HDR Test Online: Check Highlight Clipping, Black Level and Peak Brightness
An HDR display can fail in ways an SDR screen never shows: highlights that clip abruptly above reference white, shadows that crush into flat black, and tone mapping that washes out one mode while oversaturating the other. This free online HDR test runs the three patterns that expose those faults — a near-white clipping wedge, a black-level detail staircase, and APL brightness windows — directly in your browser, with nothing to install.
What an HDR test checks
Highlight clipping is the signature HDR fault. SDR content anchors its white at roughly 200 nits; HDR reserves headroom above that anchor for specular highlights, sunlit skies, and light sources. The clipping wedge steps through RGB 244-255 — the range just above reference white — and every step should stay visually distinct. When the top patches merge into a single flat white, the display is discarding highlight detail that the source material contains, and bright scenes will look plastic and flat instead of luminous.
Black-level integrity is the mirror fault at the other end. HDR panels can reach true zero-nit blacks, but aggressive tone mapping often crushes the steps between RGB 0 and 16 into an undifferentiated void. The black staircase reveals whether shadow detail survives: on a healthy display you can still pick out the boundary of the darkest patches. The opposite failure — near-black content rendered as elevated gray — is the classic "washed out HDR" complaint, and it shows up in this same pattern.
APL brightness windows expose how brightness behaves under load. OLED panels apply ABL (automatic brightness limiting) as the average picture level rises, so a 2% white window can measure several times brighter than a full white field — by design, not defect. Mini-LED displays hold their brightness across window sizes but may bloom around the small windows. Comparing the 100%, 50%, 25%, 10%, and 2% windows side by side tells you which technology you are looking at and how much peak headroom it really has.
How to run the HDR test
Set up first: enable HDR in the operating system (Windows: Settings - System - Display - HDR; macOS enables it automatically on built-in XDR panels), dim the room, and let the panel warm for ten minutes — OLED ABL behavior is measurably different on a cold panel. Then run the entire pattern set twice, once with HDR on and once with it off. The differences between the two passes are the tone-mapping behavior you are auditing.
On the white wedge, count how many patches remain distinguishable counting down from 255. A well-behaved display separates steps down to roughly 250; below that, expect gradual merging. The smooth gradient strip beneath the wedge should show no visible banding above RGB 220 — banding there means the panel or driver is quantizing the highlight range. On the black staircase, patch borders should remain visible down to roughly step 4; if the four darkest patches are a single black rectangle, shadows are being crushed.
On the APL windows, note the brightness relationship rather than absolute values. A small window clearly brighter than the full field indicates peak headroom — normal and desirable on OLED. A full white field that is dramatically dimmer than the 25% window confirms ABL throttling. If every window has identical brightness, you are likely looking at a well-driven mini-LED panel. And if you spot specks or dots on the white patterns that stay fixed in place, stop — those are pixel defects, and the dead pixel test is the right tool for them.
The brightness numbers that matter
Brightness is measured in nits (candela per square meter). The numbers worth memorizing: SDR reference white sits around 200 nits; HDR "paper white" — the brightness of a normal white page in HDR content — lands between 200 and 300 nits; and marketing peaks of 400, 600, or 1000+ nits refer to small-window, short-burst highlights, not full-screen output.
Window size dominates real brightness. A DisplayHDR 1000 badge means 1000 nits in a small window (typically 2-10% of screen area) and often 400-600 in larger fields. OLED televisions frequently push 1000-1500 nits in a 2% window while full-field white settles at 150-300 nits because sustained full-screen output generates heat and wears the emitters — this is the ABL trade-off the window test visualizes.
One honest caveat: a plain web page cannot signal in the PQ HDR format the way a game or streaming video does — browsers render standard content at the display's paper white. That is precisely what makes this test useful: it reveals how your display maps standard levels in its current mode, which is exactly the pipeline that breaks when HDR settings, GPU drivers, or color profiles are misconfigured. For the SDR baseline — banding, uniformity, gray ramps — the screen test and color calibration pages complete the audit.
Common HDR problems and their fixes
Washed-out or dim SDR content while HDR is enabled is the most common complaint on Windows. The SDR-content brightness slider (Settings - Display - HDR) and the free Windows HDR Calibration Tool both adjust the balance; run the calibration app once and re-test — the black staircase often visibly improves. On TVs, an HDMI black-level mismatch (TV set to Low while the source sends Full, or vice versa) produces the same grayed-out result.
Clipped highlights usually trace back to the output chain rather than the panel: outdated GPU drivers, a stale custom color profile, or a game choosing the wrong tone-mapping curve. Update drivers, remove third-party ICC profiles you no longer need, and re-run the white wedge after each change. Crushed blacks, meanwhile, are frequently caused by dynamic contrast or black-boost features — disable every automatic contrast enhancement, re-test, and only then decide what to re-enable.
Two companion checks complete HDR due diligence. OLED owners should pair this page with the burn-in test, since static HDR highlights are the most aggressive aging factor an OLED faces. And because HDR signaling often caps refresh rate on constrained bandwidth (4K 144 Hz HDR needs HDMI 2.1 or DisplayPort 1.4 with DSC), verify the panel is actually running its rated Hz with the refresh rate test after enabling HDR.
Frequently Asked Questions
What is an HDR test?+
A set of patterns that verify how a display handles high-dynamic-range content: a near-white clipping wedge (RGB 244-255) for highlight detail, a black-level staircase (RGB 0-16) for shadow detail, and APL brightness windows that compare peak output at different screen-area loads. Together they expose tone-mapping faults that ordinary content only makes you feel, not see.
How do I test my HDR monitor online?+
Enable HDR in your operating system, dim the room, open this page, and press F for fullscreen. Step through the three modes: count distinguishable patches on the white wedge, verify the darkest black steps are still separable, and compare window brightness on the APL test. Run the full set once with HDR on and once with it off — the differences are your display's tone mapping.
Why do all the white patches look the same?+
That is highlight clipping: the display maps the range above reference white to the same maximum output, discarding detail. Common causes are aggressive driver tone mapping, a wrong HDR mode, or an overloaded bandwidth chain. Update GPU drivers, run the Windows HDR Calibration Tool, and re-test — patches should separate down to around RGB 250.
Why is the small white window brighter than the full white screen?+
That is ABL — automatic brightness limiting — working as designed on OLED panels. Small highlights get the full peak output (often 800+ nits), while a full white field is throttled to 150-300 nits to control heat and emitter wear. It is a trade-off of the technology, not a defect. If all windows are equally bright, you are likely on a mini-LED display holding its peak.
Can a browser really test HDR?+
A plain web page cannot send a PQ HDR signal the way a game or video does — browsers render standard content at the display's paper white. What this test does is audit the mapping pipeline in your display's current mode, which is exactly what breaks when HDR settings, drivers, or profiles are wrong: clipped whites, crushed blacks, and gray washouts all appear on these patterns. That is why running it in both HDR and SDR modes is part of the procedure.
HDR makes my desktop look gray and dim. How do I fix it?+
On Windows, adjust the SDR-content brightness slider (Settings - Display - HDR) and run the free Windows HDR Calibration Tool. On TVs, check the HDMI black-level setting on both the TV and the source — a Low/Full mismatch produces the same washed-out gray. Then re-run the black staircase here to confirm the fix.