A "transfer function" is the curve that converts stored numeric code values in a file into actual light output. sRGB, the transfer function behind ordinary SDR images, is a gamma-like curve tuned for roughly a 100-nit range. PQ was designed for something much wider: absolute values from near-zero up to 10,000 nits, using a curve shaped around how human contrast sensitivity actually works — allocating more code values to the brightness ranges your eyes are more sensitive to, rather than spacing them out evenly.
"Absolute" is the key word
This is the real distinction from HLG: a given PQ code value is supposed to mean the same real nit value everywhere, on every compliant display, regardless of how bright that particular screen's SDR content happens to be running. A PQ value corresponding to 1,000 nits means 1,000 nits, full stop — not "pretty bright." That absolute mapping is what lets HDR video and images describe consistent, specific highlight brightness across completely different hardware.
Where you'll run into it
PQ is the transfer function behind most native HDR video (HDR10, Dolby Vision) and behind PQ-tagged HDR AVIF images. It's not what gain-map JPEGs use directly — those stay as ordinary sRGB-encoded JPEGs underneath, with a separate gain map describing how much brighter each pixel should get. Our Inspect tool checks for real PQ tagging (via the file's actual CICP metadata, when present) rather than assuming — and says so plainly when it can't find a signal either way.
The honest limitation
Knowing a file is PQ-tagged tells you what it claims. Whether your specific browser and screen actually render it at true extended brightness is a separate question — one that depends on real hardware and OS support that no website can fully confirm on your behalf. We say so directly rather than pretending otherwise.