EOTF (Electro-Optical Transfer Function)

The EOTF (Electro-Optical Transfer Function) is the mathematical curve that converts an encoded signal value (electrical) into a displayed luminance value (optical) on a screen. Every display format defines its own EOTF: Rec.709 uses a gamma 2.4 EOTF (BT.1886) for reference monitors, PQ uses SMPTE ST 2084 as an absolute-luminance EOTF (signal-to-nits mapping defined to 10,000 cd/m²), and HLG uses a hybrid EOTF that combines log behavior in highlights with gamma behavior in shadows. The EOTF is the display side of a transfer function chain — the encoding side, applied at capture or in compositing, is the OETF (Opto-Electronic Transfer Function) which is mathematically related but not necessarily inverse to the EOTF. For Rec.709 the OETF (camera) and EOTF (display) are not strict inverses, an intentional asymmetry that compensates for the dim viewing environment of broadcast television (the so-called 'end-to-end' system response). For PQ, the EOTF is defined absolutely so that the same signal value always means the same luminance on any compliant display. EOTF mismatch is a common source of grading errors — a master encoded in PQ but viewed on a gamma-2.4 monitor will look wildly wrong. Modern color management tools (OpenColorIO, ACES) explicitly track which EOTF applies at each point. References: ITU-R BT.1886 (Rec.709 EOTF), SMPTE ST 2084 (PQ), ITU-R BT.2100 (HDR system).