PRINCIPLE
With RASNIK, the relative position of a lens can be measured with respect to the centre between an illuminated coded mask and a CCD.
The image of the mask is projected on the CCD and the image is digitised and stored on disk. After analyses, the two transversal coordinates are obtained from the contour positions and the codes on the mask, while the longitudinal coordinate is calculated from the scale of the image.
PARTS OF THE DEMOKIT
Light source holder (earlier version)
(last version)
This device contains an infra-red LED, a diffusor, a Fresnel lens and a coded mask. It needs a 6..12 Vdc @ 120mA power supply.
The light from the LED is deflected by the Fresnel lens into a parallel beam with a diameter equal to the active part of lens. The diffusor has two functions: it makes the position of the LED with respect to the focus of the Fresnel lens less critical and it attenuates the light beam: some cameras are over-exposed without the diffusor. For large distances (> 10 m) between the mask and the CCD the diffusor may be taken out and replaced by more transparant opaque glass or thin white paper.
For Europe, a 12Vdc power supply is included in the Demo-Kit.
The lens is of a single plano-convex type. The focal length (for red LED light - 700nm) is 1355.3 mm; for the i.r. light - 880nm the focal length is about 1360 mm. The lens should be perpendicular to the optical axis of the RASNIK system to within 50 mrad.
The CCD/Video (earlier version)
(latest version)
This is a B/W CCD sensor with an infra-red filter mounted with a standard C-mount. It's shutter time should be set to 1/50s, gammacorrection off. For Europe, a power supply (identical to the one for the light source holder) is included. Camera power requirement: 12Vdc @ 200mA, refer to camera manual. The camera can be connected with a TV monitor by means of a 75 ohm cable with a BNC connector on one side and a CINCH connector on the other. The CINCH connector fits also into the frame grabber which is part of the Demo-Kit.
The frame grabber
For the Demo-kit a straight-forward type frame grabber is used that will accept all current video-standard signals. This card should be mounted in a AT/386/486 PC with an free AT-slot. The PC should have a 3.5" floppy disk drive during installation.
Connect the power supply with the camera and a TV monitor. Connect the second power supply with the light source. Point the camera to the light source at a distance of about 1 m. The (diffuse) light should be well observable in the camera image.
Position the camera and light source, facing one another, with a distance of about 5440 mm between the mask surface and the surface of the i.r. filter. Place the lens somewhere near the centre between the camera and the mask. The plane of the lense should be rather well perpendicular to the optical axis. Also see this installation drawing and cable drawing
By looking along the optical axis of the system, the alignment can be verified withing 22 mm, being the range of the system. An image of the mask should now appear on the TV monitor. By moving either the camera or the light source holder in the direction of the optical axis, the sharpness of the image can be maximised.
If the image looks OK, then the video signal cable can be connected to the frame grabber in a PC. Follow the instructions for the installation of the frame grabber itself and its software. After storing image files on disk, they can be processed by analyses-software, made available here (fortran code; ascii 70 kB).
last update: 980119
You can mail
go to CCD_Rasnik main page.
suggestions for this page to:
Jan Koopstra.
go to the
NIKHEF homepage.
