How Do Holographic Projectors Work? By Bert Markgraf; Updated March 14, Holographic projectors use holograms rather than graphic images to produce projected pictures. They shine special white light or laser light onto or through holograms. The projected light produces bright two- or three-dimensional images.
Although holographic image formation was demonstrated some forty years ago , efforts at realizing a real-time 2-D video projection system based on this technique have not been successful, principally due to the computational complexity of calculating diffraction patterns in real time and the poor quality of the resultant images.
In this paper, a new approach to hologram generation and display is presented which overcomes both of these problems, enabling—for the first time—a high-quality real-time holographic projector.
Index Terms—Diffraction, displays, displays human factors, holography, laser. Not only does this approach remove the need for polarizers and provides high optical efficiency by forming images by routing, rather than blocking light, it also allows complete control over the optical wavefront.
This opens up the possibility for incor- Despite the potential advantages of a holographic display, porating novel system capabilities such as optical aberration previous attempts at constructing such a system — have correction and novel projection geometries .
A Fraunhofer or far-field holographic display is based The first difficulty is that of calculating a hologram such on the result that, when a hologram is illuminated Holographic laser projection that, when illuminated by coherent light, a high-quality image coherent collimated light of wavelengththe complex field is formed.
It is not possible to simply invert the Fourier formed in the back focal plane of the lens of focal transform relationship of 2 to obtain the desired hologram length due to Fraunhofer diffraction from the patternsince the result of this calculation would be fully com- is the 2-D spatial Fourier transform of the hologram pattern  plex and there is no material in existence that can independently modulate both amplitude and phasewhere.
Even if such a material became available, the re- 1 sult contains amplitude components that would absorb incident light and reduce system efficiency. A much better approach is This relationship is illustrated in Fig. If the continuous hologram pattern is then replaced by an ele- Performing this operation on whilst maintaining high image ment with pixel sizethen the image formed or replayed quality in is absolutely nontrivial and requires computation in the focal plane of the lens is Holographic laser projection to the pixellated hologram to mitigate the effects of information lost in the quantization.
Until recently, as there was no hologram-generation method in existence that could simultaneously produce images of sufficient quality for 2 video style images whilst calculating the holograms quickly enough to allow real-time image display.
In this paper, it is shown that the twin barriers to the realiza- Manuscript received March 01, ; accepted March 26, Date of pub- tion of a real-time, high-quality holographic display can be over- lication June 01, ; date of current version February 02, Color versions of one or more of the figures in this paper are available online method of displaying phase holograms that is optimized with at http: The DFT of these sam.
Typically, such algorithms employ the mean-squared error ples is obtained from 4 and is also complex so that MSE measure where 6 3 Following some length calculations and using the CLT, the sam- ples of the DFT are found to be distributed as and is a normalizing constant chosen to minimize 3.
For certain types of images, such as sparse spot patterns employed for telecommunications applications, this measure works well, since zero MSE implies a perfect reconstruction. Unfortunately, this metric is particularly insensitive to the low MSE values typ- ically encountered in holographically generated video images as has been previously shown .
In order to determine an improved optimization metric, it is 7 necessary to derive the properties of noise in holographic replay and, in particular, the noise resulting from the approximation of the complex FT information by a phase only hologram.
General Properties of Holographic Replay replay, it is necessary to determine the effects of quantizing the hologram. Let the samples represent the error in- Without loss of generality, the 1-D imagewhich is the troduced into the hologram by quantization, and be the discrete Fourier transform DFT of the corresponding pixel resultant noise introduced into the image.
It is clear from 7 hologram is defined as that, regardless of the PDF of the error in the samplesthe image error samples are always normally distributed in 4 real and imaginary parts and, hence, the amplitude of this error is Rayleigh distributed and is given by Since the form of is not known a priori, because it is the re- 8 sult of some unspecified calculation, it can only be assumed that is a random variable with some, as yet unknown, random dis- tribution.
The quest to determine the properties of holographic where.
It follows that the noise in any replay therefore begins by considering the properties of samples holographically formed image—regardless of the algorithm of the DFT of a random sequenceproceeding to determine used to generate the hologram—will always be Rayleigh dis- the properties of the absolute value of the signal and noise com- tributed and dependent only upon the noise variance.
A holographically generated image will consist of a desired Consider a vector of independent identically dis- signal component of average value plus additive noise due tributed i.
The central limit theorem CLT states that the sum of these i. The samples resulting from the DFT operation will therefore 10 be governed by the CLT and, regardless of the distribution ofthe real and imaginary parts of the DFT will be normally distributed provided that is sufficiently large.
This is an im- Finally, the mean and variance of the intensity samples portant result in determining the properties of noise occurring can be calculated and are found to be in holographic replay.
In what follows, the shorthand notation will be used to indicate that the samples are normally distributed. Surprisingly, 11 shows that holographically generated images can be completely characterized by just two parameters, andregardless of the algorithm used to create the hologram.
By appropriate manipulation of these parameters, therefore, it is possible to control the noise properties of a holographic display. Perceptual Significance of Noise in a Holographic Display Although 11 characterizes the statistics of holographic re- play in just two parameters, the relationship between the choice of values for each parameter and the resultant perceived image Fig.
Temporal frequency response of the eye to luminous and chromatic quality is not clear. Since it is not obvious what values a human flicker after Kelly et al. The curves show that the eye can be modeled as a brick-wall filter function with temporal bandwidth of approximately 25 Hz.
Since the rod and cone minant of the perceptual significance of noise, which suggests structures respond slightly differently to flicker, there are that a hologram generation algorithm optimized for the display disparities between pure luminous and chromatic red—green of video-style images should attempt to minimize noise variance flicker responses.
Nevertheless, the frequency response of therather than noise energyin order to generate im- human eye can be well approximated by a brick wall filter ages with perceptually pleasing properties. This result immedi- function with a temporal bandwidth of approximately 25 Hz.
Reduction of Noise Variance a ms time window and can be expressed as The conclusion that noise variance is an improved determi- nant of the perceptual significance of noise in a video image 14 suggests a method for perceptual reduction of noise by ex- ploiting temporal averaging.
Consider a holographic display which generates video subframes which are the result of If a suitable microdisplay is used to show subframes within some, as yet unspecified, hologram generation algorithm.
Each this ms period, then the integral of 14 becomes the sum- subframe contains the same image content but with different mation of Hence, by displaying frames quickly enough and independent additive noise, of some arbitrary distribution. If the average of all subframes which is, from 13substantially noise-free.Holography is the science and practice of making initiativeblog.comlly, a hologram is a photographic recording of a light field, rather than of an image formed by a lens, and it is used to display a fully three-dimensional image of the holographed subject, which is seen without the aid of special glasses or other intermediate initiativeblog.com hologram itself is not an image and is usually.
Holographic projectors use holograms rather than graphic images to produce projected pictures. They shine special white light or laser light onto or through holograms.
In laser holography, the hologram is recorded using a source of laser light, Miku, along with other Crypton Vocaloids, performing on stage as "holographic" characters.
These concerts use rear projection onto a semi-transparent DILAD screen to achieve its "holographic" effect. Shop for laser motion projection lights online at Target. Free shipping on purchases 5% Off W/ REDcard · Same Day Store Pick-Up · Free Shipping $35+Goods: Floor Lamps, Desk Lamps, Light Fixtures, Lamp Sets, Novelty Lamps, Lamp B.
(11) BUCKLEY: HOLOGRAPHIC LASER PROJECTION Equation (11) is the crucial result for deriving an improved holo- gram generation algorithm, because it describes the statistical properties of the images produced by any holographic display. Phase-only holographic projection has prompted a great deal of research and has often been cited as a desirable method of 2-D image formation, since such a technique offers a number of advantages over conventional imaging projection technology,.
Although holographic image formation was demonstrated some forty years ago, efforts at realizing a real-time 2-D video projection .