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`29380,210
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`July 10, 1945.
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`H. F. BENNETT
`TELECENTRIG LENS SYSTEM
`
`Filed Dec. 14, 1943
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`DIAPHRAGM
`
`en]
`
`FIG. 2.
`
`
`
`
`EF = 100 mm.
`
`|LENS|_N|v_|RADII__|THICKNESSES|
`
`I
`[1.518
`|59.6|R,; = +71.2|
`ty = 7.2
`
`
`
`IT|1.649| 33.8| Ro = -31.2| to = 3.8
`R3 = -740| S; = 144.
`I|1.517|64.5| Rg =
`
`DIAPHRAGM POSITION 82.5 mm. FROM R3
`CONJUGATES AT 10X MAGNIFICATION
`SHORT CONJUGATE 74mm. FROM FR,
`LONG CONJUGATE 928 mm. FROM R5
`
`FIG. 3.
`LINEAR FIELD “H' AT SHORT CONJUGATE
`
`
`
`DISTORTIONAHASOFSHORT
`CONJUGATE
`
`H=5 mm.
`
`_
`
`H=lOmm.
`
`H=13mm.
`
`\
`
`HAROLD F. BENNETT
`INVENTOR’
`
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`BT
`
`Patented July 10, 1945
`
`ae
`
`2,380,210
`
`UNITED
`
`STATES PATENT OFFICE
`
`2,380,210
`TELECENTRIC LENS SYSTEM
`
`Harold F. Bennett, Rochester, N. Y¥., assignor to
`Eastman Kodak Company, Rochester, N. Y., a
`corporation of New Jersey
`Application December 14, 1943, Serial No. 514,231
`4 Claims.
`(Cl. 88—57)
`the
`nent. Preferably the refractive index of
`This invention relates to telecentric lens sys-
`negative element is greater than 1.62.
`It is also
`tems such as used in contour projectors.
`preferable that the rear component be a simple
`Telecentric objectives were independently dis-
`element whose refractive index is less than 1.55.
`covered by Porro in 1848 and Abbe in 1878. The
`Tt is advantageous to make the front compo-
`distinguishing feature of this type of lens is that
`nent approximately equi-convex, that is, neither
`the pupil is at infinity on one side of the lens
`of its outer surfaces should have a radius greater
`That_is_tosay._theDrincinalTavsoothat side“of thesystemareparalleltotheaxis.
`system.
`than twice that of the other one.
`
`In the accompanying drawing:
`“To avoid ambiguity in this discussion, this side of
`10
`Figures 1 and 2 show a lens according to the
`the lens system will be considered as the short
`invention and with all the preferred features
`conjugate side and also as the front. An object
`thereof.
`placed on this side of the lens and viewed from
`Figure 3 shows the distortion as of the short
`the other side, or, alternatively, projected onto a
`conjugate side at 10 times magnification.
`screen on the other side of the lens, is seen with
`The data for this example on a focal length of
`the same magnification regardless of its being
`100 mm.is as follows:
`moved in and out of focus. Also, an object of
`considerable thickness can be projected, and the
`parts nearer to the lens will have the same mag-
`nification as those farther from the lens.
`Ri=+71,.2mm.|t=7.2mm’"
`The diaphragm, of course,
`is not actually
`Fy=-—31.2mm.|43.8 mm.
`Ry= —74,0 mm
`4=144 mm.
`placed at infinity but rather the diaphragm is
`R= ©
`t&y=2.4 mm.
`placed in the conjugate plane which is at the
`Ry=—240 mm
`principal focus of the lens system so that its
`image is at infinity. An alternative arrange-
`ment is to illuminate the object with collimated
`light from a small source. The image of that
`small source is then at infinity in front of the
`telecentric system, and its image in the focal
`planeof the latter acts in about the same manner
`as a physical diaphragm would.
`Lenses of this type tend to suffer from pin-
`cushion distortion on the projection screen,
`which becomes barrel distortion if computed as
`of the object side. This pincushion distortion is
`caused by the aberrations of the lens system, that
`is, the principal rays farther from the axis are
`‘bent disproportionately so that they cross the
`axis at the diaphragm position at too steep an
`angle and strike the projection screen too far
`from the axis on the otherside.
`According to the present invention, the dis-
`tortion is very highly corrected in a telecentric
`projection lens consisting of a front positive
`component and a rear positive component with a
`diaphragm enclosed therebetween, in which the
`front component consists of a biconvex element
`cemented to the front of a meniscus element
`whose refractive index is higher by at least 0.1,
`and the rear positive component is between 0.4
`and 0.9 times as far from the diaphragm as is the
`front component and has a focal length between
`5 and 12 times its distance from the diaphragm,
`wherein the power of that surface of the rear
`component which faces the diaphragm is numer-
`ically less than the power of the whole compo-
`
`40
`
`45
`
`50
`
`Assumed AH
`
`at 0018s
`
`In this table the first column gives the height
`H of the object point above the axis, and the sec-
`ond column gives the distortion AH, all in milli-
`meters.
`It will be noted that the distortion is
`not proportiona] to the cube of the height, as is
`theoretically true of the first order or Seidel dis-
`tortion, but has a zonal shape due to the effect
`of higher order distortion. As a result it follows
`more closely a straight line than the theoretical
`distortion is supposed to.
`It can be assumed
`
`15
`
`
`
`Radii
`
`Thicknesses
`
`25
`
`4
`
`This example was designed for a magnification
`of 10, but, of course, the magnification can be
`varled considerably. When used at this magni-
`fication the object is placed at a distance of 74
`mm. in front of the Jens and the projection screen
`at about 900 mm. behind the rear surface of the
`lens. The diaphragm is placed at a distance of
`82.5 mm, behind the front component.
`The distortion has been computed on the short
`conjugate side of the objective and is shown in
`the following table also graphically in Figure 3
`of the drawing:
`
`
`
`2,880,210
`
`2 t
`
`hat this produces an average effective magnifi-.
`cation of 10.018. The third column of the table
`was obtained by computing the deviation of the
`actual image points from the positions which
`they would have if the magnification were 10.018
`and the system were free from distortion. These
`data are also reduced to the short conjugate side
`of the lens system and it will be observed that a
`smooth curve drawn thru these points deviates
`from the assumed theoretical 10.018 magnifica-
`tion by less than plus or minus 0.01 mm. Thus
`the final image appears undistorted.
`This lens is useful in checking the manufac-
`ture of certain very accurately made products
`even when the thickness of the object is greater
`than 40 mm. The lens not only has sufficient
`freedom from distortion but also the principal
`Ceti ooRLeececcemewebem 1.52|60|Ri=+0.7F__| h=0.07F.
`rays on the object side of the lens system are very
`Ti naenneseracnensennsshennns L65|34|Ro=—O.3F__| t= 0.04F,
`Ry=—0.7F._|
`¢ =1.4F
`strictly parallel to the axis so that the front edges
`20
`WMcacninnonceanesenateantepes 1.52|64 BEaes t3=0.02F.
`and the back edges show up clearly with the
`same magnification. The curvature of fleld of
`the lens is also negligible.
`It had not previously
`where the first coulmn shows the lenses num-
`been thought possible to achieve all of these de-
`bered from front
`to rear,
`the second column
`sirable ends especially with a. lens structure so
`shows the respective indices of refraction for the
`simple as this.
`D line of the spectrum, the third column shows
`What I claim is:
`the respective dispersive indices, the fourth col-
`1, A telecentric projection lens consisting of
`umn shows the radii of curvature also numbered
`front and rear positive components withgdia-
`from front to rear, the fourth radius being given
`in absolute value and the others with + or —
`signs respectively pertaining to surfaces which
`are convex or concaveto the front, and the fifth
`column shows the respective thicknesses and
`spacing, and where F is the focal length of the
`objective.
`:
`HAROLD F, BENNETT.
`
`the diaphragm as is the front component, has a ~
`focal length between 5 and 12 times its distance
`from the diaphragm, and the power of that sur--
`face of the rear component which faces the dia- .
`phragm is numerically less than the powerof the
`whole component,
`2. A telecentric lens according to claim 1 in
`which the index of refraction of said negative
`element is greater than 1.62.
`3. A telecentric lens according to claim 1 in
`which the rear component is a simple element
`whose refractive index is less than 1.55,
`4, A telecentric objective substantially accord-
`ing to the following table:
`
`Lens
`
`N
`
`|V
`
`Radii
`
`Thicknesses
`
`10
`
`15
`
`25
`
`30
`
`35
`
`phragm enclosed.
`
`therebetween,
`
`in wiiohes
`
`front component consists of a biconvex element
`cemented to the front of a meniscus negative
`element whose refractive index is higher by at
`least 0.1, the separation of the two components
`being greater than the focal length of the front
`component and in which the rear positive com-
`ponent is between 0.4 and 0.9 times as far from
`
`aclase td
`
`—<—
`
`
