Internal transmittance falls exponentially with path length, τ = exp(−αd); doubling thickness squares τ, and the loss doubles only while absorption is weak. Uncoated glass at n = 1.5 transmits at most 92.3% at normal incidence (calculated); that gap is reflection, not absorption. A transmission…

A useful custom optical window specification should define more than material and dimensions. At minimum, it should describe the operating wavelength, substrate or material grade, geometry, thickness, clear aperture, optical quality, surface imperfections, parallelism or wedge, coating performance, angle of incidence, environment, inspection conditions, and…

Scratch-dig limits discrete local defects; surface roughness is the RMS height of fine texture across the whole surface. The MIL-PRF-13830B scratch number is a visibility grade, not a width; the dig number is diameter in 0.01 mm. Roughness-driven scatter scales with (σ/λ)², so the same…

A cone half angle shifts an interference filter’s spectrum toward shorter wavelengths and softens its edges; a pure tilt only shifts it. With the chief ray normal and a uniformly filled pupil, the mean shift equals half the marginal-ray shift. The same cone produces far…

A beamsplitter does not treat s- and p-polarized light the same way. At any angle other than normal incidence, the reflectance of a surface or coating is higher for s-polarization than for p-polarization, and the difference grows with the angle of incidence. At 45°, the…

Choose fused silica when the window must work in the deep UV, must not disturb polarization or wavefront, or must survive fast thermal transients; choose sapphire when the window faces abrasion, high pressure, chemical attack or high temperature, or when the band extends past roughly…

Choosing a Raman longpass edge filter should start with two numbers: the excitation laser wavelength and the lowest Stokes Raman shift you need to measure. The nominal cut-on wavelength alone is not enough. A useful Raman edge filter must simultaneously provide strong rejection at the…

A filter described as “850 nm,” “550 nm longpass,” or “10 nm bandpass” is not fully specified by those nominal numbers alone. Real optical filters are manufactured and measured against an allowable range. A nominal 850 nm bandpass filter, for example, may require its actual…

A machine vision system may use an 850 nm or 940 nm LED, a monochrome camera with good near-infrared sensitivity, and stable exposure settings—yet still produce images whose contrast changes with factory lighting, sunlight, or reflections from nearby equipment. The problem is often not the…

Edge steepness describes how quickly an optical filter transitions between its passband and its blocking region. It is measured as the wavelength interval between two defined reference points on the transition — for example between the 50% transmission point and the wavelength where transmission drops…

Protected aluminum and enhanced aluminum start from the same vacuum-deposited aluminum film. The difference is what goes on top: protected aluminum adds a single dielectric overcoat that stops oxidation and makes the surface cleanable, with almost no change to the reflectance curve, while enhanced aluminum…

Choosing a narrow bandpass filter manufacturer comes down to three questions: can their coating process hold your center wavelength tolerance across the full clear aperture, do they measure the filter under your actual angle and cone conditions, and is the blocking specification real across your…