Why cladding RI is the lever for field of view
In a diffractive AR waveguide, the core guides image light by total internal reflection. The angle range the core can guide — and therefore the field of view (FOV) the display can deliver — is set by the index contrast between core and cladding. Higher contrast means wider angle, wider angle means larger FOV.
The relationship is governed by the critical angle:
sin(θc) = ncladding / ncore
For a fixed core RI, every reduction in cladding RI widens the angular range guided by TIR. A core at n = 1.85 with a glass cladding at n = 1.52 gives a critical angle of 55 degrees. The same core with a Kriya LRI cladding at n = 1.16 gives 38.8 degrees — a 16 degree widening per side. In a binocular AR system, that is the difference between a 35 degree diagonal FOV and one approaching 60 degrees.
The cladding is not a passive structural layer. It is an optical component, and lowering its RI is one of the few formulation levers available to expand FOV without redesigning the grating couplers.
Kriya LRI: RI 1.16 PFAS-free, overcoatable
Kriya's overcoatable PFAS-free low refractive index material reaches RI 1.16 — the lowest value commercially available from a single PFAS-free supplier. Three properties matter for a waveguide cladding application:
- PFAS-free — the legacy route to ultra-low RI was fluoropolymers or fluorinated additives. Both are within scope of upcoming PFAS restrictions in the EU, US, and Japan. Kriya LRI achieves comparable RI without fluorine. See PFAS-free coatings.
- Overcoatable — most ultra-low RI coatings cure to a surface that resists adhesion of any subsequent layer. Kriya LRI is engineered to accept overcoats, enabling the cladding-core-cladding stacks that AR waveguides require.
- Stable — RI value is achieved through engineered porosity within a robust hybrid network. Mechanical durability and environmental stability are designed in, not traded for low RI.
RI platform: cladding and core in one supplier
The cladding question cannot be separated from the core question. Index contrast is what matters; either layer in isolation is just a number. Kriya supplies both ends of the contrast equation from a single chemistry platform.
| Layer role | Kriya grade | RI @ 589 nm | Chemistry | PFAS-free |
|---|---|---|---|---|
| Ultra-low cladding | LRI 1.16 | 1.16 | Hybrid sol-gel | Yes |
| Low cladding | LRI 1.34 | 1.34 | UV-curable 100% solids | Yes |
| Mid cladding | LRI 1.40 | 1.40 | Thermal-cure latex | Yes |
| Mid core | HRI 1.65 | 1.65 | Hybrid UV-curable | Yes |
| High core | HRI 1.85 | 1.85 | Hybrid sol-gel | Yes |
| Ultra-high core | HRI 2.00 | 2.00 | Solvent sol-gel | Yes |
Index contrast and field of view: what the numbers buy you
The table below shows critical angle and approximate single-sided angular range for common core-cladding pairings, including legacy glass cladding (n = 1.52) for reference. Wider angular range translates to wider FOV when paired with appropriate diffraction grating designs.
| Core RI | Cladding | Cladding RI | Critical angle | TIR angle range |
|---|---|---|---|---|
| 1.65 | Glass (reference) | 1.52 | 67.0° | 23.0° |
| 1.65 | Kriya LRI 1.34 | 1.34 | 54.2° | 35.8° |
| 1.65 | Kriya LRI 1.16 | 1.16 | 44.6° | 45.4° |
| 1.85 | Glass (reference) | 1.52 | 55.2° | 34.8° |
| 1.85 | Kriya LRI 1.34 | 1.34 | 46.4° | 43.6° |
| 1.85 | Kriya LRI 1.16 | 1.16 | 38.8° | 51.2° |
| 2.00 | Glass (reference) | 1.52 | 49.5° | 40.5° |
| 2.00 | Kriya LRI 1.16 | 1.16 | 35.5° | 54.5° |
Going from a glass-clad n = 1.85 core to a Kriya LRI 1.16 clad n = 2.00 core nearly doubles the TIR angular range — from 34.8 to 54.5 degrees per side. That headroom is what enables wide-FOV consumer-grade AR/VR optics on a manufacturable form factor.
Optical loss and transparency through the cladding
A cladding layer affects two loss mechanisms: bulk absorption within the cladding (relevant where evanescent fields penetrate) and scattering at the core-cladding interface. Kriya LRI grades are formulated for low bulk loss and smooth interfaces.
| Grade | RI @ 589 nm | Transmission (1 µm) | Haze | Interface Rq |
|---|---|---|---|---|
| LRI 1.16 | 1.16 | >98.5% | <0.4% | <1.5 nm |
| LRI 1.34 | 1.34 | >99.0% | <0.2% | <1.0 nm |
| LRI 1.40 | 1.40 | >99.0% | <0.2% | <1.0 nm |
Transmission values are measured on 1 micrometre coating layers; interface roughness Rq is measured by AFM on coated test substrates. The trade-off pattern is clear: lower RI requires higher engineered porosity, which costs a small amount of bulk transmission and adds a small amount of interface roughness. LRI 1.16 stays well within usable specifications for waveguide cladding.
Pairing with the core: process compatibility
A waveguide stack is only as good as the interfaces between layers. Kriya cladding and core grades are formulated for compatibility:
- Overcoatability — LRI grades accept HRI overcoats without primer or surface treatment
- Shared chemistry families — hybrid UV-curable and sol-gel grades cure under the same processing windows, enabling sequential coating without parameter changes
- NIL compatibility — both LRI and HRI 100% solids grades support nanoimprint lithography for grating couplers. See NIL-compatible coatings.
- R2R-ready — gravure and slot-die coating compatible across the platform
Why this matters for R2R waveguide manufacturing
The economics of consumer AR/VR depend on moving waveguide manufacturing off wafers and onto rolls. Wafer-based waveguide production currently runs at high cost per unit; roll-to-roll nanoimprint with appropriate cladding and core materials is targeted at roughly an order of magnitude cost reduction at scale.
The cladding material is one of the gating items. A glass cladding is essentially incompatible with R2R: it requires bonding, polishing, and rigid substrate handling. A polymeric cladding with comparable optical performance enables flexible substrate processing, continuous coating, and inline NIL grating replication. Kriya LRI 1.16 is the lowest-RI polymeric cladding option currently available PFAS-free — and it is overcoatable, which means the rest of the stack can be built on top of it in the same line.
Quality control and supply
Every batch is filtered to 0.8 micrometres and tested on up to 10 critical-to-quality parameters. ISO 9001:2015 certified manufacturing. Designed capacity exceeds 100,000 kg/year nanoparticle dispersions and 1,000,000 kg/year coatings — sufficient to support volume AR/VR programmes once they ramp. A Certificate of Analysis accompanies every delivery, documenting RI, viscosity, solids content, and appearance. Six-month minimum shelf life unopened.