Hans Bjelkhagen

Color holography to produce highly realistic three-dimensional images

The 1964 publication by Emmett Leith and Juris Upatnieks [J. Opt. Soc. Am. 54, 1295 (1964)] introduced the possibility of using holograms to record three-dimensional (3D) objects. Since then, there has been an interest in creating display holograms, i.e., holograms primarily produced to show objects in 3D. More recently, full color holography has become a reality, which was predicted in the 1964 paper. To record a hologram in which both the 3D shape and the color of the object are accurately reproduced, at least three laser wavelengths are needed. By computer simulation of the holographic color rendering process, the required amount of laser wavelengths and their distribution within the visible electromagnetic spectrum have been investigated. The quality of a color hologram also depends on the properties of the recording material. The demand on a panchromatic material for color holography is described. Recording techniques for color holograms are presented as well as the future of color holography as the perfect 3D imaging technique.

Dynamics of Human Teeth in Function by Means of Double Pulsed Holography; an Experimental Investigation

Investigation in vivo of small deformation and mobility processes in the masticatory system of man has been until now a very intricate problem. Mechanical as well as noncontact methods have been utilized earlier in order to record the mobility pattern of teeth and prosthodontic appliances. In this paper holographic interferometry will be presented as a solution of some odontological measurement problems. The method was first tested in a simulator arrangement and then used in a number of clinical experiments. A special, totally reflecting paint was used for surface preparation prior to holography. A Q-switched double-pulsed ruby laser was combined with an electronic subminiature force sensor for pulse triggering, which was actuated by the masticatory force of the patient. Force increases and pulse positions were registered synchronously on the screen of an oscilloscope. The applied force exerted by the patients masticatory muscles could thus be defined according to its point of application, direction, amplitude, and duration. The corresponding surface deformation was evaluated by means of a synchronized, double-exposed hologram. Conclusions could be drawn regarding the relative and absolute mobility of the teeth and related structures of the holographed jaw section.

Pulsed sandwich holography

A new method is introduced which utilizes sandwich holography combined with a pulsed laser. In a special rotating hologram holder two plates sandwiched together are exposed through a slit during the double pulse operation of a laser. The two pulses are separated by less than 1 msec. Afterward, in an evaluating holder, one plate is rotated in relation to the other so that the images from the two pulses are overlapped. Then the two plates are glued together. If the emulsions are separated, fringes caused by object tilt between the two pulses can be eliminated by an analogous, but much larger, tilt of the sandwich hologram during reconstruction. Even the direction of tilt, forward or backward, is found this way.

Sandwich hologram interferometry. 5: Measurement of in-plane displacement and compensation for rigid body motion.

Sandwich holography has been used for measurements of in-plane displacement of an object. The sign of the displacement is found by tilting the sandwich hologram during reconstruction. Fringes caused by inplane rigid body motion can be compensated for, and local displacements evaluated. It is shown that an inplane motion of more than 1 mm of the object placed at a distance of about 1 m from the plates can be compensated for and a local tilt of 1.5 x 10(-3) degrees evaluated. A comparision between conventional evaluation and evaluation of sandwich holograms by measuring the tilt angle of the plates to calculate an objects displacement is shown with a series of experiments.

Pulsed sandwich holography. 2: Practical application

The vibration of a small electric drilling machine was studied holographically. The machine was held by hand, and drilling was done on a steel plate during two exposures from a double pulsed ruby laser. The recording was made on a rotating sandwich hologram. The influence of rigid motion was eliminated, and the local vibration evaluated. The phase relation between different areas was studied using three methods, all based on the simple idea that interference fringes are formed when a modified object surface is intersected by interference planes which tilt in the same direction as the observer tilts the sandwich hologram during the reconstruction.

Selection of optimum wavelengths for holography recording

Holography is an imaging technique which accurately can record both the amplitude and the phase of the scattered light from an object. However, to obtain a hologram in which both the 3D shape and the color of the object are required to be accurately reproduced, the recording of the hologram has to be performed by using at least three laser wavelengths. A mathematical model has been generated in order to simulate the holographic color rendering process by assuming ideal laser recording and reconstruction conditions which ignores the influence caused by the recording material and the processing. Based on this mathematical model a computer program with appropriate graphical user interface was implemented. The required amount of laser wavelengths and their distribution within the visible electromagnetic spectrum has been investigated in order to obtain the best possible color rendering. Simulations using three to seven laser wavelengths have been performed to better understand the sampling nature of color holography and by performing multiple simulations for all possible laser selections the optimum wavelengths have been obtained. We have found that three wavelengths are only sufficient if chosen carefully, but for improved color rendering four to five wavelengths are recommended.

Sandwich holography for storing information interferometrically with a high degree of security

An entirely new method of storing confidential data is presented. The method is based on sandwich holography, and its-main principle is the following: The basic state of a surface is recorded in one plate of a sandwich pair; in the other plate the same surface is recorded slightly deformed. When the two holograms are combined an interference fringe pattern is formed. By controlling the deformation of the surface it is possible to obtain exactly the interference pattern wainted. This controlled pattern can be used to create information that is either directly readable digital information or a binary coded information, suitable to automatic readout. The data stored in the holograms are not possible to read out from one hologram alone, but the two holograms must be put together to make the information available. The system is primarily meant to be applied on ID cards but can also be used for other applications where data have to be stored securely.

Color Reflection Holography

A method for recording color reflect ion holograms is presented. The spectral and angular selectivity, signal-to-noise ratio and brightness of the reconstructed color images are improved greatly. Silver halide light-sensitive materials are utilized. Water-soluble substances are introduced in the gelatin layer both in recording and reconstruction.

Deformation, displacement and vibration investigations in manufacturing applications using a new hologram interferometry technique

Abstract The use of sandwich hologram interferometry makes it possible to investigate objects of interest to an engineer that is was not possible to investigate before. The paper presents the sandwich technique as well as some different applications of the technique. Static deformation of a milling machine and a dynamic study on a vibrating hand-drill are included. Another application is a study of a test object one metre in length, where the object was removed from the holographic set-up between exposures and holes were drilled in one wall of the object. The deformation around the drilled holes could be studied, after that a second recording of the object repositioned in the holographic set-up was performed. Finally it is shown how an in-plane rigid body motion of 1 mm could be compensated for, and a tilt of 1.5 × 10 -3 degrees of a special test object evaluated in spite of the large translation motion.

Industrial Holographic Measurements

Examples of industrial holographic measurements. Some of the holograms were exposed in workshop environment. Holograms were made of detached machine parts that, between exposure and real-time observation, were taken out of the holographic rig and used in the machine.