How Hot Mirror Compares to a second-surface mirror in Spectroscopy

For engineers working in Spectroscopy, the choice of a reflective surface is rarely an afterthought. Hot Mirror components sit at the heart of systems where directing…

For engineers working in Spectroscopy, the choice of a reflective surface is rarely an afterthought. Hot Mirror components sit at the heart of systems where directing and analyzing narrow wavelength bands, and a small improvement in coating quality can change the result of an entire measurement or process.

Think of the Hot Mirror as a precisely made float or borosilicate glass plate whose working surface is a dichroic (transmits visible, reflects IR). The result is > 90% visible transmit reflection across IR reflect / visible pass, which is exactly what most Spectroscopy builders are looking for.

The working principle is the law of reflection applied to a coated plane. Mount the Hot Mirror at 45° and a beam turns 90°; stack several and you fold a long path into a short box. That simplicity is why mirrors remain the fastest way to route light in Spectroscopy.

The dichroic (transmits visible, reflects IR) is where performance is won or lost. Deposited by vacuum processes, it is tuned so the reflected wave adds constructively across IR reflect / visible pass, reaching > 90% visible transmit. Getting the layer thickness right is a precision task: a few nanometers off and the reflectivity curve shifts.

Behind the coating sits the float or borosilicate glass substrate. Its job is mechanical: hold flatness, survive cutting and mounting, and stay stable with temperature. For many Spectroscopy uses, float or borosilicate glass hits the right balance of cost, flatness (4–6λ) and workability.

A practical Hot Mirror datasheet reads: float or borosilicate glass substrate, 4–6λ flatness, 60-40 quality, 1–3 mm thick, > 90% visible transmit over IR reflect / visible pass. Those five lines settle most design reviews for Spectroscopy. See the standard size list for what we stock and what we cut to order.

Where directing and analyzing narrow wavelength bands, a Hot Mirror earns its place by doing one job reliably: turning the beam without adding noise. In Spectroscopy that reliability is the difference between a prototype and a shippable product. More application examples are worth a look if the use case is close to yours.

Choosing among options

Within the mirror family, the Hot Mirror trades some peak reflectance for bandwidth and price. If Spectroscopy demands the very highest reflectivity at one wavelength, a dielectric part wins; if it needs > 90% visible transmit across IR reflect / visible pass at sensible cost, the Hot Mirror with its dichroic (transmits visible, reflects IR) is the pragmatic choice.

A short checklist covers most Spectroscopy cases: what band (IR reflect / visible pass)? at what angle? how much loss is allowed (> 90% visible transmit)? then pick dichroic (transmits visible, reflects IR) on float or borosilicate glass at 1–3 mm. Getting these four right avoids the most common rework — the application notes show how each sector resolves them.

A Hot Mirror is tougher than it looks but softer than you think. Fingerprints on the dichroic (transmits visible, reflects IR) are the usual cause of field failures, so edge-handling and capped storage pay off. A little discipline keeps > 90% visible transmit where it belongs.

Our production of a Hot Mirror follows a simple, repeatable route: laser-cut the float or borosilicate glass to ±0.01 mm, smooth the edges, deposit the dichroic (transmits visible, reflects IR), and inspect to 4–6λ / 60-40. Thickness options span 1–3 mm, and the same Hangzhou line that builds first-surface mirrors also runs windows, substrates and custom coatings.

The Hot Mirror is not exclusive to Spectroscopy. Universities, service centers and R&D groups use it wherever a beam must turn, which makes a flexible, customizable part a quiet workhorse across the optics world.

Typical specs worth putting on a drawing: surface flatness 4–6λ, surface quality 60-40 (scratch-dig), substrate float or borosilicate glass, thickness 1–3 mm, and reflectivity > 90% visible transmit over IR reflect / visible pass. Stating these up front saves rounds of sampling later. Our full technical specifications and standard sizes list the tolerances we hold routinely.

One term worth knowing

"Reflectivity" on a Hot Mirror is the fraction of incident light returned by the dichroic (transmits visible, reflects IR). Quoting > 90% visible transmit without the band (IR reflect / visible pass) is meaningless, because the same coating can be excellent at one wavelength and poor at another — always pair the number with the range.

A Hot Mirror is an optical component built so that reflection happens at the coated front face rather than through a substrate. With a dichroic (transmits visible, reflects IR) on a float or borosilicate glass base, the part delivers > 90% visible transmit reflectivity across IR reflect / visible pass while keeping the useful aperture clean and ghost-free.

Beyond Spectroscopy, the same Hot Mirror shows up in laboratories, teaching setups and OEM builds where directing and analyzing narrow wavelength bands. Its value is generality: one well-made part serves many breadboards, which is why stocking a few standard sizes pays off.

Coating a Hot Mirror means laying down a dichroic (transmits visible, reflects IR) whose optical thickness is controlled to a fraction of a wavelength. Done well, the part holds > 90% visible transmit over IR reflect / visible pass; done carelessly, it drifts and the system loses light it cannot afford to lose.

Substrate choice for a Hot Mirror is a trade between optical grade and budget. float or borosilicate glass is a common pick because it can be cut and polished to 4–6λ flatness and a 60-40 surface, which is plenty for the reflection quality most Spectroscopy systems require.

Durability is part of the spec, not an afterthought. For Spectroscopy the Hot Mirror should survive shipping, installation and the occasional wipe. The protective overcoat on the dichroic (transmits visible, reflects IR) is what lets it do that without losing > 90% visible transmit over time.

The Hot Mirror is not exclusive to Spectroscopy. Universities, service centers and R&D groups use it wherever a beam must turn, which makes a flexible, customizable part a quiet workhorse across the optics world.

Behind the coating sits the float or borosilicate glass substrate. Its job is mechanical: hold flatness, survive cutting and mounting, and stay stable with temperature. For many Spectroscopy uses, float or borosilicate glass hits the right balance of cost, flatness (4–6λ) and workability.

At JYOPTO we make Hot Mirror parts by cutting float or borosilicate glass with laser accuracy of ±0.01 mm, then applying the dichroic (transmits visible, reflects IR) under vacuum. Standard blanks run 1–3 mm thick, edges are smoothed for safe handling, and every shipped mirror meets 4–6λ flatness with a 60-40 surface — the same disciplines we apply across our optical glass, vacuum-coating and precision cold-processing lines since 2020.

Wrapping up

A Hot Mirror is a small part with an outsized effect on Spectroscopy. Get the dichroic (transmits visible, reflects IR), float or borosilicate glass and flatness right and the rest of the system behaves. If your drawing calls for something specific, the team at JYOPTO can cut and coat it to match — start from the specifications and standard sizes, then tell us the wavelength and angle.