Domain: Engineering — Optical Engineering
Target concept: Laser Beam Expansion

Video generation prompt: A laser beam (λ=632.8 nm) is incident on a beam expander (4× expansion). After expansion, the beam is incident on a collimating lens. A white screen is placed at different distances behind the collimating lens to observe the spot variation. The laser beam before expansion is marked with a red thin line, and the beam after expansion and collimation is marked with a red thick line. The spot is indicated by a circular marker.

Evaluation guide:
(i) Target concept: Visual demonstration that beam expansion followed by collimation produces a beam whose far-field spot size increases with observation distance due to finite collimation quality.
Minimum mechanism: Beam must originate from a coherent source; expansion must precede collimation; collimation cannot be perfect (hence spot growth with distance); red line thickness difference must encode beam diameter change; spot must be geometrically linked to beam path intersection with screen.
(ii)Storyline: Frame 1–2: Red thin line extends from laser source to input of beam expander. Frame 3–4: Beam expander visibly widens beam (transition to red thick line) and directs it toward collimating lens. Frame 5–6: Red thick line enters and exits collimating lens along same axis. Frame 7–10: White screen moves backward in fixed plane; circular spot appears where red thick line strikes screen, and spot radius visibly increases across successive screen positions, all observed from fixed frontal viewpoint with no camera motion or zoom.

Manual:
Experimental Procedure
1. Illuminate He-Ne laser (λ=632.8 nm) onto 4× beam expander.
2. Pass expanded beam through collimating lens.
3. Place white screen at varying distances behind collimating lens.
Observation of Experimental Phenomena
- Red thin line marks original beam before expansion.
- Red thick line marks expanded & collimated beam after lens.
- Circular spot appears on screen; its size increases as screen moves farther from lens.
Summary of Experimental Principles
Beam expansion increases diameter while reducing divergence; collimation preserves beam parallelism; spot size on distant screen reflects residual divergence after collimation.

Prompt Grounding rubric:
level_1: No laser beam, beam expander, collimating lens, white screen, red thin line, red thick line, or circular spot marker appears anywhere in the frame.
level_2: Only one or two of the required elements appear in isolation (e.g., a white screen with no beam path leading to it, or a red line with no expander or lens nearby), and the beam expander, collimating lens, and spot marker are largely absent or unrecognizable.
level_3: The laser source, beam expander, collimating lens, white screen, a red thin line, a red thick line, and a circular spot marker are all present and identifiable, but their order or placement is off, e.g., the red thick line appears before it reaches the beam expander, or the spot shows up on the screen before the screen itself is shown in place.
level_4: The red thin line correctly runs from the laser into the beam expander, the red thick line correctly runs from the beam expander's output through the collimating lens to the screen, and the circular spot sits only on the white screen, with the expander and lens rendered as two visually distinct optical elements and the screen positioned behind the lens; one small setup flaw remains, such as the beam path being slightly non-straight or the spot's edge briefly hidden behind a lens mount.
level_5: The red thin line is visibly narrower than the red thick line at every point where both could be compared, the beam expander reads as a two-lens element and the collimating lens as a single separate element, the white screen sits flat and fully visible perpendicular to the beam axis, and the circular spot is centered on the beam's axis on the screen, with no stray props, no color bleeding between the thin and thick line segments, and no part of the optical path hidden from view.

Scientific and Causal Correctness rubric:
level_1: No causal link between the optics and the spot is shown: the spot appears with no visible beam path reaching it, the spot stays the same size regardless of screen distance, or the red thin and thick lines are drawn identically with no visible expansion effect.
level_2: The order of cause and effect is wrong: the red thick line appears before the beam reaches the beam expander, the spot shows up before the red thick line has reached the screen, or moving the screen farther away makes the spot shrink instead of grow.
level_3: The red thin line correctly precedes the beam expander and the red thick line correctly follows the collimating lens, and the spot only appears once the thick line reaches the screen and does change size across different screen distances, but the direction of that change is not consistently one-way (e.g., it grows then shrinks again without the screen moving back closer).
level_4: The full causal chain holds (thin line into the expander, thick line out of the collimator, spot appears on the screen exactly where the thick line strikes it), and the spot's radius grows as the screen moves farther from the collimating lens, matching the residual divergence expected after imperfect collimation; one small inconsistency remains, such as the spot appearing slightly off the beam's central axis despite the beam itself being centered.
level_5: The spot's radius increases monotonically and smoothly as screen distance increases, with no distance step producing a smaller spot than the previous one; the thick line is visibly and consistently wider than the thin line across its full length; the spot is never shown without the thick line actively reaching the screen at that moment, and the thick line is never shown without having first passed through both the beam expander and the collimating lens.

Spatiotemporal Consistency rubric:
level_1: The screen, beam expander, or collimating lens jumps to a new position with no motion shown between frames, the red beam line breaks apart mid-path and reforms, or the spot blinks on and off with no screen movement to explain it, the lens or beam expander appears rotated or flipped in a way inconsistent with a fixed optical setup, or the overall lighting changes abruptly between frames with no cause.
level_2: The beam path shows visible gaps rather than one continuous line, the red thick line does not appear to continue from where the thin line ended, the spot appears detached from the beam path it should sit on, or the collimating lens changes size or shape partway through the clip.
level_3: The overall layout stays recognizable and stable, but one clear glitch occurs: the thick line briefly separates from the collimating lens's output, the spot momentarily overlaps the lens or expander housing, the screen's motion jumps rather than sliding smoothly, or the beam path bends at an angle with no lens or mirror to justify it.
level_4: The beam expander, collimating lens, and screen keep consistent relative positions and orientations, the thin and thick red lines stay unbroken and aligned along one straight axis, and the spot moves smoothly as the screen's distance changes, with one small flaw remaining, such as a brief flicker at the spot's edge or slight blur as the screen moves.
level_5: The red thin line flows into the beam expander and the widened beam continues into the collimating lens with no visible seam; the thick line emerges and travels to the screen along one consistent axis; the screen slides smoothly backward and forward with no rotation or skew; the spot grows or shrinks smoothly and stays centered as the screen moves; and lighting stays even throughout with no stray flash or shadow jump breaking the sense of one continuous shot.