Black levels in multiprojector displays

Black level is the light that remains visible when the signal requests a black image. In a multiprojector display, residual contributions add in overlap regions. A seam can therefore look bright even when alignment and blending appear correct in a bright scene.

Digital compensation equalises dark areas by raising the less luminous regions. This improves uniformity but may reduce available contrast. Scalable Display Technologies’ black-level guide describes this limitation.

Identify the black you are observing

Black on the projection surface depends on the projector, the room and the other beams. Useful diagnosis separates individual-projector residual light, overlapping contributions and ambient illumination. A dark photograph alone cannot distinguish them.

A simplified model for a point on the screen is:

observed light = ambient light + sum of residual black
                 + sum of image contributions

Consider this hypothetical numerical example, with no ambient light. Each projector contributes 0.02 cd/m² when displaying black. A region covered by one projector receives 0.02 cd/m²; a two-projector overlap receives 0.04 cd/m². Multiplying the black signal by zero does not remove optical leakage. Raising the first region to 0.04 cd/m² can equalise them while making the overall black floor brighter.

These figures illustrate addition only. Actual values depend on the installation and require suitable measurement. RGB readings from an automatically exposed webcam are not directly luminance measurements in cd/m².

Distinguish black, gamma and signal range

Residual black, midtone response and signal range are different properties. Before adding compensation, check that near-black steps remain distinguishable and that all projectors interpret the incoming signal consistently.

Control	What it affects	Mistake to avoid
Projector brightness or black-level control	Low-level threshold and tone reproduction, depending on the model	Lowering it until shadow detail disappears
Light-source power	Emitted light and operating conditions	Changing it after measurement without rechecking
Gamma or tone curve	Signal-to-light relationship through intermediate tones	Treating it as a universal cure for residual black
HDMI/video range	Interpretation of digital levels	A mismatch between source, splitter and projector
Multiprojector black compensation	Uniformity of dark regions	Applying it before geometric alignment

Use near-black bars to detect clipping. If several steps become indistinguishable, detail is being lost in the signal chain. Correct signal interpretation before pursuing a uniform seam.

Prepare a repeatable measurement

Useful measurement requires stable projectors, correct geometry, controlled illumination and a camera that can distinguish the observed levels. Changing those conditions during comparison makes the result difficult to interpret and reproduce.

Igloo recommends completing warp and blend and allowing projectors to reach operating temperature before black-level adjustment. Igloo, Black Level Tab.

For a practical check:

Record picture presets, light modes and room conditions.
Disable projector automation that changes light output with scene content.
Observe full black and several low greys with all projectors active.
Where the installation procedure allows it, compare individual projector contributions without changing their configuration.
Inspect reflections, safety lighting, nearby surfaces and visible light sources.
Check several audience positions. Measurement from one position may not describe every visible difference.

In a dome, bright images can indirectly illuminate other parts of the surface. Test a mixed scene with bright objects against a dark background as well as uniform black.

Use the TholusOS black-level stage

The black-level stage follows alignment and resolution of the blend step. It provides guided black and low-grey inspection, with camera measurement or an explicit visual-review choice. Keep the camera in the same position and framing used for geometric capture.

Open the live preview and inspect seams on the surface. On black, look for brighter regions. On low grey, look for dark patches and halos. A correction that passes the first check may still need work on the second.

If the camera is stopped, return to the camera stage and restart it without moving it. Visual black-level review can be an operator choice, but the final quality check still needs the camera.

If measurement leaves local defects, the app may offer visual refinement or another measurement. When low grey is unverified, it can request an explicit choice between repeating black-and-grey measurement and using black only. Read the limitation before accepting it: the result must suit the intended content, especially very dark scenes.

Complete the save or confirmation requested by the stage, then perform the final calibration check. Temporary preview is useful for comparison; verify that the chosen result belongs to the session you intend to activate.

Assess uniformity and contrast together

A useful result retains shadow detail and makes seams less conspicuous under real operating conditions. Uniform black alone is insufficient: excessive compensation can flatten the scene or produce discontinuities at slightly higher grey levels.

Epson explains that black-level adjustment matches the brightness and tone of non-overlapping regions and that differences can remain after adjustment. Epson, Adjusting the Black Level.

Before accepting the result, inspect at least:

uniform black, to locate residual light and overlap regions;
low grey, to reveal halos and discontinuities;
a grey scale, to inspect near-black detail;
a scene with stars, captions or small bright objects;
a representative sequence from the complete programme.

Record remaining differences and the conditions in which they appear. If the residual limitation is optical or environmental, address the room design or adapt content after an actual projection test. For a seam that is most visible in midtones, return to edge blending.