Optical filter (thin-film interference filter)

01

Set the filter type and spectral features

Alluxa defines cut-on wavelength as a rising edge, as in a longpass filter, and cut-off as a falling edge, as in a shortpass filter, each taken where transmission reaches 50% of peak. Centre wavelength (CWL) is the midpoint between adjacent cut-on and cut-off points of a passband or notch, and full-width at half-maximum (FWHM) is the distance between those 50% points; bandwidth can also be specified at other levels, such as 80% of peak. [1]

Chroma's handbook of optical filters uses the same 50% convention: the centre wavelength is the arithmetic mean of the wavelengths at 50% of peak transmission, and the FWHM is the bandwidth at that level. Longpass and shortpass filters are denoted by their 50% cut-on or cut-off wavelengths, and average transmission is calculated over the useful transmission region of the filter rather than over the entire spectrum. Slope, the sharpness of the transition from transmission to blocking, can be specified by stating the wavelength at which the filter must reach a given blocking level. [6]

Chroma's ET525/50m emission filter, for example, is specified with a 525 nm centre wavelength and 50 nm FWHM. Chroma designs its emission filters for 0° angle of incidence and its dichroics for 45°, and warns that changing the angle shifts spectral performance. [5]

Alluxa specifies 0° filters — edge, bandpass and notch — with transmission in percent and blocking in optical density, and dichroic filters and mirrors used at larger angles with reflection bands, since they direct the beam within the system. It asks buyers to state whether a band specification is an average or absolute value, because absolute specifications with tight transitions raise cost or reduce feasibility. [1]

The handbook explains how the coating works: a stack of layers, each around a quarter of a wavelength thick (about 1/10,000 of a millimetre), whose reflections at each interface combine through interference to reflect some wavelengths and transmit others. The number of layers and the thickness of each set the nominal wavelength, the bandwidth and the blocking level. [6]

Give the supplier the filter type, CWL and FWHM (or edge wavelengths) with tolerances, transmission level and band (average or absolute), blocking OD and range, ripple, angle of incidence and cone half-angle, operating temperature range, substrate, size, clear aperture, thickness, surface quality, mounting and quantity.

02

Check blocking, angle of incidence and temperature

Alluxa defines optical density as OD = −log₁₀(T). Tilting an interference filter away from normal blue-shifts its spectrum towards shorter wavelengths, more strongly at larger angles, following λθ = λ₀√(1 − (n₀ sin θ / n_eff)²) for collimated light. Temperature also moves the spectrum — a red shift as temperature rises — and Alluxa states its hard-coated filters shift between 2 and 5 pm/°C over instrument operating temperatures, depending on substrate and design. [1] [2] [3]

Chroma's handbook defines blocking level and blocking range in optical density, OD = −log(T) or OD = −log(%T / 100), and gives the example OD 4.5 = 3 × 10⁻⁵ T (0.003 %T). It notes that thin-film interference filters block mainly by reflection, not absorption. Cross-talk is the minimum attenuation of two filters placed in series over a stated range, which matters when matching excitation and emission filters. [6]

Editorial calculation: OD 6 blocking means T = 10⁻⁶, one millionth of the light, while OD 4 passes 10⁻⁴. For a 525 nm filter and an assumed effective index of 2.0, a 10° tilt moves the centre to 525 × √(1 − (sin 10° ÷ 2.0)²) ≈ 523.0 nm, a 2 nm blue shift. A 60 °C swing at 5 pm/°C moves it 0.3 nm. [1] [2] [3]

Semrock's technical note on non-normal incidence describes two effects as the angle increases: spectral features shift to shorter wavelengths, and separate spectra emerge for s- and p-polarized light. On its RazorEdge longpass designs the p-polarized edge shifts more than the s-polarized edge (the reverse for shortpass), giving a shelf near 50% transmission in unpolarized light. On StopLine U- and S-grade notch filters, the notch moves to shorter wavelengths, loses depth and narrows. Semrock's FAQ adds that at larger angles the spectrum can become highly distorted and that the shift depends strongly on filter design. [7] [8]

Chroma's handbook states that a filter or beamsplitter used at any angle other than the usual 0° or 45° must be specified explicitly, and that in a converging or diverging beam the half-cone angle may also need to be specified; it can be given as the f-number or the numerical aperture, which equals the sine of the half-cone angle. [6]

Coatings and absorbing glass react to temperature on different scales. The handbook cites sharp-cut longpass filter glasses shifting their cut-on by about 0.1 to 0.15 nm/°C, against Alluxa's 2–5 pm/°C for its hard-coated filters. Editorial calculation: 0.1–0.15 nm/°C is 100–150 pm/°C, 20 to 75 times the 2–5 pm/°C range. [6] [3]

Alluxa defines clear aperture as the area over which specifications must be met and states that more than 90% can be achieved for unmounted filters, though it may be smaller for large parts with demanding spectra; parallelism and wedge are set by substrate polishing. [4]

03

Read a catalogue specification

Edmund Optics' 355 nm hard-coated OD 4.0 10 nm bandpass filter shows how a catalogue sheet states each field: centre wavelength 355.00 ±2.0 nm, FWHM 10.00 ±2.0 nm, minimum transmission ≥85%, average OD ≥4.0 over a blocking range of 200–1200 nm, angle of incidence 0°, diameter 25.00 +0.0/-0.1 mm, clear aperture 21.0 mm, surface quality 80-50, substrate thickness 3.5 ±0.5 mm and mount thickness 5.0 ±0.1 mm, mounted in a black anodized ring. [9]

Edmund states these filters are optimized for spectral performance rather than laser damage threshold, quoting 1 J/cm² at 532 nm, 10 ns for the series, and lists durability tests to MIL-PRF-13830B (adhesion, moderate abrasion) and MIL-STD-810H (humidity, temperature). The mount carries an arrow pointing to the filter-coated surface S1; the anti-reflection coating is on S2. [9]

Editorial calculation: a 21.0 mm clear aperture on a 25.00 mm mounted filter is (21.0 ÷ 25.0)² ≈ 71% of the face area, below the more than 90% Alluxa cites for unmounted filters. Editorial calculation: a ≥85% passband against OD 4.0 blocking (0.01%) gives a pass-to-block ratio of at least 8,500 to 1.

Blocking range has limits. Chroma's handbook states that interference coatings block over a finite wavelength range, and that adding interference or absorbing-glass blockers extends the range at the cost of transmission and thickness. On its product page Chroma says its excitation filters block 300–1000 nm or beyond outside the passband, and that its emission filters may not block that far out of band. [6] [5]

Ask each supplier for measured spectra at your angle of incidence, the blocking range in OD, thermal shift, clear aperture and the test method for each specification. [1] [2] [3]

04

Trace what each part controls

This is a functional reference built from the sources cited; it is not a full specification.

05

Fill the specification fields with values for your system

A filter quotation needs values for your optical system. The table explains each field, gives published values where a reviewed source states them, and says what to ask the supplier for.

06

Send the spectral, angle and size data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Type: [bandpass; longpass; shortpass; notch; dichroic]
Spectral: [CWL; FWHM; edges; tolerances]
Transmission/blocking: [T% avg or abs; OD; range]
Angle: [AOI; cone half-angle]
Temperature: [°C; max shift]
Mechanical: [size; CA; thickness; mount]
Quantity: [units]

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