KEYWORDS: Optical transfer functions, Spatial frequencies, Cameras, Sensors, Fourier transforms, Printing, Digital imaging, Light sources and illumination, Modulation transfer functions, Standards development
We present a novel method for accurately measuring the optical transfer function (OTF) of a camera lens by digitally
imaging a tartan test pattern containing sinusoidal functions with multiple frequencies and orientations. The tartan
pattern can be tuned to optimize the measurement accuracy for an adjustable set of sparse spatial frequencies. The
measurement method is designed to be accurate, reliable, and fast in a wide range of measurement conditions, including
uncontrolled lighting. We describe the design of the tartan pattern and the algorithm for estimating the OTF accurately
from a captured digital image. Simulation results show that the tartan method has significantly better accuracy for
measuring the modulus of the OTF (the modulation transfer function, or MTF) than the ISO 12233 standard slanted-edge
method, especially at high spatial frequencies. With 1% simulated imaging noise, the root mean square (RMS) error of
the tartan method is on average 5 times smaller than the RMS error of the slanted-edge method. Experiments with a
printed tartan chart show good agreement (0.05 RMS) with MTFs measured using the slanted-edge method and that, like
the slanted-edge method, our method is tolerant to wide variations in illumination conditions.
We have developed methodologies to study the usability of documents. This methodology has been applied to the study
of the impact of visible security patterns in the usability of typical office documents. Two specific information retrieval
tasks have been examined - the retrieval of text-based information from a written report and the retrieval of numerical
information from tables and graphs. The methodology we have developed aims to minimize sources of uncontrolled
variability in the measurements while simultaneously avoiding a systematic bias from learning effects and maintaining
task equivalence across all documents. We believe the methodologies developed in this work may prove useful in future
studies of document usability.
Ray tracing simulations of LED lighting systems typically use the smooth angular intensity profiles supplied by the LED manufacturer. However, measurements of a range of 5 mm LEDs presented in this paper show bright regions in the LEDs' angular distributions. The intensity patterns and bright regions vary between different LEDs, even when the measured angle for 50% integrated light output (as measured using an integrating sphere) is similar. When non-diffuse or partially diffuse optical elements such as clear light guides are part of a lighting system design, this source profile unevenness is intensified, so that bright caustic rings are formed. We have performed lighting simulations using coloured LEDs coupled into a clear light guide, and compared the light output using smoothed LED profiles with that using actual measured profiles. The simulated light patterns projected from the end of a light guide onto a screen are compared with that obtained by experiment. It is shown that the uniqueness of individual LED beam patterns needs to be taken into account for simulation accuracy. This is particularly important when the lighting system combines the output from several LEDs. It is also shown to be crucial in optimising the amount and type of diffusion required for homogenising the light distribution.
Colour mixing of red, green and blue (RGB) LEDs is demonstrated for a 6 cm long PMMA cylindrical rod with a transparent refractive index matched micro particle (TRIMM) diffuser sheet at the output end. Ray tracing simulations have been performed, and the output light distributions, transmittances and losses modelled and compared with experiment. Photographed and modelled colour mixing results are presented for rods with and without TRIMM sheet mixers. The TRIMM particles homogenize the light output of plain PMMA rods to form white light, with negligible backscattering. A simple method for measuring the concentration of the particles in the diffuser sheet is described, and computer modeling and analysis of TRIMM particle systems is discussed.