# Sat / Lum

The Sat/Lum scope plots each pixel in a 2D space where the **X axis** is **luma (Y')** — the weighted-sum of the non-linear R'G'B' channels used in Y'CbCr — and the **Y axis** is **saturation**, computed as the length of the CbCr (or UV) chroma vector from the neutral axis. A fully desaturated pixel (grey, white, black) sits on the bottom edge regardless of its luma; a fully saturated primary sits at the top. This makes the scope a compact at-a-glance view of how saturated the picture is across the tonal range.

![](/files/-MHGq-3bbHE1WiPorU2H)

## Reading the plot

* **Shadows on the left**, **highlights on the right**
* **Low saturation at the bottom**, **high saturation at the top**
* A tight, narrow trace at low saturation → the image is nearly neutral
* A trace that bulges toward the top-right → saturated highlights (common cause of clipping perceived as "burnt" colour)
* A trace that bulges toward the top-left → saturated shadows (often indicates chroma noise or crushed blacks with a colour cast)

Because naturally exposed footage has most pixels in the midtone range, the trace usually hugs the centre. Outliers are what you're looking for.

## Colour Space Selection

The CbCr coefficients used to compute the chroma vector depend on the active colour space — **Ccir 601** for SD, **Rec. 709** for HD, **Rec. 2020** for UHD. This affects both where pixels land horizontally (luma coefficients differ) and how far they extend vertically (chroma scaling differs).

| Colour space  | Usage                                              |
| ------------- | -------------------------------------------------- |
| **Ccir 601**  | Legacy SD broadcast (Kb = 0.114, Kr = 0.299)       |
| **Rec. 709**  | HD broadcast / most SDR (Kb = 0.0722, Kr = 0.2126) |
| **Rec. 2020** | UHD / HDR container (Kb = 0.0593, Kr = 0.2627)     |

## Options

* **Colorize** — tint each trace sample with the pixel's own colour
* **Colour space selection** — Ccir 601 / Rec. 709 / Rec. 2020

## Related

* [Vectorscope](/nobe-omniscope/scopes/vectorscope.md) — raw CbCr plane without the luma axis
* [Histogram](/nobe-omniscope/scopes/histogram.md) — saturation mode


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