MaxCLL and MaxFALL
MaxCLL and MaxFALL summarize the highest content light level and highest frame-average light level reported for a video.
One small highlight and one broadly bright frame can determine different content-light fields. MaxCLL is the reported maximum content-light bound; MaxFALL is the reported maximum frame-average bound. Both use cd/m², commonly called nits.
Calculate the two summaries from three simple frames
Use neutral-gray pictures so component-light and luminance values coincide. The constructed sequence contains:
| Frame | Picture contents | Highest level | Frame average |
|---|---|---|---|
| A | 1% at 1,000 nits; 99% at 10 nits | 1,000 | 19.9 |
| B | 50% at 300 nits; 50% at 20 nits | 300 | 160 |
| C | Entire frame at 50 nits | 50 | 50 |
Frame A's average is 0.01 × 1,000 + 0.99 × 10 = 19.9. Frame B's average is 0.5 × 300 + 0.5 × 20 = 160.
Using the measured maxima as the declared bounds gives MaxCLL 1,000 and MaxFALL 160. A determines the content peak; B determines the largest frame average. No averaging across the three frames is needed to obtain MaxFALL.
This explains the distinction physically. A tiny reflection can be intense while most of its frame remains dark. A broadly illuminated scene can have a lower peak while placing a larger average-light demand on the display.
What the measurement definition adds for colored pictures
The standardized content-light messages describe bounds for linear-light RGB component intensities. A strong colored component can consequently give a value different from true CIE luminance calculated with color-dependent weights. Production measurement methods must be interpreted with their component definitions and picture-area treatment.
The neutral-gray table above deliberately avoids that distinction. For a real program, keep the measuring tool and its definition identified when comparing reported values. The field records a declared bound; it is not a live measurement made by the television during playback.
Unspecified bounds and the mastering display
In the standardized messages, zero indicates that the corresponding upper bound is not supplied. It does not describe a movie whose light levels are all zero. A media report can also omit fields or carry values that were not correctly measured.
Mastering maximum identifies the grading display's configured range. It can exceed the program's content peak, so a 1,000-nit mastering record and a lower MaxCLL can be consistent.
A tone mapper may use reported content bounds, mastering context, measured picture statistics, or a combination. When an Emby conversion changes across different source editions, recording these fields beside the exact release helps identify whether the mapper began with different source assumptions.