# US 11,112,601 B2 · Head mounted display including a reverse-order crossed pancake lens > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 11,112,601 B2 - Document ID: US11112601B2 - Status: Granted - Kind: B2 - Application number: 16/160,128 - Filed: 2018-10-15 - Published: 2019-12-12 - Granted: 2021-09-07 - Earliest claimed: 2018-06-07 - Assignees: Facebook Technologies, LLC - Applicants: Facebook Technologies, LLC - Inventors: Gary Dean Sharp - Web page: https://patentgrep.com/patent/US11112601B2 ## CPC - G02B 27/0018: {with means for preventing ghost images (anti-reflection coatings G02B1/11)} - G02B 5/3016: {involving passive liquid crystal elements (optical properties of liquid crystals G02F1/0063; polarising elements associated with active liquid crystal devices G02F1/133528)} - G02B 5/3058: {comprising electrically conductive elements, e.g. wire grids, conductive particles} - G02B 5/3066: {involving the reflection of light at a particular angle of incidence, e.g. Brewster's angle} - G02B 5/3083: {Birefringent or phase retarding elements (G02B5/3008, G02B5/3016 take precedence; systems for polarisation control G02B27/286; manufacturing phase modulating patterns by lithographic processes G03F7/001)} - G02B 27/0025: {for optical correction, e.g. distorsion, aberration} - G02B 27/0037: {with diffracting elements (G02B27/0056 takes precedence; holographic optical elements G02B5/32; zone systems G02B5/1876)} - G02B 27/0093: {with means for monitoring data relating to the user, e.g. head-tracking, eye-tracking} - G02B 27/0172: {characterised by optical features} - G02B 27/0983: {being curved} - G02B 27/283: {used for beam splitting or combining} - G02B 27/286: {for controlling or changing the state of polarisation, e.g. transforming one polarisation state into another (G02B5/3083 takes precedence; light guide coupling means utilising polarising elements G02B6/34)} - G02B 2027/012: {comprising devices for attenuating parasitic image effects} - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS ## Abstract In various embodiments, a pancake lens block may include (i) a first compound retarder comprising a first plurality of retarders oriented to a plurality of first axes of orientation, respectively, where the first compound retarder is configured to selectively transmit a portion of the emitted light, (ii) a partial reflector that receives the portion of emitted light from the first compound retarder and transmits the portion of the emitted light, (iii) a second compound retarder comprising a second plurality of retarders oriented to a plurality of second axes of orientation that are substantially orthogonal to the respective plurality of first axes of orientation, and (iv) a reflective polarizer configured to reflect the portion of the emitted light selectively transmitted by the second compound retarder back to the second compound retarder. ## Claims ### Claim 1 A pancake lens block comprising: a first compound retarder comprising a stack of at least three quarter wave plates whose optical axes are oriented in a specific order relative to one another, where the first compound retarder is configured to selectively transmit a portion of radiation from a radiation source; a partial reflector is configured to receive the portion of radiation from the first compound retarder and transmits the portion of radiation; a second compound retarder comprising an additional stack of at least three quarter wave plates whose optical axes are orthogonal and oriented in a different order that reversely maps to the specific order, where the second compound retarder is configured to selectively transmit the portion of radiation from the partial reflector; a reflective polarizer configured to reflect the portion of radiation selectively transmitted by the second compound retarder back to the partial reflector through the second compound retarder, where the partial reflector reflects a second portion of the radiation from the second compound retarder back to the reflective polarizer; and an eye-reflection portion configured to: receive the second portion of radiation from the reflective polarizer after the second portion of radiation has passed through the second compound retarder; and reduce back reflections emanating from an eye of a user, the eye-reflection portion comprising: a polarizer; a third compound retarder comprising another stack of at least three quarter wave plates; and an anti-reflective material. ### Claim 2 (depends on 1) The pancake lens block of claim 1, further comprising a display polarizer that polarizes the radiation from the radiation source before the radiation reaches the first compound retarder, and wherein the portion of radiation selectively transmitted from the first compound retarder has a circular polarization state. ### Claim 3 (depends on 1) The pancake lens block of claim 1, wherein the at least three quarter wave plates of the stack include a first quarter wave plate and a second quarter wave plate that collectively serve as a half wave plate to a third quarter wave plate. ### Claim 4 (depends on 3) The pancake lens block of claim 3, wherein: the first quarter wave plate and the second quarter wave plate have axes of orientation that are parallel to one another; and the third quarter wave plate has an axis of orientation that is approximately 58.9 degrees relative to the axes of orientation of the first and second quarter wave plates. ### Claim 5 (depends on 1) The pancake lens block of claim 1, wherein at least one of the first compound retarder or the second compound retarder comprises a dispersion-controlled quarter-wave retarder. ### Claim 6 (depends on 5) The pancake lens block of claim 5, wherein the dispersion-controlled quarter-wave retarder comprises a co-polymer of polycarbonate and polystyrene. ### Claim 7 (depends on 1) The pancake lens block of claim 1, wherein at least one of the first compound retarder or the second compound retarder comprises a cyclic olefin material. ### Claim 8 (depends on 1) The pancake lens block of claim 1, wherein the first compound retarder has a first surface facing a display and a second surface facing the partial reflector, and the second compound retarder has a first surface facing the partial reflector and a second surface facing the reflective polarizer, and wherein the pancake lens block further comprises a first c-plate coupled to the second surface of the first compound retarder and a second c-plate coupled to the first surface of the second compound retarder. ### Claim 9 (depends on 8) The pancake lens block of claim 8, wherein the first c-plate has a first retardation value and the second c-plate has a second retardation value that is substantially equal to the first retardation value. ### Claim 10 (depends on 9) The pancake lens block of claim 9, wherein the first retardation value and the second retardation value increase with an increasing number of components in the first compound retarder and an increasing number of components in the second compound retarder, respectively. ### Claim 11 (depends on 1) The pancake lens block of claim 1, wherein the first compound retarder is coupled to a display polarizer of a display. ### Claim 12 (depends on 11) The pancake lens block of claim 11, wherein the display polarizer comprises a linear polarizer and a polarization axis of the display polarizer is matched to a polarization axis of the first compound retarder. ### Claim 13 (depends on 11) The pancake lens block of claim 11, wherein the display polarizer is coupled to the first compound retarder, the display polarizer and the first compound retarder collectively serve as a circular polarizer. ### Claim 14 (depends on 1) The pancake lens block of claim 1, wherein the anti-reflective material comprises a shaped anti-reflective lens. ### Claim 15 A head mounted display (HMD) comprising: an electronic display configured to emit light; and a pancake lens block comprising: a first compound retarder comprising a stack of at least three quarter wave plates whose optical axes are oriented in a specific order relative to one another, where the first compound retarder is configured to selectively transmit a portion of the emitted light; a partial reflector configured to receive the portion of emitted light from the first compound retarder and transmits the portion of emitted light; a second compound retarder comprising an additional stack of at least three quarter wave plates whose optical axes are orthogonal and oriented in a different order that reversely maps to the specific order, where the second compound retarder is configured to selectively transmit the portion of emitted light from the partial reflector; a reflective polarizer configured to reflect the portion of emitted light selectively transmitted by the second compound retarder back to the second compound retarder, where the partial reflector reflects a second portion of emitted light from the second compound retarder back to the reflective polarizer; and an eye-reflection portion configured to: receive the second portion of emitted light from the reflective polarizer after the second portion of emitted light has passed through the second compound retarder; and reduce back reflections emanating from an eye of a user, the eye-reflection portion comprising: a polarizer; a third compound retarder comprising another stack of at least three quarter wave plates; and an anti-reflective material. ### Claim 16 (depends on 15) The HMD of claim 15, wherein at least one of the first compound retarder or the second compound retarder comprises a dispersion-controlled quarter-wave retarder. ### Claim 17 (depends on 15) The HMD of claim 15, wherein the first compound retarder has a first surface facing the electronic display and a second surface facing the partial reflector, and the second compound retarder has a first surface facing the partial reflector and a second surface facing the reflective polarizer, and wherein the pancake lens block further comprises a first c-plate coupled to the second surface of the first compound retarder and a second c-plate coupled to the first surface of the second compound retarder. ### Claim 18 A method of assembling a pancake lens block, the method comprising: positioning a partial reflector adjacent to a first compound retarder across an air gap, wherein: the first compound retarder comprises a stack of at least three quarter wave plates whose optical axes are oriented in a specific order relative to one another, where the first compound retarder is configured to selectively transmit a portion of radiation from a radiation source; and the partial reflector is configured to receive the portion of radiation from the first compound retarder and transmit the portion of radiation; positioning a second compound retarder adjacent to the partial reflector, the second compound retarder comprising an additional stack of at least three quarter wave plates whose optical axes are orthogonal and oriented in a different order that reversely maps to the specific order, where the second compound retarder is configured to selectively transmit the portion of radiation from the partial reflector; positioning a reflective polarizer adjacent to the second compound retarder, the reflective polarizer configured to reflect the portion of radiation selectively transmitted by the second compound retarder back to the second compound retarder, where the partial reflector reflects a second portion of the radiation from the second compound retarder back to the reflective polarizer; and positioning an eye-reflection portion adjacent to the reflective polarizer, the eye-reflection portion configured to: receive the second portion of radiation from the reflective polarizer after the second portion of radiation has passed through the second compound retarder; and reduce back reflections emanating from an eye of a user, the eye-reflection portion comprising: a polarizer; a third compound retarder comprising another stack of at least three quarter wave plates; and an anti-reflective material.