Another way to highlight features is the use of ultraviolet radiation which has a wavelength of 10 to 400 nm. By means of fluorescent lamps ("black light") or ultraviolet LEDs it is relatively simple to generate UV light with a wavelength of 365 nanometres in the near "UV-A range".
Example: spectrum of a typical UV LED
Nevertheless normal glasses absorb large quantities of the emitted light even in this range. This also applies to standard optics of the camera system, filter glasses and also the protective glass on the sensor. Many applications can still be solved using standard components, as the excitation due to UV radiation causes many materials to fluoresce in the visible range. Real UV applications in shorter-wave light ranges, however, require special optics, e.g. made of quartz glass, and special camera sensors with suitable detectors.
Why use ultraviolet radiation for inspection?
Short-wave radiation increasingly tends to reflection compared to long-wave radiation, as due to the short-wave oscillation there is much interaction even in case of minimum penetration depth. UV radiation is therefore ideal to inspect surfaces. Small features, dust and scratches are clearly visible in this way.
Furthermore there are many applications in which the UV radiation causes the material to be inspected to glow.
Typical inspection possibilities using UV radiation
- When inspecting PCBs and solder joints, the strong reflection is advantageous. Defects, scratches or features are clearly visible.
- Inspection of fibre optics and fibres: due to the short-wave light, the intersection can be inspected as it reflects especially well and only glows diffusely.
- Transparent glues on various backgrounds, adhesive beads, sealing compounds, etc. are UV-active: the excitation using ultraviolet light often causes a bluish or greenish fluorescence in the visible wavelength range of light.
- UV-active inks and dyes in the field of marking and ID. Printed typefaces and safety features (e.g. on credit cards, bank notes, passport stamps, etc.) can be excited, detected and evaluated using UV radiation. Sometimes such dyes can be added in industrial applications in order to simplify the readability of these markings or similar.
- Chemical markers in biochemistry are affixed to gene sequences, proteins etc. to be inspected and made visible by means of UV radiation.
Important for imaging
- UV radiation is not visible to the human eye. Particularly the short-wave UV radiation can very easily cause harm to the eyes, skin and materials as it is especially rich in energy. Industrial applications using UV radiation possibly require the same safety measures like laser protection class 4 if certain energy values are exceeded. Safety zones, beam timing, switch-off when opening the guard doors, respective safety officers can become necessary in the company. Please thoroughly acquaint yourself with the topic!
- If possible, clock the industrial UV illumination. In this way unnecessary light emission is avoided and critical energy quantities, which lead to a high protection class, do not arise at all. Particularly UV LED illumination (very expensive: metal socket LEDs with quartz glass housing!) would otherwise typically become very hot and prematurely defective.
- The use of narrow-banded band-pass filters prevents interference caused by daylight. It should exactly be adapted to the fluorescence.