Planar parabolic refractive lenses are one important kind of compound refractive lenses which are used in the third generation synchrotron radiation sources as an x-ray focusing device.
In order to test the influence of the number of the individual lens on the transmission and gain, a set of 30 micro-focusing planar parabolic lenses with different number (from 1 to 30) of the individual lens was designed, which had equal apertures of 250 μm and equal focal length of 30 cm at 8.05 keV. The parameters of the lenses were optimized for performing focusing test at x-ray diffractometer (XRD). The optical simulation was completed by ray tracing by the commercial program of ZEMAX. The radius of curvature at the tip of the parabola ranged from 1.23 μm to 37 μm and the theoretical transmission from 43.3 % to 33.2 %. The theoretical width of focal line was about 3.4 μm. Ultra violet lithography was used to fabricate the planar lenses from SU-8 photoresist which
thickness was 224 μm. Measured dimension of lenses is coincident with the theoretical one. Focusing properties of the lenses were studied both at XRD and at U7B beamline at the National Synchrotron
Radiation Laboratory (NSRL) at the energy of 8.05 keV. The detector was a Peltier-cooled, 1380 x 1030 format (6.45 x 6.45 μm<sup>2</sup> pixels) charge coupled device with a fiber-optic-coupled scintillator (Photonic Sciences Ltd., "X-ray Fast Digital Imager"). The width of focal line was about 27.4 μm with a gain of 7.17 for N=8 lenses which was achieved in the NSRL. The reasons for the big difference between theoretical values and measured ones were discussed.
A focusing test of hard x-rays has been done using spherical compound refractive lens which was composed of 123 biconcave microlenses with a size of 200 μm in diameter. Each microlens was
formed by the epoxy between two bubbles which were injected into an epoxy-filled glass capillary. The focal length of the lens was 114 mm at 8 keV. The optical simulation was completed by ray tracing by
the commercial program of ZEMAX. The estimated size of focal spot was 17.2 μm. The focusing experiment has been achieved at U7B beamline in the National Synchrotron Radiation Laboratory (NSRL) at the energy of 8 keV. The measured focal spot size (FWHM) was 33.3x65.9 μm<sup>2</sup> (VxH).
Compound refractive x-ray lens is a unique device to form image of opaque object which is
illuminated by x-rays. It is consisted of a lot number of placed in-line concave microlenses and works
like ordinary refractive lens for visual light. In contrast to other x-ray optical devices, it could achieve
satisfying resolution without complicated equipment. The spherical compound refractive x-ray lens
used in this experiment is composed of 123 biconcave microlenses of 200 μm diameter. Each microlens
was formed by the epoxy between two bubbles which were injected into an epoxy-filled glass capillary.
There are three advantages ensuring good image quality of the lens for using in hard x-rays: (1) The
epoxy (C<sub>100</sub>H<sub>200</sub>O<sub>20</sub>N, 1.08 g/cc) is composed of carbon, hydrogen and nitrogen, each of them is
characterized by a low absorption coefficient for 5-30 keV x-rays. (2) Because of the nature of physics
forming the bubble, the lens surface quality is extremely good. (3) The capillary makes sure that the
series of unit lenses are well aligned coaxially. The lens focal length is 114 mm at 8.05 keV which is
adjusted according to thick-lens analysis. X-ray images of grid mesh are compared between using a
copper anode x-ray tube without filter and a synchrotron radiation source with monochromator. It could
be found that the resolution and contrast are improved a lot by using monochromatic x-rays. The field
of view and geometrical distortion around the edges of the field of view are reduced because of using a
synchrotron radiation source. For x-ray tube as source the lens achieved a spatial resolution of 5 μm
and field of view of about 700 μm. For a synchrotron radiation source, that is 3.8 μm by 2.3 μm
resolution and field of view of about 316 μm by 128 μm.