Maumita Chakrabarti

Replication of optical microlens array using photoresist coated molds.

A cost reduced method of producing injection molding tools is reported and demonstrated for the fabrication of optical microlens arrays. A standard computer-numerical-control (CNC) milling machine was used to make a rough mold in steel. Surface treatment of the steel mold by spray coating with photoresist is used to smooth the mold surface providing good optical quality. The tool and process are demonstrated for the fabrication of an ø50 mm beam homogenizer for a color mixing LED light engine. The acceptance angle of the microlens array is optimized, in order to maximize the optical efficiency from the light engine. Polymer injection molded microlens arrays were produced from both the rough and coated molds and have been characterized for lenslet parameters, surface quality, light scattering, and acceptance angle. The surface roughness (Ra) is improved approximately by a factor of two after the coating process and the light scattering is reduced so that the molded microlens array can be used for the color mixing application. The measured accepted angle of the microlens array is 40° which is in agreement with simulations.

High-flux focusable color-tunable and efficient white-light-emitting diode light engine for stage lighting

Abstract. A color mixing light-emitting diode (LED) light engine that can replace 2-kW halogen–Fresnel spotlight with high-luminous flux in excess of 20,000 lm is reported for applications in professional stage and studio lighting. The light engine focuses and mixes the light from 210 LEDs of five different colors through a microlens array (MA) at the gate of Ø50  mm. Hence, it produces homogeneous color-mixed tunable white light from 3000 to 6000 K that can be adjustable from flood to spot position providing 10% translational loss, whereas the corresponding loss from the halogen–Fresnel spotlight is 37%. The design, simulation, and optimization of the light engine is described and compared to the experimental characterization of a prototype. The light engine is optimized through the simulated design of reflector, total internal reflection lens, and MA, as well as the number of LEDs. An optical efficiency of 59% and a luminous efficacy of 33  lm/W are achieved, which is three times higher than the 2-kW halogen–Fresnel spotlight. In addition to having color rendering of color rendering index Ra>85 and television lighting consistency index 12>70, the dimmable and tunable white light can be color controlled during the operational time.

A color management system for multi-colored LED lighting

A new color control system is described and implemented for a five-color LED light engine, covering a wide white gamut. The system combines a new way of using pre–calibrated lookup tables and a rule-based optimization of chromaticity distance from the Planckian locus with a calibrated color sensor. The color sensor monitors the chromaticity of the mixed light providing the correction factor for the current driver by using the generated lookup table. The long term stability and accuracy of the system will be experimentally investigated with target tolerance within a circle radius of 0.0013 in the uniform chromaticity diagram (CIE1976).

The dynamic speckle-based wavemeter

Based on a previously devised speckle-based set-up for probing minute wavelength changes for a coherent field [1], [2] we will here present the first experiments where these changes are resolved on a millisecond time scale. The setup is based on probing the lateral shift of a speckle pattern arising from a slanted rough object, the speckle displacement being linearly proportional to the wavenumber change. Thus, by shearing the speckle pattern across a grating-like structure [3],[4] and [5], a frequency proportional to the frequency of the wavelength change can be derived as will the irradiance. Thus, a cordial display of the complex field amplitude may be obtained with a high temporal resolution and a reasonable spectral resolution. The spatial filter is here preliminarily implemented by recording the speckle pattern with a CMOS array with subsequent digital image processing mimicking the use of a spatial filter.

Monte Carlo analysis of a control technique for a tunable white lighting system

A simulated colour control mechanism for a multi-coloured LED lighting system is presented. The system achieves adjustable and stable white light output and allows for system-to-system reproducibility after application of the control mechanism. The control unit works using a pre-calibrated lookup table for an experimentally realized system, with a calibrated tristimulus colour sensor. A Monte Carlo simulation is used to examine the system performance concerning the variation of luminous flux and chromaticity of the light output. The inputs to the Monte Carlo simulation are variations of the LED peak wavelength, the LED rated luminous flux bin, the influence of the operating conditions, ambient temperature, driving current and the spectral response of the colour sensor. The system performance is investigated by evaluating the outputs from the Monte Carlo simulation. The outputs show that the applied control system yields an uncertainty on the luminous flux of 2.5% within a 95% coverage interval which is a significant reduction from the 8% of the uncontrolled system. A corresponding uncertainty reduction in Δu ′v ′ is achieved from an average of 0.0193 to 0.00125 within 95% coverage range after using the control system.

A white–cyan-red LED system for low correlated colour temperature lighting

A white LED complemented by cyan and red LEDs is a good candidate for achieving high colour rendering at low correlated colour temperatures. This is usually very difficult with commercially available white LEDs. In addition, the system is able to replace incandescent lighting in many applications; for example, the lighting for museum display cases. To investigate and optimize the colour and light distribution properties, both spectral and geometrical modelling are used. Mapping of the possible combinations of LEDs is used to locate the optimal solutions within the colour gamut, with emphasis on chromaticity and colour rendering indices. A geometric optical model is used to design and optimize the homogeneity of the colour and light intensity distribution as a function of angle. The resulting system produces diffused homogeneous white light with a tunable correlated colour temperature from 2000 K to 2400 K. Within this range the white light is characterized by a high general colour rendering index (Ra > 90), special colour rendering indices for saturated red objects (R9 > 85), and low chromaticity distance (Duv) from the Planckian locus (Duv < 2 × 10−3).

Finding small displacements of recorded speckle patterns: revisited

An analytical expression for the bias effect in digital speckle correlation is derived based on a Gaussian approximation of the spatial pixel size and array extent. The evaluation is carried out having assumed an incident speckle field. The analysis is focused on speckle displacements in the order of one pixel, thus having no speckle decorrelation. Furthermore, sensitivity is a main issue wherefore we need speckles close to the pixel size, which means that speckle averaging becomes important, and that Nyquist’s criteria may not be fulfilled. Based on these observations, a new correlation method is introduced, which alleviates the need to know the expected shape of the crosscovariance between the original and the off-set recorded speckle pattern. This concept calls for correlating the crosscovariance with the auto covariance, which essentially carries information on the expected shape of the crosscovariance.

Speckle-based wavemeter

A spectrometer based on the application of dynamic speckles will be disclosed. The method relies on scattering of primarily coherent radiation from a slanted rough surface. The scattered radiation is collected on a detector array and the speckle displacement is monitored during a change in the incident wavelength. The change of wavelength gives an almost linear phaseshift across the scattering surface resulting in an almost linear shift of the speckle pattern, which is subsequently monitored. It is argued that frequency changes close to 100 MHz can be probed using a common CMOS array. Experiments showing agreement with theoretical predictions will be given. An extension of the method, with which fast wavelength changes in the GHz regime can be probed, will be discussed but not experimentally verified. This method relies on shearing the dynamic speckle pattern across a cylindrical lens array as it’s well-known within spatial filtering velocimetry.