An off-axis three-mirror optical system with freeform surfaces comprised an aperture, a primary mirror, a secondary mirror, a tertiary mirror, and a detector. The aperture is located on an incident light path. The primary mirror is located on an aperture side. The secondary mirror is located on a primary mirror reflected light path. The tertiary mirror is located on a secondary mirror reflected light path. The detector located on a tertiary mirror reflected light path. The primary mirror and the tertiary mirror have a same fifth-order polynomial freeform surface expression. The primary mirror reflected light path, the secondary mirror reflected light path and the tertiary mirror reflected light path overlap with each other.

An off-axis three-mirror optical system with freeform surfaces includes a primary mirror, a secondary mirror, a tertiary mirror, and an image sensor. The primary mirror receives light rays first and the secondary mirror is located on a path of light reflected from the primary mirror. The tertiary mirror receives light reflected from the secondary mirror. The image sensor is located on a path of light reflected from the tertiary mirror. Each reflecting surface of the primary and tertiary mirrors is a sixth order xy polynomial freeform surface. The secondary mirror reflecting surface is a spherical surface. A field of view of the off-axis three-mirror optical system with freeform surfaces in Y-axis direction is greater or equal to 65. A field of view of the off-axis three-mirror optical system with freeform surfaces in X-axis direction is greater or equal to 0.8.

An inspection device for inspecting a panel, in particular a display, or a PCB, includes a first mirror, a second mirror, a third mirror, and a sensor. The first mirror, the second mirror, and the third mirror are arranged to display a section of the panel to be inspected on the sensor with a magnification factor greater than one. At least two of the group of the first mirror, the second mirror, and the third mirror, have both a first type of curvature, and a remaining mirror has a second type of curvature, opposite to the first type of curvature. The first mirror, the second mirror, and the third mirror form a telecentric system which is telecentric on a panel facing side and/or on a sensor facing side.