A high pressure cell for viewing high pressure fluid passing through the cell, comprising: a first housing member and a second housing member; a spacer member sandwiched between substantially planar surfaces of the first housing member and the second housing member, the spacer member having a thickness less than the first and second housing members, the spacer member having a hole therethrough with an edge of the hole defining a flow cavity between the first housing member and the second housing member; each of the first housing member and the second housing member having a groove (112, 122) for a gasket (180, 181) on a surface of the housing member that faces the spacer member, each groove surrounding and being larger than the hole through the spacer member; an inlet passage formed in either the first housing member or the second housing member extending from an outer face of the first or second housing member to the flow cavity; an outlet passage formed in either the first housing member or the second housing member and extending from an outer face of the first or second housing member to the flow cavity; an optical window in a mounting hole of the first housing member positioned with a surface of the optical window exposed to the flow cavity, wherein the optical window has a first portion at a first end with a substantially constant smaller cross sectional area, a second portion at a second end with a substantially constant larger cross sectional area, and a shoulder between the first and second portions (197, 195), and wherein the optical window is positioned in the mounting hole with the first end facing the flow cavity; a compliant gasket positioned around the first portion of the window between the shoulder of the window and a corresponding first shoulder surface in the mounting hole; an externally threaded retaining member having a central opening configured to let light pass through the externally threaded retaining member along a main axis, the externally threaded retaining member being positioned in a threaded portion of the mounting hole, such that tightening the externally threaded retaining member compresses the gasket against the first shoulder of the mounting hole, creating a static seal against fluid leaking past the optical window and positioning the optical window with the first end at the surface of the flow cavity; and a spacer that is pressed against a second shoulder in the mounting hole by the threaded retaining member, the spacer having a through hole that is smaller than the cross-sectional dimension of the first portion of the optical window, the spacer having an outer width that is larger than the second portion of the optical window, the spacer being more compliant than the threaded retaining member. a first housing member and a second housing member; a spacer member sandwiched between substantially planar surfaces of the first housing member and the second housing member, the spacer member having a thickness less than the first and second housing members, the spacer member having a hole therethrough with an edge of the hole defining a flow cavity between the first housing member and the second housing member; each of the first housing member and the second housing member having a groove (112, 122) for a gasket (180, 181) on a surface of the housing member that faces the spacer member, each groove surrounding and being larger than the hole through the spacer member; an inlet passage formed in either the first housing member or the second housing member extending from an outer face of the first or second housing member to the flow cavity; an outlet passage formed in either the first housing member or the second housing member and extending from an outer face of the first or second housing member to the flow cavity; an optical window in a mounting hole of the first housing member positioned with a surface of the optical window exposed to the flow cavity, wherein the optical window has a first portion at a first end with a substantially constant smaller cross sectional area, a second portion at a second end with a substantially constant larger cross sectional area, and a shoulder between the first and second portions (197, 195), and wherein the optical window is positioned in the mounting hole with the first end facing the flow cavity; a compliant gasket positioned around the first portion of the window between the shoulder of the window and a corresponding first shoulder surface in the mounting hole; an externally threaded retaining member having a central opening configured to let light pass through the externally threaded retaining member along a main axis, the externally threaded retaining member being positioned in a threaded portion of the mounting hole, such that tightening the externally threaded retaining member compresses the gasket against the first shoulder of the mounting hole, creating a static seal against fluid leaking past the optical window and positioning the optical window with the first end at the surface of the flow cavity; and a spacer that is pressed against a second shoulder in the mounting hole by the threaded retaining member, the spacer having a through hole that is smaller than the cross-sectional dimension of the first portion of the optical window, the spacer having an outer width that is larger than the second portion of the optical window, the spacer being more compliant than the threaded retaining member.