OPERATING MANUAL LPSF-100

Jodon's Pinhole Spatial Filter Model LPSF-100

Oescr ipt ion

A lens pinhole spotial filter is used to improve the quality ofa source by removing interference and other off axis light from the beam. The filter consists of a calibrated micrometric mechanism capable of translation in the X, Y, and Z axes, one 10X microscope objective and a stainless steel drilled 25~+ 2.5~ precision pinhole mounted in an easily removable magnetic holder. otherpinhole sires and microscope objectives are available from stock for Immediate shipment.

Theory of Operation .< The spatial filter is essentially a beam diverging device and a filter. The pinhole or filter serves to remove interference in the beam caused by diffraction from dust, lint, iens imperfections, etc, that are jMrt of my laser optical system. Oiffroction interference degcades the laser beam by prodwing phase and amplitude variations or modulatiorr, on the uniphase output of the loser beam. Thedvlation.Interference propagates along with the carrier, and vnless it is removed from the carrler, .fresnel zone patterns will occur in the laser be*. The modulation or diffraction interference is removed from the beam In the following manner, The laser output appears as a source at infinity. However, the interferenee producing sources appear as Huygen generators a finite distance from the filter. Due to the difference in the point of origin, the laser beam can be focused on the small pinhole and pass through it while the image of the interference producing Huygen soutce will be defocused at the pinhole and be severely attenuated. Attenuations of 40db or greater are readily produced through this manner of filtering. Operation

The apparatus should be setup initially as follows:

1, With the magnetic pinhole mount removed, odjust the laser and LPSF-100 such the diverging beam illuminates the subject in a manner suitable for photographie purposes. Care should be taken to insure that the beam. passes through the lens cell cm or closely aligned to the optical axis. This will reduce aberrations, provide optimum light economy, and ease alignment of the pinhole., Reinstall the magnet pinhole assembly. 2- Adjust the Z axis or lens focusing knob until the pinhole is substantially outside the lens . This will produce a large area light spot on the pinhole plane and ease locatiorr of the pinhole. The light psing through the pinhole will be small in this configuration so a white card about E inches square should be placed a foot or so in front of the filter. Adjust the X and Y axes controls in a raster sweep until a light spot is seen on the white card. Now adjust the Z axis knob until the lens is more nearly focused on the pinhole. The spot image will brighten and grow in sire as this is done. It will be necessary to retouch the X and Y axes adjustments as the pinhole comes into focus. The adjustments become more critical as the focus is approached. Try to keep the spot in sight on the card at all times. ' As the focus is reached the sharply defined fresnel zone patterns will become fuzzy and disappear ond the card or subject will become illuminated with a uniformly clean light.

Lens Power - Pinhole Size Relationship

The chart shows the lowest recommended for use with popular pinhole sizes. The lenses are standard achromatic icroscope objectives employed with a 1.5 to 2.5 mm diameter laser beam ot (53283 . In most cases the next lower power lens may be used if some diffraction can be tolerated in the beam output.

PINHOLE SIZE RECOMMENDED MINIMUM LENS POWER

The following mathematical relationship agrees closely with the above empirical data. D = 2~f where D = diameter of the pinhole A = of the laser light f = lens divided by the diameter of the laser beam at the lens (.OOl" = 25y) (fl 10X = 16mm, 20X = 8mm, etc.)

If the disturbances are small with respect to the beam diameter, a larger pinhole can be tolerated. '. 1 :$

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