US 4592636

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Let me check for any litigation or CAFC docket activity involving this patent.

Summary of US Patent 4,592,636

Important caveat up front: I ran targeted web searches rather than a direct, credentialed query of the USPTO Patent Center / Patent Trial and Appeal Board APIs or the CAFC's PACER/CM-ECF docket system. No CAFC 2026 docket entry, appeal, or IPR referencing 4,592,636 surfaced in any search. Given the patent's age and status (see below), that absence is expected rather than meaningful. I do not have authoritative confirmation of zero litigation — only that nothing appeared.

Bibliographic data

Note: the patent number is 4,592,636 (not 4,592,635 or any neighboring number). A search for the bare string "4592636" also returns an unrelated Argentine trademark docket ("Acta 4592636"), which I excluded.

Abstract (as issued)

A view finder optical system comprises an optical path split mirror which divides a second optical axis from a first optical axis of an objective optical system; a predetermined image forming plane arranged on the second optical axis to have an image coming from the objective optical system formed thereon; a first prism arranged along the second optical axis to bring about reflection twice on the second optical axis and to have the incidence plane thereof and a second reflection plane on the same plane; a second prism which is disposed behind the first prism and is provided with a roof type reflection plane for a first reflection and a reflection plane for further reflecting the second optical axis after the second optical axis is reflected by the roof type reflection plane; and an eye-piece on the second optical axis after it comes out from the second prism. The spacing distance between the first optical axis and the optical axis passing through the eye-piece is shorter than a distance between the first optical axis and the predetermined image forming plane. An optical axis which passes through the exit plane of the first prism is arranged to be tilting to an axis which is perpendicular to the exit plane of the first prism.

Plain-language overview of the independent claim

Claim 1 is the sole independent claim. It defines a viewfinder optical system with these required elements, in combination:

1. Optical path splitting means — e.g., a movable reflex mirror — that diverts a second optical axis off the first optical axis of the objective (taking) lens, creating the finder path.
2. A predetermined image forming plane on that second axis — in the embodiment, the focusing-screen plane where the objective forms an image.
3. A first prism behind that image plane, which reflects the beam an even number of times (twice in the embodiment). Two structural conditions are recited: (a) the axis emerging from the first prism's exit plane is slanted relative to a normal to that exit plane (i.e., the exit beam is oblique, not perpendicular), and (b) the first prism's incidence plane and its second reflection plane lie on the same plane.
4. A second prism placed behind the first, having a roof-type reflection plane that makes the first reflection and a further reflection plane that redirects the axis after the roof reflection. Together the two prisms invert the image to a correct, erect, non-mirrored orientation.
5. An eyepiece whose optical axis coincides with the second axis exiting the second prism.
6. The spatial/dimensional limitation that is the heart of the claim: the distance between the first (objective) optical axis and the eyepiece optical axis is shorter than the distance between the first optical axis and the image forming plane.

The practical point of the claim is compactness: by folding the finder path so the eyepiece sits closer to the taking-lens axis than the focusing screen does, the optical path between the screen and the eyepiece is kept short, which preserves a high field ratio (≥90% in the described examples) and a high finder magnification (~0.6) even for a small image format such as a 2/3-inch sensor — something the conventional pentagonal roof prism cannot do in that situation.

Dependent claims at a glance

• 2 — first prism's incidence plane parallel to a plane containing the first optical axis.
• 3–6 — information display: a light-guiding prism injecting display light into the first or second prism, cemented to (4) the first reflection plane of prism 1, (5) the second reflection plane of prism 1, or (6) the incidence plane of prism 2.
• 7–11 — a positioning notch on a prism for accurate mounting on the camera framework: (8) at both ends of the edge between prism 1's incidence and first reflection planes; (9) in prism 2's second reflection plane; (10) at both ends of the edge between prism 2's second reflection plane and exit plane; (11) at the edge between prism 2's second reflection plane and roof plane.
• 12 — the axis reflected by the roof-type plane travels in a hindward (rearward) direction.

Distinguishing prior art cited on the face

Only two references were cited by the examiner: US 3,541,919 (Zeiss Stiftung, 1970, view finder with roof prism) and US 4,021,823 (Asahi Kogaku, 1977, photometric device in an SLR viewfinder). The specification also discusses, as background, Japanese Laid-Open Utility Model 56-53372 (a viewfinder for a much larger film format) and 49-117841.

Practical note

Because the patent expired on May 1, 2005 (fee years 4, 8, and 12 were all paid, so it ran its full term), it is not an enforceable right today and is useful primarily as prior art. Its disclosure has been cited in later patents including US 5,166,718, US 5,541,768, US 6,281,381, and US 6,430,375. Conversely it cites US 4,592,636-family material only via the two US references above.

If you need certainty on the CAFC/litigation question, that requires a direct PACER or Docket Navigator query rather than open web search — say the word and I can frame the exact query fields to use.

I'll search for litigation involving US patent 4592636 across patent litigation databases and court sources.

Initial searches returned irrelevant matches. Let me search dedicated patent litigation sources.

Let me try a few more targeted searches on litigation-specific databases.

Proceedings overview

Zero AIA trial proceedings on file — the USPTO ODP structured block reports no IPR, PGR, or CBM petitions for US 4592636 (0 active / 0 claims invalidated / 0 claims sustained / 0 settled / 0 institution-denied), and independent web searches (PTAB E2E / PTAB Decisions, CourtListener, and general search) surfaced no petitions, FWDs, rehearings, or Federal Circuit appeals involving this patent number. The defensive posture is therefore not "the patent has survived a validity attack" and not "the claims are dead" — it is that the patent is long expired (anticipated expiration 2005-05-01, ~17 years before the first AIA trial could even be instituted against it), so the asserting party cannot be a live patent owner and any demand letter citing it deserves scrutiny before you spend a dollar responding. (Web search corroboration is thin by design here — the absence of results, not a found document, is the evidence. Flag any recently-filed proceeding you locate; ODP is the canonical list and it shows none.)

Why there is nothing to report, proceeding by proceeding

There is no {PROCEEDING_NUMBER} — {Petitioner} v. {Patent Owner} heading to write. Rather than fabricate a docket, here is the structural reason the list is empty, with the record citations that ground it:

No proceeding can exist — the eligibility windows closed before the forum opened

• Patent identity (verbatim from the structured block): US 4592636 A, "View finder optical system"; inventors Masatake Kato, Tetsuharu Nishimura, Hideo Yokota; assignee Canon K.K.; priority 1984-05-07 (JP 59-090762/3/4 and related); filed 1985-05-01; published 1986-06-03; legal status "Expired - Lifetime"; "Anticipated expiration 2005-05-01"; 12 claims issued.
• Type: none — Inter Partes Review, Post-Grant Review, and Covered Business Method review are the only AIA trial types, and no petition of any type exists.
• Filed: N/A.
• Status: N/A (no proceeding). The patent itself carries the ODP status Expired – Lifetime.
• Judge panel: N/A — no panel was ever assigned.
• Petition grounds: N/A — no § 102 / § 103 / § 112 grounds were ever advanced at the Board.
• Institution decision: N/A — no § 314(a) or § 324(a) determination was ever made.
• Final Written Decision: N/A — no FWD exists. Important: the absence of an FWD means no claim of 4592636 has ever been canceled, confirmed, or even construed by the PTAB. Do not treat any claim as "held valid" — it was simply never tested there. The only Board-side record activity for this family is ordinary ex parte prosecution appeals (the "PTAB Appeals" side), which are not AIA trials and do not produce claim-level validity rulings.
• Settlement / termination: N/A.
• Appeal: No appeal to the U.S. Court of Appeals for the Federal Circuit from any PTAB decision on this patent, because there was no PTAB decision to appeal. (The only Federal Circuit activity touching this family would be unremarkable — searches returned nothing.)
• Defensive value: The meaningful point is temporal and jurisdictional, not substantive: AIA trials became available 2012-09-16, and this patent's term lapsed 2005-05-01. There was a ~7-year gap in which no petitioner could file and the patent was already dead. A demand letter asserting US 4592636 today is asserting an expired patent — and an expired patent cannot be infringed (infringement requires an unexpired term; post-expiration "damages" for past acts would require the patent to have been in force during the accused conduct, i.e., before 2005-05-01, which also implicates the 6-year § 286 damages lookback and laches/§ 282 defenses). That, not an IPR, is your cleanest dispositive argument.

Strategic summary

Claim status: all 12 claims UNTESTED at the PTAB, and all 12 expired. Because no AIA trial was ever filed, there is no claim-cancellation ledger to report. Claims 1–12 of 4592636 — including independent claim 1 (the two-prism/roof-prism viewfinder with the eye-piece optical axis closer to the objective optical axis than the image-forming plane), and dependents 2 (incidence plane parallel to the first optical axis), 3–6 (information-display light-guiding prism cemented to the first reflection plane, second reflection plane, or second prism's incidence plane), 7–11 (various positioning notches), and 12 (hindward reflection by the roof-type plane) — all stand exactly as issued, but that is a statement about prosecution, not about adjudicated validity. Nothing has been narrowed, and nothing has been struck. A defendant should not over-read the zero count as either a threat or a shield.

Estoppel landscape: empty. Section 315(e)(2) estoppel only attaches to a petitioner that obtains a final written decision. With no IPR, PGR, or CBM, no party is estopped from anything before the PTAB regarding this patent, and no prior-art ground has been "used up." Every § 102/§ 103 combination of patents and printed publications remains theoretically available — but that availability is academic, because the expiration of the patent (not any estoppel) is the controlling obstacle, and the Board cannot cancel claims of a patent whose term has already run for the purpose of freeing an accused product that post-dates the term. Practically, the invalidity fight for this patent lives in district court § 282, not at the Board.

Pattern signals: none present. No petitioner filed once, let alone repeatedly, because the procedural door never opened during the patent's life. The patent owner (Canon K.K.) has not "pursued PTAB appeals aggressively" with respect to this patent — there is no PTAB appeal to pursue. And there is no defensive aggregator (Unified Patents, RPX, etc.) in the chain; the only named assignment in the record is the original 1985-05-01 grant to Canon Kabushiki Kaisha (REEL/FRAME 004402/0107), with fee payments logged through the 12-year window (1997-12-03, Year 12) and then expiration. In other words, this is a 37-year-old, naturally expired, never-litigated-at-the-Board Canon patent — not an NPE assertion vehicle and not a post-2012 IPR target.

Recommended next steps

• If you are a defendant and have received a demand citing US 4592636, lead with expiration, not invalidity. The structured ODP record states the patent is "Expired – Lifetime" with an anticipated expiration of 2005-05-01. Cite that status and the 1985-05-01 filing date (20-year term under pre-URAA/§ 154). An expired patent is not infringed going forward, and any backward-looking theory confronts 35 U.S.C. § 286's six-year damages bar (accrued damages, if any, would have to have accrued before 2005-05-01 — now more than two decades stale) plus laches/equitable defenses. There is no FWD to quote because none exists; the disposition you cite is the expiration event and legal status in the ODP/Google Patents record, not a Board ruling.
• There are no active proceedings, so there are no trial-stage milestones to track (no § 314(a) institution deadline, no oral hearing date, no statutory 1-year FWD clock). Any opposer attempting to resurrect validity activity would be met at the threshold with a § 311(c)/§ 321(c) timeliness problem and the practical reality that AIA review of a term-expired patent yields no exclusionary relief.
• If a representative or letters-rope aggregator is invoking this number, treat it as a red flag on their diligence, not a sign of a hardened patent. Well-asserted live patents eventually attract IPRs; this one never could — and the reason is that its enforceable life ended in 2005. Verify the number literally (US 4,592,636; application US 06/729,292; issued 1986-06-03) against the demand before responding, but the expected answer is that the asserted asset is functionally unassertable.

Bottom line for a defendant: there is no PTAB history to exploit and no PTAB estoppel to worry about — the dispositive point is cleaner than any IPR: US 4592636 expired on 2005-05-01 and has never been the subject of an AIA trial, so any live infringement theory built on it should be met with a straightforward expiration/no-standing objection.

I'll verify the assignment record against the USPTO Assignment Center and cross-reference ownership sources.

Assignment / Ownership-Chain Analysis — US 4,592,636

Method note and limitation up front. My queries were routed through the open web and a Google Patents legal-events mirror of the USPTO assignment data, not a credentialed session against assignmentcenter.uspto.gov. The Google Patents "Legal Events" block for this patent reproduces USPTO assignment data (transaction code AS, reel/frame, assignor list, effective date), and it shows exactly one assignment event. No further recorded conveyance of any type (change of name, merger, security agreement, license, release) appears. I could not retrieve the correspondent-of-record field, which is not exposed in the mirrored data; I flag that as a gap rather than guessing at an attorney name.

Inventors

All three are named in the recorded assignment instrument as assignors to CANON KABUSHIKI KAISHA, 3-30-2 Shimomaruko, Ohta-ku, Tokyo, and the assignment was executed 1985-04-24, i.e. before the US filing date of 1985-05-01 — the normal pre-filing employment-assignment pattern for a Japanese corporate camera/optics R&D group.

Pattern check: no unusual signals. There is no evidence any inventor left Canon within 12 months of filing or assigned to a side entity. To the contrary, third-party inventor-leaderboard data shows Masatake Kato as a career Canon inventor with ~45 Canon-assigned US patents (https://www.patentleaderboard.com/canon/masatake-kato/[828193](/patent/828193)?page=2). This is the opposite of the "all inventors peel off early" precursor to a portfolio fire-sale.

Original assignee

Canon Kabushiki Kaisha / Canon Inc. (TSE: 7751) — named both as original assignee on the face and as current assignee. Canon is a major operating manufacturer of cameras, lenses, office imaging equipment, and semiconductor lithography tools; it is not an IP-holding shell.

Product embodiment: the claims are directed to a TTL electronic-camera viewfinder — specifically a two-prism roof-prism finder with a slanted exit axis, sized for a small (2/3-inch) image sensor, as described in the specification (Figs. 3–5, Tables 1–3). Canon was and remains a volume manufacturer in exactly this technology space (its 1980s video/electronic camera line and subsequent EOS-family finder optics). I found no evidence of a specific commercial model publicly mapped to this patent number, so I characterize "shipped a product embodying the claims" as highly likely by corporate context rather than documented per-model.

Current status: operating, publicly listed, no bankruptcy, no acquisition. The US patent itself expired 2005-05-01 (20-years-from-filing term under the URAA transition; the fee years 4 / 8 / 12 records confirm the full term was run out rather than abandoned).

Assignment timeline

1985-04-24 (executed) / recorded 1985-05-01 — Reel 004402/0107

• Conveyance: Assignment — "ASSIGNMENT OF ASSIGNORS INTEREST" (USPTO transaction code AS)
• Assignor: Masatake Kato; Tetsuharu Nishimura; Hideo Yokota
• Assignee: Canon Kabushiki Kaisha, 3-30-2 Shimomaruko, Ohta-ku, Tokyo, Japan
• Correspondent: Not determinable from the sources I could reach. The mirrored legal-events record exposes the reel/frame and the assignor list but not the recording attorney or agent. This is a data gap, not a null finding — the underlying assignment-center record almost certainly carries a correspondent, but I did not retrieve it and will not invent one.
• Context: Pre-issuance employment assignment (inventors → employer), executed seven days before the US filing date and covering the US national application together with its seven Japanese priority filings (JP 59-090762/63/64, JP 59-075564/65/66, JP 59-119473, JP 59-090478).

Non-assignment legal events (for completeness, from the same legal-events block):

• 1986-04-02 — patent grant recorded
• 1989-12-01 — fee-payment procedure, payor number assigned; entity status: large entity
• 1990-01-04 — maintenance fee, year 4 (with same-day late-payment surcharge SULP)
• 1993-12-03 — maintenance fee, year 8
• 1997-12-03 — maintenance fee, year 12
• 2005-05-01 — anticipated expiration; status "Expired – Lifetime"

No post-issuance assignment of record exists. There is no transfer to a holding company, no security interest, no name-change filing (Canon's "Canon Kabushiki Kaisha" → "Canon Inc." usage change in English-language documents did not generate a recorded US assignment here), and no release. Canon has been the sole recorded owner from 1985 through expiry.

Timeline diagram

timeline
    title Ownership of US 4592636
    1984 : Inventors file seven JP priority cases
    1985 : Inventors assign rights to Canon
         : US application filed
    1986 : Patent issued to Canon
    1990 : Maintenance fee year 4 paid
    1993 : Maintenance fee year 8 paid
    1997 : Maintenance fee year 12 paid
    2005 : Patent expires

NPE / troll-pattern signals

Cross-reference result: SEC filings, RPX/Unified-type asserter directory searches, and open litigation searches produced no entity other than Canon associated with this patent. No assignment record, no successor-in-interest, no assertion campaign.

Minor inconsistency to flag against the prior sections: the earlier bibliographic table lists Int'l class G03B 13/02, whereas the Google Patents classification block for this document prints G02B 5/04 (prisms) and G03B 19/12 (reflex cameras). These are different classification vintages/authorities rather than a substantive conflict, but the two lists should not be quoted interchangeably as "the" classification.

Verdict

Insufficient data — in the specific sense the rubric defines it: only the original assignment exists. But the substance here is stronger than "unknown." The record is a single pre-filing employment assignment executed 1985-04-24 and recorded 1985-05-01 at reel 004402/0107, moving the patent from three Canon R&D inventors to Canon Kabushiki Kaisha, with no subsequent conveyance of any kind before the patent's 2005-05-01 expiry. Zero of the eight NPE signals is present; the chain never left the original operating-company assignee, so this patent was never an NPE asset. Signal 3 (repeat correspondent) is the only unclear entry, and that is purely a retrieval limitation on my side, not evidence of anything.

Verification link: https://assignmentcenter.uspto.gov/ (search patent number 4592636; equivalent mirror at https://patents.google.com/patent/US4592636/en under Legal Events). Anyone needing the correspondent-of-record for reel 004402/0107 must pull the assignment document itself from the Assignment Center, since the mirrored data does not carry that field.

I'll research each cited reference for US 4,592,636. Let me run targeted searches.

Let me get the actual claim text and details for US 3,541,919, plus verify the USPTO record.

Prior-Art Analysis — US Patent 4,592,636 ("View finder optical system")

0. USPTO record confirmation (patent number treated literally)

Note on scope of this task: US 4,592,636 has an unusually thin citation record. The USPTO front page carries exactly two U.S. patent citations, both cited by the examiner:

1. US 3,541,919 (Zeiss Stiftung)
2. US 4,021,823 (Asahi Kogaku Kogyo K.K.)

There are no other U.S. patent citations on the face of the patent. The specification additionally discusses two Japanese Utility Model Laid-Open publications in its "Description of the Prior Art," which I treat separately in §3 because they are discussed prior art rather than cited references.

I flag one inconsistency with the earlier-generated sections: the prior "Patent summary" listed the international class as G03B 13/02 and US classes 354/225 and 354/155, whereas the authoritative full text supplied here shows the Google Patents classifications as G02B 5/04 (prisms) and G03B 19/12 (SLR reflex cameras). The classifications in the authoritative text should control; the earlier class numbers are not reproduced verbatim anywhere in the supplied patent text.

1. Cited reference A — US 3,541,919

Brief description

A reflex-camera viewfinder built around a roof-edge (roof-type) prism — the classic pentaprism/roof-prism erecting system in which the finder beam enters a bottom face, is reflected by the two roof surfaces, then by a further reflecting face, and exits toward the eyepiece. Weyrauch's contribution is information injection into the finder field: one or more small auxiliary reflecting prisms are cemented to the prism's entrance face (and/or a reflecting face is ground on the prism base) so that an indication beam — e.g., the moving needle/hand of an exposure meter, or diaphragm/aperture and shutter-speed scales — can be projected into the viewfinder image without enlarging the camera body. The auxiliary prism picks up the display light, reflects it into the roof-prism interior path, and the display image is superimposed on the finder field. The patent explicitly states the teaching is applicable to "all types of prisms which have a roof edge for the purpose of reversing the sides of the finder image, i.e. particularly penta roof edge prisms."

Potential § 102 relevance to specific claims of 4,592,636

Claim 1 (independent) — NO anticipation. Weyrauch discloses a roof-prism finder but does not disclose the combination that defines claim 1: (i) a two-prism system (a first prism performing an even number of reflections with its incidence plane and second reflection plane coplanar and with the exit beam slanted relative to the exit-plane normal, followed by a second prism), nor (ii) the claimed dimensional relationship — "a distance between said first optical axis and the optical axis of said eye-piece being arranged to be shorter than a distance between said first optical axis and said predetermined image forming plane." At most this reference is § 103 background on roof-prism erecting finders.

Claims 3–6 (information display by a light-guiding prism) — the most probative potential art. This is where US 3,541,919 has real force:

• Claim 3 ("information display means … and a light guiding prism for guiding said light at least to one of said first and second prisms") — Weyrauch discloses information display means (exposure-meter hand and scales) and a light-guiding prism feeding display light into a viewfinder prism. The only gap is that Weyrauch's receiving prism is a single roof-edge prism rather than a claim-1 "first/second prism" pair; anticipation would require the petitioning party to read the two-prism claim-1 structure out of the preamble, which is not available.
• Claim 4 ("light guiding prism is cemented to the first reflection plane of said first prism") and Claim 5 ("cemented to the second reflection plane of said first prism") — Weyrauch squarely teaches cementing the auxiliary prism to a reflecting/entrance face of the finder prism (prism 2 cemented to entrance face E in his FIG. 1; prism 2a and reflecting faces 3a on other faces in his FIG. 2). The doctrine issue is whether Weyrauch's single prism face maps onto the claimed "first" or "second" reflection plane of the claimed first prism. Because claims 4 and 5 are dependent on claim 3 and therefore inherit claim 1's two-prism architecture, anticipation is not established; US 3,541,919 is best characterized as § 103 art against claims 3–6 (display-light injection into a finder prism by cementing a light-guiding prism to a prism face).
• Claim 6 (light-guiding prism cemented to the incidence plane of the second prism) — Weyrauch does not disclose a second prism at all, so this claim is not anticipated.

Claims 7–11 (positioning notch) — NO. US 3,541,919 contains no notch/mounting-datum teaching.

Claim 2 (incidence plane parallel to the first optical axis) — NO. Weyrauch's entrance face is described as perpendicular to the incident finder axis in the conventional roof-prism layout; there is no teaching of the claimed parallelism of the incidence plane to a plane containing the taking-lens axis.

Claim 12 (rearward reflection off the roof plane) — NO anticipation on the record reviewed. Not a distinguishing disclosure found.

Net: US 3,541,919 does not anticipate any claim of 4,592,636. It is strongest as § 103 art on the information-display/light-guide-prism features (claims 3–6) and as general background on roof-prism erecting finders.

2. Cited reference B — US 4,021,823

Brief description

A TTL photometric arrangement for an SLR with an eye-level/waist-level selectable, interchangeable finder. The finder optical path runs from the focusing screen through a condenser lens into a first prism having a roof face and a total-reflecting face; light is reflected first by the roof face and then by the reflecting face — i.e., twice reflected within the first prism — and then enters a second (replaceable) prism and an eyepiece. The photometric function is achieved by defining a separate optical path for light emerging from the first prism at a peripheral position outside the finder field: a second prism is adhered to a region of the reflecting face (the second region, outside the finder field) and directs light to a light-receiving photometric element aligned with the light path from the center of the focusing screen. The stated design goal is that the photometric elements sit at conjugate positions and equal angular positions relative to the imaging plane so that interchange of finders does not disturb photometry.

Potential § 102 relevance to specific claims of 4,592,636

Claim 1 (independent) — NO anticipation. Superficially the reference shares vocabulary with the claim ("first prism," "second prism," "reflecting face," twice-reflected light). But it does not disclose:

• a first prism whose incidence plane and second reflection plane are on the same plane;
• a first-prism exit beam slanted relative to the normal of the exit plane;
• a roof-type reflection plane in the second prism performing the first reflection and a further reflection plane after it (in US 4,021,823 the roof face is in the first prism, not the second); or
• the governing dimensional limitation that the eyepiece axis lies closer to the first (taking-lens) optical axis than the image forming plane does.

This last limitation is the point of novelty of claim 1, and US 4,021,823 — whose second prism exists only to divert a sliver of off-axis light to a photocell — does not teach or suggest it. No § 102 anticipation.

Claims 3–6 (information display / light-guiding prism) — partially analogous, but addressing a photometric beam, not a display. US 4,021,823 does disclose "a second prism … adhered to the second region of the reflecting face for causing a portion of the light … to impinge upon the light receiving element." Structurally this is a prism cemented to a reflecting face of the finder prism, which parallels the geometry of claims 4 and 5. The substantive difference is purpose: US 4,021,823's prism carries light out to a photometric light-receiving element, whereas claims 3–6 require an information display means whose display light is carried into the finder prism. Because claims 4 and 5 depend on claim 3 (which requires "information display means … to display information"), the photometric disclosure does not meet the claim language, and anticipation fails. Best characterized as § 103 art on the "separate prism cemented to a reflecting face of the finder prism" sub-feature, and as art on the general two-prism/twice-reflection finder architecture.

Claim 6 (light-guiding prism on the incidence plane of the second prism) — NO. US 4,021,823 adheres its second prism to the reflecting face of the first prism, not to the incidence plane of the second prism.

Claims 7–11 (positioning notch) — NO. No notch or prism-mounting-datum teaching.

Claim 2 — NO. Claim 12 — NO.

Net: US 4,021,823 does not anticipate any claim. Its relevance is § 103 (two-prism, twice-reflecting finder with a secondary prism affixed to a reflecting face) and as background for the interchangeable-finder/SLR context.

3. Prior art discussed in the specification (not examiner citations)

The 4,592,636 specification expressly distinguishes two Japanese Utility Model publications, which are part of the intrinsic record and should be considered if a validity challenge is mounted:

These two documents are the closest thing in the file to art directed at the same class of arrangement, and any § 102/§ 103 attack on claim 1 would most plausibly start from SHO 56-53372 combined with an SLR roof-prism reference. On the record reviewed, however, neither is shown to disclose the claim-1 dimensional limitation (eyepiece axis closer to the first optical axis than the image forming plane).

4. Conclusion

Most relevant prior art for US 4,592,636:

1. US 3,541,919 (Zeiss Stiftung, pub. 1970-11-24) — most relevant to the information-display subject matter (claims 3–6), because it teaches cementing an auxiliary light-guiding prism to a face of a roof-type finder prism to inject display light into the finder field.
2. US 4,021,823 (Asahi Kogaku, pub. 1977-05-03) — most relevant to the two-prism, double-reflection finder with a secondary prism affixed to a reflecting face subject matter (claim 1 architecture and claims 3–6 geometry), and to the SLR/interchangeable-finder context.

Anticipation verdict under 35 U.S.C. § 102:

• Neither cited reference anticipates independent claim 1. The claim-1 limitation that has no counterpart in either reference is the spatial relationship "a distance between said first optical axis and the optical axis of said eye-piece being arranged to be shorter than a distance between said first optical axis and said predetermined image forming plane," together with the first prism's coplanar incidence/second-reflection planes and the oblique first-prism exit beam.
• No cited reference anticipates any dependent claim when the dependency on claim 1 is respected.
• The cited references are properly characterized as § 103 art — US 3,541,919 against the display/light-guide claims (3–6) and US 4,021,823 against the two-prism/twice-reflection and prism-on-reflecting-face features.

Caveats. (a) I could not retrieve the complete claim set and drawing text of US 3,541,919 from the sources reachable here; the characterization above rests on the abstract/description excerpts and the citation metadata, so a full-text check of US 3,541,919's claims is advisable before relying on any § 102 statement about it. (b) The two Japanese Utility Model references discussed in the specification (SHO 56-53372 and SHO 49-117841) are part of the intrinsic record but were not examiner citations; their full texts were not retrieved. (c) The § 102 vs. § 103 characterizations are my analytical judgment as a technical analyst, not a legal opinion, and a formal validity analysis would require the complete texts and prosecution history.

I'll ground this §103 analysis in the actual prior-art references before reasoning about combinations.

Obviousness Analysis — US 4,592,636 under 35 U.S.C. § 103

Note on scope before I begin. This is a validity/prior-art analysis of a patent that expired May 1, 2005 (all three maintenance fees paid — years 4, 8, 12 per the legal-events record). It is therefore not an enforceable right. The §103 question is still answered as posed, but its practical value is (a) defensive — assessing whether the disclosure is good §102/§103 art against later filings, and (b) doctrinal — whether the Examiner's allowance was correct.

Coverage caveat, stated up front. I obtained substantive text for only one of the two Examiner-cited references (US 4,021,823). For US 3,541,919 I have only its title, assignee, and date from the face of the record; for JP 56-53372 and JP 49-117841 I could not retrieve text at all — searches for the Japanese utility-model numbers returned unrelated documents. My characterizations of those three references therefore rest on (i) their titles/faces and (ii) the '636 specification's own discussion of them, which I treat as a party admission but not as an independent disclosure check. I have marked every inference of that kind as such.

1. Legal framework applied

• §103 / Graham v. John Deere Co., 383 U.S. 1 (1966): scope and content of the prior art, differences between the claims and the prior art, level of ordinary skill, and secondary considerations.
• KSR Int'l Co. v. Teleflex Inc., 551 U.S. 398 (2007): the motivation to combine need not be found in the references themselves; it may come from "the background knowledge, creativity, and common sense of the person of ordinary skill," from a known problem, from market demand, or from design incentives. A combination of familiar elements yielding predictable results is likely obvious.
• MPEP §2141 / §2143 / §2144.04: the combination must be evaluated as a whole; the references themselves need not suggest the claimed combination, only that the elements are known and some reason existed to combine them. "Obvious to try" applies where the prior art gives a finite number of identified, predictable solutions (KSR; In re O'Farrell).
• In re Fritch, 972 F.2d 1260 (Fed. Cir. 1992): a reference is available for everything it fairly teaches, and bodily incorporation of one reference into another is not required.
• In re Ratti, 270 F.2d 810 (CCPA 1959): a proposed combination requiring a change in a reference's principle of operation is improper — the applicant's strongest available rebuttal here, addressed in §9 below.
• Analogous art: every reference discussed is a viewfinder/image-erecting prism system for a reflex camera — same field of endeavor (In re Bigio, 381 F.3d 1320).

2. Level of ordinary skill in the art (POSITA)

As of the May 7, 1984 priority date, a POSITA would be a lens/prism designer with a bachelor's degree in optics, physics, or mechanical engineering, or equivalent, plus roughly 2–4 years' hands-on design of SLR viewfinder prism assemblies (or a master's degree with less experience). Such a person would know, without invention:

• Roof (Dach) and pentagonal-roof prism geometry, and the raytrace identity that a roof reflection inverts the image in one meridian while two planar reflections invert it in the orthogonal meridian (this is the classic 5-reflection "penta" fold).
• Total internal reflection (TIR) and critical-angle budgeting (n = 1.77 → ~34.3°, as the '636 spec itself computes).
• The viewfinder magnification identity γ ≈ fθ / fe and the constraint that the eyepiece front focal point must sit at the finder image plane — i.e., that shortening the screen-to-eyepiece glass path is the recognized route to raising magnification. (Both the '636 spec and the Asahi literature state this explicitly.)
• The mechanical practice of mounting prisms in a camera shell with flat abutment faces, spring loading, and alignment screws.

The claimed subject matter sits squarely inside that skill set — no non-obvious optics theory, no new material, no new method of manufacture.

3. The prior art of record and its disclosure

4. Claim 1 mapped element-by-element

Reading of the claim. Elements (a)–(h) are, collectively, a two-part penta-roof prism finder. The only structurally distinctive recitations are (e) the coplanar incidence/second-reflection plane and (d)/(i), which are geometric results of choosing the fold angles. Note also that the two numerical examples in the specification use A = 30°/C = 22° and A = 32°/C = 24° — both giving a 16° exit slab, and Example 3 (A = 25°, C = 21°) giving 8°. The specification's own "Definitions" box concedes the only apparently critical angle ranges are broad: 20° ≤ A ≤ 40°, 20° ≤ B ≤ 60°, 10° ≤ C ≤ 40°.

5. Primary combination (the rejection I would expect to issue)

Combination A: US 4,021,823 + US 3,541,919, and optionally JP 56-53372.

US 4,021,823 supplies almost the entire mechanical skeleton: mirror-split TIR viewfinder path (a), focusing screen (b), a first prism giving an even number (two) of reflections (c), a second prism mounted behind it in a distinct, separable finder portion (f)/(g), and an eyepiece (h). US 3,541,919 supplies the express teaching of a reflex-camera viewfinder "including a roof prism", confirming both the roof-reflection and post-roof-reflection architecture as a known, conventional alternative to a solid pentaprism. JP 56-53372, which the spec admits "resembles in appearance" the embodiment, supplies the general two-prism roof-folded layout for a camera whose format differs (larger) — the difference in format is a matter of degree, not of kind.

Result: claim 1 is made obvious by the combination, with the coplanar-plane recitation (e) and the exit-closeness recitation (i) supplied as design consequences.

Combination B (stronger, if JP 49-117841 is available as art): add JP 49-117841 to Combination A. The spec's own discussion establishes that the reference teaches a first prism capable of total internal reflection at the second reflection plane (its FIG. 2), and identifies the only problems as (1) the prism is oversized when the reflection is not total (its FIG. 1) and (2) the slanted incidence plane induces astigmatism/distortion. Faced with that teaching, a POSITA optimizing for compactness has an obvious and finite set of design moves: make the second reflection plane coplanar with the incidence plane so the aperture can be shared and the prism shrunk. That is exactly the '636 expedient, and it is the predictable way to fix the very defect the reference is described as having. This is the classic "fixed against a known problem" rationale of KSR, and also an In re O'Farrell "finite number of identified, predictable solutions" case.

Combination C (the display claims' core): for claims 3–6, add US 3,387,530 (auxiliary prism cemented to the entrance face of the roof prism to inject an exposure-meter scale into the finder) and/or US 4,021,823 itself, whose second prism is adhered to a region of the reflecting face of the first prism outside the field to create a separate optical path. Together these teach, expressly, (i) cementing an auxiliary light-guiding prism directly to a prism face to inject display light, and (ii) doing so at a reflection face of the first prism.

6. Motivation to combine (why a POSITA would have done this)

The motivation is unusually well documented and comes from three independent directions, which is what makes this a strong §103 case:

1. A recognized problem in the art. The '636 background and US 4,219,526 both state the same problem: the pentagonal-roof finder path is too long, and lengthening it reduces viewfinder magnification. The specification itself says exactly this — the fix is to shorten the screen-to-eyepiece path so a short-focal-length eyepiece (fe) can be used, because γ = fθ/fe.
2. A new market/demand driver. Small-format electronic cameras (2/3″ sensor ≈ ¼ the diagonal of 35 mm) cannot use a conventional pentaprism and still reach ≥90% field ratio and γ ≈ 0.6, because the eyepiece must simultaneously be close to the prism and project the pupil to the rear of the camera (the spec's own FIG. 2 analysis; the two principal points cannot be spread far enough).
3. A design-space incentive. Splitting the pentaprism into two separate prisms is not itself new — US 4,021,823 already does it, and does so specifically to place an auxiliary prism at a reflection-face region outside the field, and the '636 spec notes the resulting freed volume "in the neighborhood of the image sensing plane 6" can be occupied by part of the second prism. KSR expressly endorses this kind of "use the available space / follow the design incentive" reasoning.

A POSITA following any one of those three threads, and starting from US 4,021,823, would have arrived at the claim-1 architecture with a reasonable expectation of success: all the constituent reflections, materials (n = 1.7725 and 1.834 glasses), and assembly techniques were conventional, and the only engineering work required was the routine prism raytrace (which the spec itself carries out with the closed-form relations B = 45° − A + C, e = A, f = 2A, g = 2A − B, h = 2A − C, j = 90° + 2(A − C)). That is textbook prism design, not invention.

7. The two "geometric consequence" recitations (claim 1(d) and 1(i))

This is the crux of the analysis, and I want to be precise rather than glib.

• 1(d) — the slanted exit axis. The specification's own formula is dispositive: j = 90° + 2(A − C). The exit axis is normal to the exit plane only in the degenerate case A = C. The moment a designer skews the roof edge (C ≠ A) to make the prism compact and to fold toward the sensor, the exit axis must depart from the normal by 2|A − C|. A limitation that is an inevitable by-product of the recited structural combination is not a separate inventive contribution; it does not render the claim patentable over art disclosing the combination. I would expect an Examiner to make precisely this argument, and it is a good one.
• 1(i) — exit closer to the objective axis than the screen. The tabulated coordinates in the '636 specification (Table 1: P0 at z = −0.30, exit point P6′ at z = −4.491; Table 6 similarly) show the offset geometry but the tables do not fix the location of the first optical axis (the tables define only planes of the prisms, not the taking-lens axis position). So this inequality cannot be independently verified from the patent's own data — and, more importantly for §103, it means the limitation is a dimensional outcome of a fold choice, not an added part. Once the designer chooses to place the eyepiece side of the assembly over/adjacent to the sensor and shutter space (which every small-format electronic-camera designer was motivated to do, per motivation #3 above), the inequality follows.

Therefore: claim 1 stands or falls on element (e), the coplanar incidence/second-reflection plane. Everything else is either disclosed or an arithmetic consequence. And element (e) is precisely the move that JP 49-117841's described defect makes obvious (§5, Combination B).

8. Dependent claims 2–12

Net: claims 2, 7–12 are, in my assessment, vulnerable to a §103 rejection on the strength of the cited art plus ordinary mechanical/optical skill and, if needed, official notice of the mundane character of the features (a notch datum, a parallel face, a rearward fold). Claims 3–6 are vulnerable to US 3,387,530 and US 4,021,823, both of which teach the very concept of a cemented auxiliary prism for injecting light into a finder at a prism face.

9. Where the rejection is weakest — and the counterarguments I would expect

I want to be straight about the limits of the case I can build here:

1. The coplanar-plane limitation (claim 1(e)) is the only real structural hook. If applicant were to show that the combination of (i) coplanar incidence/second-reflection plane, (ii) the resulting shared-aperture geometry that permits a small first prism and total internal reflection and freedom from the astigmatism of the slanted-incidence alternative, produces a result not achievable by the enumerated prior-art routes, there is an argument that this is a synergistic combination rather than an aggregation. That is a Kahn-style argument (aggregation vs. synergy). It is colorable but, in my view, not strong: the specification attributes the benefit to compactness and the short optical path, both of which are expressly identified as already known desiderata (US 4,219,526; the '636 background).
2. Teaching away. The '636 spec characterizes JP 56-53372 as designed for large film with convergent marginal rays, and asserts that for small picture planes "the marginal rays must be arranged to be as parallel as possible with each other," so that the JP 56-53372 arrangement "would result in a large prism." Whether this is a true teaching away depends on what JP 56-53372 actually says — which I could not verify. If JP 56-53372 merely discloses a large-format finder without disparaging small formats, there is no teaching away, only a difference in intended format, which is a matter of degree. If it expressly teaches that its configuration is unsuitable for a short-focal-length eyepiece, applicant has a real argument. This is the single most important factual question for the §103 determination, and I cannot resolve it from available sources.
3. In re Ratti / change in principle of operation. If the Examiner were to combine a pentaprism reference and argue for bodily substitution, applicant could invoke Ratti. But that is not the rejection I would frame: US 4,021,823 is already a two-prism, non-pentaprism finder, so Combination A above does not require bodily incorporation of any reference's system — it uses selectively taught features (In re Fritch). Ratti is therefore, in my view, unavailable as a rebuttal to Combination A.
4. Claim 1(i) can be attacked on indefiniteness/enablement grounds instead. Because the tabulated coordinates do not locate the "first optical axis," one cannot verify from the disclosure that the recited inequality is satisfied in the stated examples. That is a §112 concern, not §103, but it also means an Examiner's §103 treatment of the limitation will necessarily be qualitative rather than numerical — which an argumentative applicant can exploit.
5. This is not a §102 case. No single reference of record appears to disclose all of (a)–(i). Anticipation would require the coplanar-plane first prism and the relative-position limitation in one document. I found none, and I would not assert one.

10. Secondary considerations (Graham factor 4)

Nothing in the record supplied to me evidences: unexpected results (asserted in the specification — ≥90% field ratio, γ ≈ 0.6 for a 2/3″ format — but asserted by the patentee, without comparative data against a pentaprism at the same format, and therefore weak and arguably without nexus); commercial success; long-felt but unmet need (the pentaprism problem was known, and the Asahi documents show the industry was actively working it — i.e., simultaneous invention, which if anything cuts against non-obviousness); failure of others; copying; or industry praise. A "long-felt need" argument is also weakened by the very fact that the 1984–86 Canon family (JPS60233628/29/30, JPS60189001–189003, JPS60263101, JPS616804, and US 4,609,272/US 4,595,270) shows several closely-related solutions being pursued in parallel by the same assignee — evidence of a crowded and active design space, not of a barrier others could not cross.

Also worth flagging for completeness: this patent, the three same-day Japanese applications it claims priority from, the three May 23, 1984 utility models, and the June 11 and June 18, 1984 applications are all Canon filings sharing inventors (Kato / Nishimura / Yokota). Those co-family members are not §102/§103 prior art against '636 (same inventive entity / claimed priority), but the overlap in subject matter would have raised obviousness-type double patenting as between the resulting US patents had the claims been materially coextensive.

11. Bottom line

Confidence: Moderate-high on claims 3–12; moderate on claim 1. The uncertainty is concentrated in two places I flagged explicitly: (i) the unverified disclosures of US 3,541,919 and JP 56-53372 / JP 49-117841, and (ii) whether JP 56-53372 contains a genuine teaching away from small-format use. If you can obtain the full text of US 3,541,919 and a translation of JP 56-53372, I would revise claim 1's assessment upward or downward accordingly — it is the pivot point.

12. Contradiction flagged with the previously generated sections

One discrepancy to record, per the instructions:

• The earlier Bibliographic data table assigns US class 354/225; 354/155 and Int'l class G03B 13/02, and names Michael L. Gellner as primary examiner. The authoritative Google Patents text supplied here lists only CPC classifications: G02B5/00, G02B5/04 (prisms), G03B19/00, G03B19/02, G03B19/12 (reflex cameras with a movable reflector or partly-transmitting mirror). It contains no G03B 13/02 (viewfinder) entry, no US class, and no examiner name. This is not necessarily a true conflict — the US class and examiner appear on the printed front page, which the fetched HTML does not reproduce, and Google Patents' "Classifications" panel shows CPC rather than USPC — but note that for a viewfinder patent the absence of any finder-specific classification in the authoritative text is mildly surprising and worth verifying against the front page. The earlier section's priority list (JP 59-090762/63/64; JP 59-75564/565/566U; JP 59-119473; JP 59-90478U) matches the authoritative text exactly, as do the cited-by lists (US 5,166,718; US 5,541,768; US 6,281,381; US 6,430,375) and the expiration date. The Litigation summary section provided to me is truncated and records no findings; that does not change the analysis above, which is confined to §103 and to the prior art.