Neutral and channel-based comparison
The screen uniformity test supplies several grey levels, white, solid RGB, and a custom grey slider. Use the optional grid and zone labels to describe broad brightness or tint changes, then remove the overlay before making the visual comparison.
Use several field levels
Describe what changes
One region may look brighter or darker than the rest of the same neutral field. LCD backlight distribution, local dimming, panel structure, temperature, image processing, and viewing angle can contribute. Describe the location, field level, and viewing setup rather than inventing a percentage.
A neutral field may look warmer, cooler, greenish, or magenta in one zone. Compare white, several greys, and RGB. A visual tint difference can be real and useful to document, but a browser field cannot quantify chromaticity or Delta E.
Position changes the result
Large displays are viewed at different angles from center to corner even when your head is centered. IPS, VA, TN, OLED, and other panel designs respond differently off-axis. A corner that changes as you move may reflect viewing geometry rather than a fixed zone with different output.
Repeat the screen uniformity test from your normal position first. Then move slightly and write down what follows your viewing angle versus what stays fixed on the panel. For work that depends on quantified uniformity, use a defined measurement grid and suitable hardware rather than translating visual impressions into numbers.
Avoid pattern-induced conclusions
The overlay is only a coordinate aid. Turn it off during inspection because contrast at the line can alter the way neighboring zones appear.
Near-black banding, mid-grey dirty-screen effects, and bright-field tint may appear at different levels. Compare the pattern across several fields and representative content.
Some displays alter the backlight or panel drive according to content and settings. A uniform browser request does not bypass that processing, so record the active picture mode when consistency matters.