Flame technology explained

Because "realistic flame" appears on every product in this category, including the least convincing ones. The mechanism does not: it is a physical fact about how the light is produced, it predicts exactly which viewing conditions the unit will fail in, and you can find it on the manufacturer's own page.

The oldest and cheapest approach still in wide production. LEDs shine onto a slowly rotating reflector — typically a shaped, part-silvered drum — and the moving light is projected up onto a diffuser panel or a set of resin logs. The movement of the reflector is what your eye reads as flicker.

• Convincing when: the room is dim, you are more or less in front of it, and you are not close.
• Gives itself away when: it is daylight, or you move to one side. There is no depth to the effect, so from an angle it collapses into a flat orange glow.
• Tell-tale: a repeating rhythm. The reflector rotates at a fixed speed, and once you notice the loop you cannot stop noticing it.
• Where you find it: entry-level log sets and inserts, and most budget wall units.

The mainstream approach today. Multiple LED layers sit at different depths behind a diffuser, with a separately lit ember bed beneath. Because the layers are physically separated, there is parallax — the front of the flame moves differently from the back — and your eye reads that as three-dimensionality.

• Convincing when: normal evening viewing at normal seating distance. This is genuinely good, and it is what most people should buy.
• Gives itself away when: bright daylight, or very close inspection. The layers become individually visible up close.
• Tell-tale: the flame has a consistent maximum height. Real flames vary far more than any LED assembly does.
• Where you find it: most mid-range linear units, and anything advertising a 3D or multi-layer flame.

A recorded video of a real fire, played on a screen behind glass. The flame is literally a photograph of fire in motion, so its shape, color variation and unpredictability are exactly right — because they were real.

• Convincing when: viewed head-on from a fixed position. Still-image realism is the best in the category.
• Gives itself away when: you move off-axis, because it behaves like any LCD panel — brightness and color shift with viewing angle.
• Tell-tale: it loops. Video is finite, and once you have identified the loop point it is permanent.
• Where you find it: the upper-mid and premium tiers. Modern Flames and Dimplex both build screen-based units.

The one genuine step change. Ultrasonic transducers turn ordinary tap water into a fine cool mist, which rises through the firebox and is lit from below by LEDs. Because the mist is a real physical medium moving through real space, the flame has actual volume: it curls, it disperses, and it reads as smoke as well as fire.

Dimplex, which holds the patents on the Opti-Myst mechanism, is explicit about two properties that follow from it. The flame is cool to the touch — you can put your hand through it — and it produces no heat at all, with heaters offered as a separate option on some models.

• Convincing when: essentially always, including from an angle and from above. This is the only mechanism that works in three dimensions.
• Gives itself away when: you look for a reservoir. It needs refilling with water.
• Tell-tale: none visual. The tell is practical — it is a small appliance you maintain rather than a thing you switch on.
• Where you find it: Dimplex Opti-Myst, at several times the price of a good LED unit.

1. Dim room, evening use, you sit in front of it → layered LED. It is convincing in exactly those conditions and costs a fraction of the alternatives.
2. Bright room, seen from all angles, all day → water vapor, or accept that this is the hardest case in the category and spend the money on the housing instead.
3. A fixed viewing position, and budget → LCD screen. Best still-image realism available on mains power.
4. Flame matters, heat does not → water vapor, without hesitation.
5. Traditional masonry opening → layered LED with a raised log and a lit ember bed. The traditional cues do more work than the technology tier.