The left wing has a donor portrait of Heinrich von Werl, who kneels in prayer in the company of John the Baptist facing the missing devotional centre-panel scene, which is lost and unrecorded.
This panel contains a number of elements indebted to Jan van Eyck, notably the convex mirror in the midground, which as with the 1434 Arnolfini Marriage, reflects the scene back at the viewer.
Although the centre panel is lost with no surviving copies, inventory records or descriptions, it has been speculated that it was set in the same room occupied by Saint Barbara.
The panel's strength comes from her well-described clothing and the highly detailed objects placed around her, most of which are shaped and contrasted by the two sources of light falling on their generally golden and polished surfaces.
[2] The fireplace emits a warm reddish glow, which contrasts with the relatively hard light falling from the window and the unseen middle panel to the left.
[2] The room is obviously from a contemporary middle-class rather than biblical setting,[8] and contains many of the same details found in the centre panel of the c 1425–28 Mérode Altarpiece, also attributed to Robert Campin.
These include the latticed and shuttered window, the reading Virgin seated on a long bench, and the tilted iris (instead of lily) in a vase on a table to her side.
The painting contains a number of vanishing points stretching from the lower right hand to the open window serve to emphasise the panel's depth.
The steep angle of the panel from the viewer's point of view is achieved through the tilt of the bench, sideboard, line of the fireplace, and shutters of the window.
According to Walther Ingo, the dramatic angle of these elements serves to demote the figure of St Barbara to secondary importance to an examination of the anatomy of the space itself.
[13] Campin's early altarpieces, unlike those of van Eyck's, stick with the traditional hieratic form of a centre panel reserved for the devotional scene, and are physically and spatially removed from the wings.
The triptych further introduces the idea of the intermediary saint, again a van Eyeckian influence; here represented by John the Baptist seen holding a lamb in the same panel as von Werl, increasing the significance of the area occupied by the donor.