There is always more than one way to solve the optical problem in machine vision and industrial inspection, and the options are not equally good at everything. This article compares optical blue glass against the materials, processes and design approaches it competes with, on the criteria that matter commercially: performance, durability, unit cost and lead time.
Optical Blue Glass in Machine Vision and Industrial Inspection
In machine vision and industrial inspection, that translates into a specific set of requirements rather than a general preference for quality. Machine vision decides whether a production line passes or stops, so its optics must be repeatable rather than merely good. Illumination wavelengths are chosen deliberately, filters must block exactly what the algorithm cannot tolerate, and any change in transmission or geometry becomes a false reject. The environment is often a factory floor with coolant mist, dust and vibration.
The tolerances that actually matter
Because blue glass works by absorption, its tolerances are dominated by thickness and by the transmission curve at that thickness. A thickness tolerance of plus or minus 0.02 mm is normal and is not merely mechanical - it protects the spectral specification. Flatness and parallelism matter for the same reason they matter on any window: a wedge will steer the beam and a curved surface will change focus in a bonded stack. Chamfer and edge quality become critical when the part is bonded, since a chipped edge under adhesive creates a stress riser. In Machine Vision and Industrial Inspection, whether the part is bonded between lens elements or sits as an external window changes which tolerances dominate, and the drawing should say which of the two it is.
How it compares with the alternatives
The alternatives are a dichroic coated filter, an absorptive dye film and software correction. A dichroic filter is thinner and can be tuned more precisely, but its edge shifts with angle, which matters in wide-angle systems. A dye film is cheaper and thinner still but degrades under UV and heat. Software correction removes obvious infrared bleed but cannot restore color accuracy across a wide field. Blue glass sits between these options: angle-stable, robust, slightly thicker and more expensive than a film. In Machine Vision and Industrial Inspection, the deciding factor is usually mechanical depth - if the module can accommodate a third of a millimeter of glass, blue glass or a hybrid part is normally the most predictable solution.
Substrate and material selection
Blue glass is a family rather than a single product. Copper-ion-doped and phosphate-based grades give different transmittances, different cut-on positions and different color casts, and each supplier's designation behaves slightly differently, so the grade must be specified by transmission curve rather than by name. Thickness is a design variable as well as a mechanical one, because absorption scales with path length: a 0.3 mm plate blocks less near infrared than a 0.55 mm plate of the same grade, so thinning a part to fit a module changes its optical performance. In Machine Vision and Industrial Inspection, the sensible specification is a transmission requirement at the operating thickness, plus a permitted tolerance on that thickness, because the two interact and cannot be fixed independently after the design is frozen.
What drives cost and lead time
Blue glass is inexpensive material but expensive handling. The glass itself is a small share of the part price; what drives cost is thickness control at fine tolerances, the dicing process and the yield of thin, brittle parts. Adding an AR coating or a dichroic layer changes the part from a commodity to a coated optic and multiplies the price, which is why bare absorptive filters survive in designs long after a coating would be optically preferable. Very thin parts under 0.3 mm are the most expensive of all, because handling losses and inspection time rise faster than the material gets cheaper. In Machine Vision and Industrial Inspection, the cheapest specification is usually a slightly thicker part with a relaxed flatness, which is worth testing before the requirement is frozen.
Requirements specific to Machine Vision and Industrial Inspection
In machine vision and industrial inspection, that translates into a specific set of requirements rather than a general preference for quality. Machine vision decides whether a production line passes or stops, so its optics must be repeatable rather than merely good. Illumination wavelengths are chosen deliberately, filters must block exactly what the algorithm cannot tolerate, and any change in transmission or geometry becomes a false reject. The environment is often a factory floor with coolant mist, dust and vibration.
- Telecentric-friendly flatness and low wedge
- Band-matched coatings for the illumination wavelength
- Consistent transmission part to part
- Resistant to coolant, dust and cleaning solvents
ISO 10110 drawings, MIL-PRF-13830B surface quality, plus the machine builder's own repeatability and cleanliness requirements.
Selection data at a glance
| Parameter | Typical value | Notes |
|---|---|---|
| Type | absorptive NIR filter, uncoated or dichroic-coated | Hybrid keeps the edge sharp |
| Thickness | 0.21 - 0.55 mm common, up to 3 mm | Thin grades for camera modules |
| Visible transmission | T ≥ 90 % average | 420-620 nm |
| NIR blocking | OD2 - OD4 to 1100 nm | Depth set by thickness and grade |
| Cut-on shift with angle | negligible | The main reason blue glass is used |
| Thickness tolerance | ± 0.02 mm | Critical for bonded stacks |
| Flatness | λ/4 typical, λ/10 on request | Measured on 633 nm |
| Edge chips | ≤ 0.05 mm, laser-cut edges available | Laser cutting avoids micro-cracks |
| Environmental | humidity and temperature cycling stable | Absorptive color cannot delaminate |
Frequently asked questions
Is blue glass available in hybrid form?
Yes. Combining an absorptive substrate with a dichroic coating on one face gives a deeper, sharper cut-on in a thinner part than either approach alone, which is how modern thin camera modules are built.
Why choose blue glass instead of a coated IR-cut filter?
Because its cut-on does not shift with the angle of incidence. In a wide-angle or fast lens system a dichroic coating colors the frame edges, while blue glass keeps the spectrum consistent across the field.
What thickness should I specify?
Enough to reach the blocking depth the sensor needs. Absorption scales with path length, so a thinner part blocks less near infrared - thinner parts often need a stronger grade or a dichroic layer added on one face.
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