IR Blocking Filter

IR-blocking filter (hot mirror): short-pass quarter-wave stacks in series reflect the near IR and transmit the visible.

n_H (high index)n_L (low index)n_substraten_incidentBlocked band (near IR): from (nm)Blocked band (near IR): to (nm)Transmitted band (visible): from (nm)Transmitted band (visible): to (nm)Periods per stack (N)

Mean R, 700–1100 nm

99.67 %

Lowest R in the band: 94.4 %

Mean T, 420–650 nm

96.6 %

Lowest T in the band: 86.6 %

50 % point (edge)

690.2 nm

Between the blocked and transmitted bands

Stacks: 2 × 10 periods, (L/2 H L/2)^N, quarter waves at λ₀ = 806.6, 932.6 nm (from the incident side)

Layers: 41

One stack reflects g = λ₀/λ in 1 ± Δg, Δg = (2/π) asin((n_H − n_L)/(n_H + n_L)) = 0.1522. K = ⌈ln(λ_to/λ_from) / ln((1 + Δg)/(1 − Δg))⌉ stacks with λ₀ from λ_from(1 + Δg) to λ_to(1 − Δg) cover the blocked band with overlapping zones. Each stack also reflects near λ₀/3.

Model and references

Exact. The equations are exact within the stated assumptions, or are a standard's tabulated values. The results are as good as the inputs. About the tiers

Transfer matrix of short-pass edge-filter stacks in series covering the blocked band; lossless constant indices, normal incidence, semi-infinite substrate. A starting design: no matching layers or optimization.

Macleod H. A. (2010). Thin-Film Optical Filters, 4th ed., ch. 6 and ch. 7 (edge filters, extending the reflection zone). CRC Press. doi:10.1201/9781420073034
Epstein L. I. (1952). The design of optical filters. J. Opt. Soc. Am. 42, 806. doi:10.1364/JOSA.42.000806

Report a problem