Estimating beams and blends
Two things get guessed on site that are cheap to work out beforehand: how big a beam will be when it lands, and whether two projectors can be made to look like one picture.
First, read the data sheet properly
Beam angle is not field angle
A fixture does not project a hard-edged cone; intensity falls off from the centre outwards. So "how wide is the beam" needs a definition of where the beam stops, and the industry uses two.
Beam angle is measured where intensity has fallen to 50% of the centre. Field angle is where it has fallen to 10%. Field angle is always the larger number — usually a good deal larger — and it is closer to what your eye reads as the edge of the light.
This is why comparing two fixtures on a single quoted figure is a trap. A data sheet quoting only "beam" makes the fixture sound tighter than it looks in a room; one quoting only "field" makes it sound wider than its useful centre.
- Field angle is always larger than beam angle. If a spec sheet quotes one number and calls it both, it is quoting the flattering one.
- Zoom range is quoted at the extremes. A 7–50° fixture is not evenly useful across that range; optical quality and output usually fall off at one end.
- Intensity falls with the square of distance. Doubling the throw quarters the illuminance — the same inverse square law as sound, for the same reason.
- Beam size is not coverage. A beam that reaches the back wall does not mean a face at the back is lit to a usable level. Check the lux, not just the circle.
Sizing it before it is rigged
How to estimate a beam on paper
The calculation needs two numbers you already have: the throw distance from the fixture to the surface, and the angle from the data sheet. Everything else is one line of trigonometry — the beam is an isosceles triangle, and you are solving for its base.
For a fixture at an angle rather than square-on, the beam lands as an ellipse rather than a circle and gets larger; the figure above is the square-on case, which is the useful planning number and slightly conservative for anything raked.
Video
What a projection blend actually assumes
Edge blending overlaps two projected images and ramps each one down across the overlap, so the two add up to a single even brightness. The maths of the ramp is the easy part. What decides whether the blend is invisible is a set of assumptions nobody writes down.
The big one is black level. In the overlap, both projectors are putting up their black floor at once — and a projector's black is never truly black. Two blacks add, so the blend band is measurably lighter than the rest of the image. On bright content it is invisible; on a dark scene it is a glowing stripe down the middle, and no ramp adjustment can remove it because there is nothing to ramp down.
- Black level cannot be blended away. Plan for it: design content that avoids full black across the seam, or specify projectors with a better native contrast ratio.
- Lamp or laser age changes brightness and colour. Two identical projectors of different ages will not match, and the difference shows exactly at the seam.
- The blend needs overlap to live in. Roughly 10–25% of image width is normal — too little leaves no room for the ramp or for alignment tolerance.
- Geometry has to stay put. A projector nudged by a ladder, or a truss that moves as the room warms, ruins a blend that was perfect at 2am.
- Match the colour before the geometry. Aligning a blend between two projectors on different colour settings wastes the alignment when you then correct the colour.
Beam & throw for diameter and lux, projector throw ratio, screen brightness and the LED wall resolution tool are on the field tools page. Fixture safety standards are indexed under IEC 60598-2-17 and UL 1573.
2 questions on the mechanism rather than the trivia. Nothing is scored and you can answer again — the explanation under a wrong answer is the point of it.
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1A fixture is quoted with a 15° beam angle and a 30° field angle. What is the difference?
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Neither is a physical dimension. Both are angles defined by how far the intensity has fallen.
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Right. They are two definitions of "edge" on the same cone. Feed either into the same formula and you get a different diameter — which is why a spec sheet quoting only one number is ambiguous.
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Shutters cut the beam mechanically. These angles describe the optics before any shuttering.
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Both angles are quoted for both kinds of fixture.
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2Two projectors edge-blend and the overlap still looks like a visible band. The ramp is set correctly. What is most likely wrong?
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Right. A blend hides a seam in brightness. It cannot hide a difference in black level, colour or geometry — and black level is the one that shows most in the dark content a show usually runs.
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That shows as a sharpness difference across the join rather than a band in the overlap.
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Worth checking, and it usually shows across the whole image rather than specifically in the overlap.
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A narrow blend makes the ramp steep, but a correctly set ramp is the premise here.
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Answers are kept in this browser on this device. Nothing is sent anywhere.
- Beam angle or field angle?
- How wide will that beam be?
- What does blending not fix?
Where to go next
These pages are a chain rather than a library — the next one along usually answers the question this one raised.
Everything on this page, in the index
Each of these has its own entry with sources, ports, gotchas and an open gap where a field is missing.