Look-Up Table (LUT)

A LUT is an array of 256 (or 2^N) entries, each containing an RGB triplet. When displaying an image, each pixel's intensity value is used as an index into the LUT, returning the display color. Linear LUTs map proportionally (e.g., green: 0→black, 255→bright green). Non-linear LUTs like 'Fire' use multi-segment color ramps to enhance contrast in specific intensity ranges.

Critically, LUTs are a display operation only — they do not modify the stored pixel values. Quantitative measurements must always be performed on the raw intensity data, not the false-color display.

LUTs serve two purposes: they assign channel identity (green for GFP, red for mCherry) in multi-channel composites, and they can enhance visual contrast to reveal features that are hard to see in plain grayscale. Choosing the right LUT is important for presentation but must never be confused with actual data modification.

A 4-channel multiplex image uses four LUTs: DAPI nuclei in blue, CD3 T-cells in green, PD-L1 in red, and Ki-67 proliferation in cyan. Each channel is a separate grayscale image; the LUTs create the composite false-color overlay. The same raw data could be displayed with entirely different LUT assignments.