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A recent theory claims that some Renaissance artists, as early as 1425, secretly traced optically projected images
during the execution of some passages in some of their works, nearly a quarter millennium before historians
of art and of optics have secure evidence anyone recorded an image this way. Key evidence adduced by the
theory's proponents includes the trelliswork in the right panel of Robert Campin's Merode altarpiece triptych
(c. 1425-28). If their claim were verified for this work, such a discovery would be extremely important to
the history of art and of image making more generally: the Altarpiece would be the earliest surviving image
believed to record the projected image of an illuminated object, the first step towards photography, over 400
years later. The projection theory proponents point to teeny "kinks" in the depicted slats of one orientation
in the Altarpiece as evidence that Campin refocussed a projector twice and traced images of physically straight
slats in his studio. However, the proponents rotated the digital images of each slat individually, rather than the
full trelliswork as a whole, and thereby disrupted the relative alignment between the images of the kinks and
thus confounded their analysis. We found that when properly rotated, the kinks line up nearly perfectly and are
consistent with Campin using a subtly kinked straightedge repeatedly, once for each of the slats. Moreover, the
proponents did not report any analysis of the other set of slats-the ones nearly perpendicular to the first set.
These perpendicular slats are straight across the break line of the first set-an unlikely scenario in the optical
explanation. Finally, whereas it would have been difficult for Campin to draw the middle portions of the slats
perfectly straight by tracing a projected image, it would have been trivially simple had he used a straightedge.
Our results and the lack of any contemporaneous documentary evidence for the projection technique imply that
Campin used a simple mechanical aid-such as a minutely kinked straightedge or a mahl stick commonly used
in the early Renaissance-rather than a very complex optical projector and procedure, undocumented from that
time.
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