Autofocus for multispectral camera using focus symmetry

Abstract

A multispectral camera acquires spectral color images with high fidelity by splitting the light spectrum into more than three bands. Because of the shift of focal length with wavelength, the focus of each channel should be mechanically adjusted in order to obtain sharp images. Because progressive adjustment is quite time consuming, the clear focus must be determined by using a limited number of images. This paper exploits the symmetry of focus measure distribution and proposes a simple yet efficient autofocus method. The focus measures are computed using first-order image derivatives, and the focus curve is obtained by spline interpolation. The optimal focus position, which maximizes the symmetry of the focus measure distribution, is then computed according to distance metrics. The effectiveness of the proposed method is validated in the multispectral camera system, and it is also applicable to relevant imaging systems.

Multispectral Image Out-of-Focus Deblurring Using Interchannel Correlation

“…The spectral image of the 9th channel, which corresponding to wavelength 560 nm, is well focused. We also obtain the the well-focused lens positions for all other channels using an autofocus method with the assistance of a controllable step motor [16]. The lens distance, which is computed between the lens positions of target channel and reference channel, is proportional to the variance of Gaussian blur kernel [16].…”

“…accuracy is evaluated by the peak signal to noise ratio (PSNR) with ground truths obtained using techniques in [16]. Fig.…”

“…When the lens is well focused at the reference channel with wavelength 560 nm, we will fail to obtain sharp images in the channels with wavelengths 420 nm and 700 nm. To deal with this problem, an autofocus method [16] has been proposed to compute the focus position of lens using a controllable step motor. A limitation of this method is that, although an efficient algorithm is adopted, the autofocus process is still time consuming.…”

“…This paper proposes a multispectral image out-of-focus deblurring framework which is solely based on the captured scenarios, without using additional mechanical components [16] or calibration patterns. It is assumed that the imaging system is well focused at a reference channel.…”

Color specification of a single strand of yarn from a multispectral image

“…Recent advances in optics, electronics and image processing have resulted in a proliferation of multispectral imaging (MSI) systems for color measurement in textile and garment. A MSI system can provide not only the spectral information but also the spatial information of a sample, i.e., multispectral images.…”