Optical illusions provide powerful tools for studying how the visual system constructs our perception of the world — or, as the physiologist Jan Evangelista Purkyně famously noted, “Illusions of the senses tell us the truth about perception”.

The vision scientist Michael Bach maintains on his homepage a list of optical illusions and their explanations; however, in many of them, it is not quite clear how they "fool" our brain.

One such illusion is the panda illusion, popularized by Ilja Klemencov’s 2016 artwork They can disappear, created to raise awareness for the endangerment of the panda. The image embeds the WWF panda logo within a field of black‑and‑white zigzag lines, so that the panda remains invisible at close viewing distances but becomes clearly recognizable when stepping back or when vision is blurred, for example by removing one’s glasses. This distinctive behavior arises because the panda illusion is a special form of two‑dimensional pulse‑width‑modulated halftone, in which the figure is encoded solely through subtle modulations of line thickness within a fixed‑frequency carrier pattern. When the visual system applies low‑pass filtering — either optically (through blur) or neurally — the encoded figure emerges.

Our group asked whether the minimum spatial frequency at which the illusion becomes visible, the perception threshold, could serve as a proxy for visual acuity. In a systematic study with healthy observers and artificially degraded visual acuity, we demonstrated a strong linear relationship between spatial frequency threshold and conventional visual acuity, enabling estimation of acuity from the illusion. This counterintuitive principle — that worse acuity leads to better visibility of the illusion — makes the panda illusion uniquely suited as a complementary visual function test.

Building on these findings, a subsequent clinical study evaluated a child‑friendly version, PandAcuity, in 152 pediatric participants. The test uses illusion images with decreasing spatial frequencies and measures the lowest frequency at which children recognize hidden animal silhouettes (e.g., panda, rabbit, owl). PandAcuity showed good test–retest reliability (ICC = 0.89) and a medium‑to‑large correlation with conventional visual acuity. In 93% of cases, the classification of visual impairment agreed with standard acuity testing. 

Given its simplicity, brief testing time, and the fact that it motivates children by revealing “hidden animals”, PandAcuity is particularly valuable when compliance is limited, for example, in very young children or in cases where conventional visual acuity results are uncertain.

Software

• https://strator1.github.io/PandaIllusion/

Patent

• Method and device for determining a refractive error

Publications

1. Straßer T, Kurtenbach A, Langrová H, Kuehlewein L & Zrenner E (2020) The perception threshold of the panda illusion, a particular form of 2D pulse-width-modulated halftone, correlates with visual acuity. Sci Rep 10: 13095
2. Kelbsch C, Spieth B, Zrenner E, Besch D & Straßer T (2023) PandAcuity in paediatrics: a novel clinical measure of visual function based on the panda illusion. Br J Ophthalmol 107: 582–586