Alfred Dale Ducharme

Random transparency targets for modulation transfer function measurement in the visible and infrared regions

Measurements of modulation transfer function (MTF), particularly for staring imager systems, are affected by the position of the test target with respect to the rows and columns of the detector array. We demonstrate that random transparency targets of known spatial-frequency content allow shift-invariant MTF measurement in the visible, 3- to 5-μm, and 8- to 12-μm bands. Design criteria and verification procedures for the targets are presented.

Modulation transfer function testing of detector arrays using narrow-band laser speckle

method for measuring the modulation transferfunction (MTF) of a detector array from zero spatial frequency to twice the Nyquist frequency is presented. Laser speckle with a tunable, narrow spatial-frequency bandpass is used. The MTF measured with this method is compared to the MTF measured using sine targets. The results of the two methods agree to within 2%.

Design of an integrating sphere as a uniform illumination source

The design and fabrication of an integrating sphere to provide a uniform illumination source was the basis for a student laboratory project. The design process emphasizes several important concepts in radiometry. An integrating sphere is a useful component for other student laboratory experiments, and the design presented is cost-effective when compared with commercially available products.

Microlens diffusers for efficient laser speckle generation

Laser Speckle is the optical phenomena resulting from the random interference of coherent light. This phenomenon can be utilized to measure the Modulation Transfer Function (MTF) of detector arrays. Common devices used for speckle generation, such as integrating spheres and ground glass, suffer from low efficiencies less than 20%. Microlens diffusers are shown to be more efficient alternatives for speckle generation. An analysis of the statistical behavior of microlens diffusers is presented with emphasis on their application to MTF testing of detector arrays in the visible spectrum.

Improved aperture for modulation transfer function measurement of detector arrays beyond the Nyquist frequency

The design of an aperture for the generation of laser speckle with a flat power spectrum covering a wide band of the measurement spatial-frequency range is presented. This aperture allows for the measurement of modulation transfer function (MTF) from zero to twice the Nyquist frequency of a two-dimensional detector array. This design mitigates many of the measurement problems inherent in other aperture designs. The MTF measurement of a CCD detector array is used to demonstrate the measurement technique and illustrate the advantages of the new aperture design.

Effects of intensity thresholding on the power spectrum of laser speckle

Spatial-frequency filtering of laser-speckle patterns has proved to be a useful tool in the measurement of the modulation transfer function for focal plane arrays. Intensity thresholding of the laser-speckle patterns offers nearly an order of magnitude savings in digital storage space. The effect of this thresholding on the spatial-frequency power spectral density of the speckle pattern is investigated. An optimum threshold level is found that minimizes distortion of the power spectrum for the classes of speckle data used for modulation transfer function testing.

Random targets for modulation transfer function testing

Tests for modulation transfer function (MTF), particularly for staring systems, are affected by the position of the test target with respect to the rows and columns of the detector array. To alleviate the position-dependent nature of the measurement, we have developed a target that uses random patterns of known spatial-frequency content. In this way a phase-averaged MTF is measured, which is indicative of field performance on natural scenes.

ELIMINATION OF THRESHOLD-INDUCED DISTORTION IN THE POWER SPECTRUM OF NARROW-BAND LASER SPECKLE

The distortion in the power spectrum of narrow-band laser speckle that results from irradiance thresholding is quantified. A method for compensation of this distortion is presented. An optimal threshold level is presented that simplifies the compensation method.

Recent advances in the modulation transfer function testing of detector arrays

The increased complexity of imaging sensors and total number of discrete detector sites has challenged traditional testing methods. The importance of reliable modulation transfer function testing of imaging sensors with high uncertainty has consequently grown more difficult. In this paper we demonstrate the design of an aperture for the generation of laser speckle with a flat power spectrum covering a wide-band of the measurement spatial frequency range. This aperture allows for the measurement of modulation transfer function (MTF) from zero to twice the Nyquist frequency of a twodimensional detector array. This design mitigates many of the measurement issues inherent in other aperture designs. The MTF measurement of a charge-coupled device (CCD) detector array is used to demonstrate the measurement technique and illustrate the advantages of the new aperture design.

Holographic elements for modulation transfer function testing of detector arrays

A holographic technique is presented that increases the flux-transfer efficiency of laser speckle generation by a factor of 100 over the integrating sphere method. This makes a wider range of low-power lasers usable for the speckle MTF test method, and increases the number of wavelengths at which the test can be applied.