Measuring the modulation transfer function of an imaging spectrometer with rooflines of opportunity

Abstract

Measuring the modulation transfer function (MTF) of digital imagers focused at or near infinity in laboratory or field settings presents difficulties because the optical path is longer than a typical laboratory. Also, digital imagers can be hindered by low-resolution detectors, resulting in the resolution of the optics surpassing that of the detector. We measure the MTF for a short-wave infrared hyperspectral imager developed by Resonon, Inc., of Bozeman, Montana, which exhibits both characteristics. These difficulties are overcome with a technique that uses images of building rooflines in an oversampled, tilted knife-edge-based MTF measurement. The dark rooftops backlit by a uniformly cloudy sky provide the high-contrast edges required to perform knife-edge MTF measurements. The MTF response is measured at five wavelengths across the imager's spectral band: 1085, 1178, 1292, 1548, and 1629 nm. The MTF also is observed at various distances from the roof to investigate performance change with distance. Optimum imaging is observed at a distance of 150 m, potentially a result of imperfect infinity focus and atmospheric turbulence. In a laboratory validation of the MTF algorithm using a monochrome visible imager, the roofline MTF results are similar to results from point-source and sine-card MTF measurements. C 2010 Society of Photo-Optical Instrumentation Engineers.

Measurement of the optical transfer function using a white-dot pattern presented on a liquid-crystal display

“…These two approaches are generally referred to as the diffractive MTF and the geometric MTF respectively [4]. Any MTF quantifies the system's capacity to spatially resolve a sinusoidal intensity pattern with discernible contrast [4,7]. Previous studies have proposed several methods for measuring the MTF of optical systems based on detector arrays of charge-coupled devices (CCDs) [8] and most of them differ essentially in the type of target or pattern used as the object pattern [9].…”

Intrapixel measurement techniques on large focal plane arrays for astronomical applications: a comparative study

“…To overcome the aliasing effect, the knife-edge is tilted slightly and the construction of the oversampled ESF is realised at the subpixel spacing [14]. Experimentation showed that a subpixel bin of one-quarter pixel had the best results [15]. Recently virtual knife edge has been proposed by Karcher et al [16].…”

“…10562 105623D-4 The tilted edge was used to construct a superresolution 1-D ESF, which was resampled at quarter-pixel increments to provide data with four times the resolution of the detector. Image courtesy [15] We notice that the estimation of the PRF through the ESF must be done carefully to avoid bias in the result as the intensity profile (erf function) or its derivative (Gaussian function) are very sensitive to noise. Among all these techniques, the spot scan is the most effective for the investigation of the intrapixel sensitivity analysis.…”

MTF Estimation Based on an Improved Step edge Method

“…According to different signal patterns, three common methods can be used to estimate the absolute MTF: step edge method [1], point source method [2], and rectangular pulse method [3]. Since the step edges are easy to find in practice, the step edge method is the most widely adopted one.…”