious from the diagrams in figure 8 that such a source will produce considerable levels of scatter noise and chromatic blur. As the white point temperature of a tungsten halogen spotlight cannot be varied electronically it is very hard to make color corrections to a hologram with this kind of illuminating source.

3.2 Holograms reconstruction with the Holofos III and Holofos IV

In the setup depicted in figure 9 we substituted the pure white target with color holograms and took readings of the spectrum reflected using the Specbos 1211. The holograms had to be tilted a little for proper reconstruction so we raised and tilted the Specbos to keep the readings clear of specular reflections. With Holofos III, Holofos IV and tungsten spotlight, we illuminated a 20x25cm color hologram of a replica of a model soldier titled “Evzonas” and a 10X12 color hologram of a “NBS 1963A” glass resolution target . The NBS 1963A has been recorded at 5 cm behind the hologram’s plane. Both holograms have been recorded with our Z3RGB color holography camera at 457, 532 and 638nm in single beam reflection mode on Ultimate plates. The resolution target was piggy backed with a piece of white painted piece of wood. The resolution markings are etched in the front face of a glass substrate. Their shadows on the white backing spoil the visual perception of the markings in the final hologram. In the case of the Evzonas hologram we aimed the Specboss at a small white area at the right knee of the model’s virtual image while we left the color balance of the Holofos III and IV at the CIE points depicted in figures 5, 6.

Figure 9. Left: setting the spotlight to illuminate the resolution target. Right: layout for the measurements.

Snapshots of the measurements taken with the Specbos spectro-radiometer are presented in Figures 10, 11, 12.

Figure 10. Evzonas target, Holofos III RGB illumination. Spectrum and corresponding CIE point.

Figure 11. Evzonas target, Holofos IV RGB illumination. Spectrum and corresponding CIE point.

Figure 12. Evzonas target, tungsten halogen illumination. Spectrum and corresponding CIE point.

The inherent filtering action of reflection holograms is clearly shown in the above spectrograms. The spectrum reflected from the Evzonas hologram under tungsten halogen illumination is clearly shifted from the recording wavelengths of 457, 532 and 638nm. Comparing the spectrums of figure 8 and figure 12 we can see that a large part of the tungsten halogen emission is missing due to absorption and scattering by the Evzonas hologram. The blue shift of the white point under Holofos illumination can be easily adjusted by varying the relative intensities of the RGB channels. In the case of

Holofos illumination the dominant wavelengths in the spectrograms of figure 10 and figure 11 clearly match the recording wavelengths. In figures 13 and 14 we present the photos of the reconstructed images of the Evzonas and NBS 1963 resolution target holograms under Holofos and spotlight illumination.

Figure 13. Left: Holofos III illumination. Center: Holofos IV illumination. Right: Tungsten halogen illumination.

Figure 14. Left: Holofos III illumination. Center: Holofos IV illumination. Right: Tungsten halogen illumination

Under Holofos illumination both holograms reconstruct deep, crisp images with enhanced contrast and saturated colors. The tungsten halogen illumination reconstructs images that have less contrast, unsaturated colors and considerable levels of blur and scattering.

4. CONCLUSIONS The Holofos III and Holofos IV hologram illuminating devices reconstruct deep single beam color holograms with enhanced contrast and saturated colors. The dominant wavelengths of their RGB LEDs can match commonly used sets of recording wavelengths and thus provide enhanced color reconstruction. The narrow band emissions of the LEDs and the small dimensions of the LEDs chips (1x1 mm up to 2x2 mm) minimize the chromatic blur and the source size blur as described by equations (2) and (3). Color balance can be easily adjusted by the precise control of the LEDs driving currents.

REFERENCES

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[3] Denisyuk Y.N., ‘On the reproduction of the optical properties of an object by the wave field of its scattered radiation’, Optical Spectroscopy (USSR) 1963 14 279–284.

[4] Hans Bjelkhagen and Jill Cook, ‘Colour holography of the oldest known work of art from Wales’, The British Museum Technical Research Bulletin 2010 4 1-9.

[5] Collier, Burckhardt and Lin, ‘Optical Holography’, Academic Press, London, 1971 170-174. [6] K. Bazargan, ‘Techniques in display holography’, 1986 Phd Thesis Optics Section Blackett Laboratory Imperial

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