In the context of a virtual retinal display, the exit pupil of the optical device is small, and for this reason, even small eye
movement of the user will induce losing the virtual image. In order to increase pupil size, we work on devices such as
diffuse surfaces, intended to expand emission angles of sources. Unfortunately, because of laser coherence, this type of
device will create speckle noise, which will degrade image quality.
In previous papers we have described in details how this noise is generated by a classical diffuser plate, and how to
specify devices in order to decrease this noise.
In this paper, we analyze another type of pupil expander: faceplates. In addition to this modeling, we compare both
devices and conclude that the faceplate is a better device than a classical diffuser.
A model for digital image computation of a driving scene during daytime fog has been implemented in order to evaluate targets' visibility. Using a semi-Monte Carlo method to solve the radiative transfer equation inside fog, we establish a physically based rendering with an accurate lighting simulation. The fog is modeled as a layer of water droplets described by a Deirmendjian size distribution, and the scattering properties of each droplet are computed using Mie theory. The boundary conditions are sky luminance and sun illuminance on top of the fog layer, and a bidirectional reflectance distribution function (BRDF) model derived from measurements on road samples. The veiling luminance inside the layer in all directions and for several altitudes is computed and allows us to evaluate the attenuation of every surface existing in the scene. As an example, the visibility of a Lambertian reflective silhouette is given.
Since Sarajevo's sadly famous events (sniper alley), the military tried and hoped to detect snipers before they hit. The principle of the detection is based on the 'cat's eyes' effect according to which the light emitted by the system and incident on the sniper's sight reflects backward in the direction of the source. The system is thus composed of a laser emitter and a CCD array detector. Already existing equipment has been tested in operations and they present too low a probability of detection for the false alarm rate we want to reach. In order to specify equipment characteristics to industrials, it has been necessary to develop a sight laser detector model. The model presented here takes into account all the various elements of the system, from the laser emission to the CCD detection, and atmospheric propagation (ie attenuation and turbulence). The signal and noise probability density functions are calculated by combining the different elementary probability density functions encountered on the double-pass propagation. This Matlab coded model gives the probability of detection of the system for given geometrical (monostatic or bistatic) and electronic characteristics of the system and for a given probability of false alarms. In addition to this, measurements in the field made it possible to validate the budget link of the model and improve it. Those measurements also permitted to underline the importance of the target optical signature, namely its Laser Cross Section. The most significant parameters necessary to the validation of the model are measured. This study allows us to answer the question 'why is the probability of detection of existing systems too low and how could we increase it's efficiency?'
The thermo-optic coefficient (delta) n divided by (delta) T of CaF<SUB>2</SUB>, ZnS and Ge single crystals have been measured in the infrared from 20 degrees C to 100 degrees C. The laser interferometric method employed allows a determination of (delta) n divided by (delta) T with an accuracy close to 10<SUP>-6</SUP>K<SUP>-1</SUP> in the case of nonabsorbing materials. For Ge the uncertainty is increased by a factor of 3 and is mainly due to its increasing absorption coefficient with temperature. The behavior of ZnS was examined at 1.06 micrometers and 10.6 micrometers laser radiations; CaF<SUB>2</SUB> and Ge were investigated respectively at 1.06 micrometers and 10.6 micrometers.