Illumination

Illuminate features & make defects visible

UV illumination

Another option for highlighting features is the use of ultraviolet radiation, which can have a wavelength of 10 to 400 nm. Using fluorescent lamps ('black light') or ultraviolet LEDs, it is quite easy to produce UV radiation with a wavelength of 365 nm in the 'near UV-A' range.

Please check the photobiological safety of UV lighting. As the human eye can no longer detect UV radiation, there is no eyelid closure reflex. UV LEDs are therefore often no longer in LED Risk Class 1, but in LED Risk Class 2, for example. Shielding, enclosure and labelling are mandatory.

Which hardware for inspecting in the UV?

However, even in this UV-A range, standard lenses absorb a large proportion of the light that is emitted. This also applies to standard camera system optics, filter glasses and even the protective glass on the sensor. Nevertheless, many applications can be solved with standard components, as excitation with UV radiation causes fluorescence in the visible range in certain materials.

However, real UV applications in shorter wavelength light ranges require special optics, e.g. made of quartz glass, and special camera sensors with suitable detectors.

Normal CMOS sensors are only UV sensitive up to 370nm. Normal CMOS sensors are only UV sensitive up to 370nm. However, there is now a simple solution, as SONY has launched the 'Pregius S IMX 487' sensor with 8 megapixels (2.74µm, 193 fps). Its sensitivity ranges from 200 to around 1000 nm and is in the product portfolio of many camera manufacturers.

Emission spectrum of a UV LED

Why use UV radiation for inspection?

Short-wave radiation tends to be reflected more strongly than long-wave radiation because the short-wave vibrations cause strong interaction even at low penetration depths into the material.

UV radiation is therefore ideal for surface inspection. Small features, dust and scratches are particularly visible.

There are also many applications where the material to be inspected is made to glow in the visible by UV radiation.

Surface effects using IR and UV radiation

Typical inspection possibilities using UV radiation

The high reflectance is an advantage when inspecting printed circuit boards and solder joints. Defects, scratches or features are clearly visible.
Inspection of fibre optic cables and fibres: The short wavelength light allows inspection of the cut surface as it is highly reflective and not just diffusely illuminated.
Transparent adhesives on various substrates, adhesive beads, sealants, etc. are UV active: When excited with ultraviolet light, they often produce a bluish or greenish fluorescence in the visible wavelength range of light.
UV-active inks and dyes for labelling and identification. Printed inscriptions and security features (e.g. on credit cards, banknotes, passport stamps, etc.) can be excited, detected and analysed by UV radiation. Sometimes such dyes are added industrially to improve the readability of such and similar markings.
In biochemistry, chemical markers are attached to gene sequences, proteins, etc., to be observed and visualised by UV radiation.

Visible light

White glue on white plastic bottle is hard to see.

UV radiation

The adhesive fluoresces and appears bright.

UV radiation

Security features on official documents become visible when exposed to UV radiation. Metal security strips also shine (as with visible light).

Important for machine vision

UV radiation is not visible to the human eye. Due to its high energy, short-wave UV radiation in particular can easily cause damage to eyes, skin and materials. Industrial applications using UV radiation may require the same safety measures as lasers (Laser Safety Class 3B) if certain energy levels are exceeded. Safety zones, beam limitation, shutdown when safety doors are opened, safety officers in the company may be required. Please find out in detail!
Where possible, led based UV lamps should be cycled. This avoids unnecessary light emissions and critical energy levels that lead to a high protection class. UV LED lighting in particular (very expensive: metal base LEDs with quartz glass housings!) would otherwise typically become very hot and fail prematurely.
The use of narrow-band bandpass filters prevents disturbing light influences from daylight. These should be precisely matched to the fluorescence.

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