Arturo González-Vega

Flicker minimization in an LCoS spatial light modulator

We present a method for reducing the phase flicker originated by the pulsed modulation of a Liquid Crystal on Silicon (LCoS) Spatial Light Modulator (SLM). It consists in reducing the temperature of the LCoS in a controlled way, in order to increase the viscosity of the liquid crystal. By doing this, we increase the time response of the liquid crystal, and thus reduce the amplitude of phase fluctuations. We evaluate the efficacy of this method quantifying the temporal evolution of phase shift using an experiment that is insensitive to optical polarization fluctuations. Additionally, we determine the effect of the temperature reduction on the effective phase modulation capability of the LCoS. We demonstrate that a reduction of up to 80% of the flicker initial value can be achieved when the LCoS is brought to -8 °C.

Flicker reduction in an LCoS spatial light modulator

The spatial light modulators based on liquid crystal (LC-SLMs) have found applications in areas like beam shaping, optical tweezers, and microscopy. The use of these devices for pupil engineering has been an active research field. Many experiments with LC-SLMs involve the use of a digital camera, or even an additional LC modulator. Discrepancies in the refreshing rates of the modulator and the camera, or even between two nominally equal modulators, give rise to beating and intensity fluctuations, known as flicker, in the point spread function generated by the engineered pupil. In this paper we present a method for reducing the flicker caused by a liquid crystal on silicon (LCoS) SLM. It consists in reducing the temperature of the LCoS in a controlled form, in order to increase the viscosity of the liquid crystal, and with this reduce the amplitude of the intensity fluctuations due to the refreshing mechanism. We demonstrate that the flicker has been reduced to only the 25% of its initial value, when the LCoS is brought at 0 °C at a given gray level. Additionally, we found that the proposed reduction of temperature does not affect the dynamical range of phase control.

Study of emissivity changes presented by inorganic and organic soil under drying at ambient temperature

Thermal emissivity can be used to determine the moisture content in soils, but it is strongly influenced by the kind of soil and the organic matter content. These experiments were performed by recording infrared images of the wet soils as a function of water loss. Samples with different organic matter content were wet until reach the field capacity; then, a sequence of thermal images was acquired to follow the different stages of drying process of the studied samples. The emissivity was calculated indirectly by measuring the reflection and absorption of the samples.

Study of variations in soil water potential with the incorporation of charcoal and carbon nanotubes through infrared thermal images

Different concentrations of charcoal and carbon nanotubes were incorporated in different mix types of soil samples, these were previously chemically characterized, and physically grain standardized, then the water potential was measured by traditional procedures, which need to consider the water composition and the soil salinity to achieve an accurate measurement, and by infrared thermal images where the water potential was correlated with the superficial emissivity. It was observed that the organic incorporation increases the water potential but it depends of soil gradation, a biggest increment of the water potential was observed in a poorly graded soil than that observed in a well graded soil; the nanotubes in low concentrations do not present considerable changes in the water potential, and in high concentrations the cost is not profitable. It was analyzed the minimum concentration changes of charcoal and nanotubes in the soil that can be measured with thermal emissivity, and the deepness at which the infrared thermal images can measure, also it was studied the rate of water drain in the different soils, and the ability of follow this with thermal sequence of images.

Volumetric liquid flow measurement through thermography to simulate blood flow in an artery

Encouraged to improve the procedure to measure the blood flow in cases with peripheral artery disease using thermography, that allows to evaluate several arteries simultaneously, it was developed an alternative to measure the volumetric flow through a conduit, it was studied the variation of the thermal energy computed from thermal images due to changes in flow at different temperatures, and it was observed that the measurement is not strongly influenced by the emissivity of the conduit, the ambient temperature and humidity, but that is necessary to establish an adequate calibration of the camera to can use it as measurement instrument.

Location of foot arteries using infrared images

In this work are presented the results of localization of foot arteries, in a young group of participants by using infrared thermal images, these are the dorsal, posterior tibial and anterior tibial arteries. No inclusion criteria were considered, that causes that no strong statistical data about the influence of the age in the arterial localization. It was achieved to solve the confusion when veins present a heat distribution similar to the artery and in the position of this. it contributes to enhance the rate of location of arteries. In general it is possible to say that the use of infrared thermal images is a good technique to find the foot arteries and can be applied in its characterization in a future. The procedure proposed is a non-invasive technique, and in certain fashion does not requires specialized personnel to achieve locate the arteries. It is portable, safe, and relatively economical.

Hybrid super-resolving pupils with smooth profile

One form of achieving super-resolution consists in reducing the size of the Point Spread Function (PSF) of a diffraction-limited optical system. For this end, good results have been obtained by means of pupils with discontinuous profiles. When devices such as deformable mirrors are used to generate such pupils, however, these cannot be accurately reproduced. To overcome this limitation, we developed a method based on a Bessel series expansion to design pupils with smooth profiles, and found that, properly designed, pupils with continuous profiles will perform equally well, and in some respects better, than pupils with discontinuous ones.