Illumination

With the advent of sophisticated software, designing of the optics has almost become a matter of "turning the crank". Illuminating the object so as to capture the wanted detail, and not the surrounding "noise", is still more of a learned art than a science.

To achieve an accurate image, it is necessary to turn off any gain control on the camera. This feature tries to even out the signal over the full image. Hot spots will drive the camera to reduce the signal level, possibly erasing desired data. Conversely, a large amount of dark area will drive the camera to boost the signal and over drive certain areas. Video imaging is further complicated by the camera's ability to see lower light levels and longer wavelengths than the human eye. In a machine vision application, one takes great pain in eliminating illumination artifacts which the software will interpret as object detail.

Historically, the old tungsten bulb technology was improved with the addition of halogen gas which permitted a higher operating temperature and longer life, because the halogen gas would redeposit the tungsten back onto the filament, instead of the bulb envelope.

Then, with the advent of fiber optics, the bulb was able to be placed in a remote location without worrying about heat buildup, thereby permitting higher wattage and forced cooling.

Recently, they have been doing the same with the high pressure mercury bulbs used to provide UV illumination for use in fluorescence applications (the glass fiber had to be replaced with a "liquid light guide", as the glass would absorb the UV).

And along comes the LED. What started out as a few photons being emitted from a semiconductor junction has gone from a low cost indicator light to a "light engine" that is used for traffic lights, store fronts and buildings, and automotive.

The advantages are:

  • A long life (100K hours, if you control the operating current and temperatures).
  • Robust - you can drop them without breaking a fragile filament.
  • Energy efficient -- only the desired wavelength is produced, with minimal excess heat.
  • Cost -- the initial outlay is significantly more due to the electronic components, but in areas where difficulty of replacing or cost of downtime is significant, the financial picture may be positive.


Backlight

The backlight "shadows" internal (holes) or external (boundaries) much like an opaque projector. Useful in measurement or edge defect detection.

Image: Backlight



Ringlight

Ringlights provide a 360 degree illumination angle that minimizes the formation of shadows normally associated with oblique illumination. Mostly used with relatively flat, diffused surfaces.

There is a specialized application where the angle of illumination is very shallow causing the light rays to reflect off shiny (specular) surfaces and not re-enter the lens. In this case, any defects on the shiny surface, (scratches, contamination, etc) will show up against a dark background (darkfield illumination).

Image: Ringlight    Image: Ringlight




Image: LED Ringlight

LED Ringlights with auxiliary lenses for working at 50, 100, and 150mm working distances.

Limit of useable working distance about 200mm.



Image: Fiber Optic Ringlight    Image: Fiber Optic Ringlight Lamphouse

Fiber Optic Ringlights and lamphouse.

Limit of useable working distance about 200mm.



Coaxial (Incident) Illumination

Illumination is sent down the viewing axis, striking the object and reflecting back into the lens, and on to the image plane. The most common design uses the last optic in the viewing lens as the condenser lens. If well controlled, it provides an excellent image of a flat specular object.

Image: Coaxial Illumination

Image: Zoom 6000 / Fiber Optic Coax

Zoom 6000 / Fiber Optic Coax
Image: Zoom 6000 / LED Coax

Zoom 6000 / LED Coax


Image: Coaxial Illumination Image: Coaxial Illumination Image: Coaxial Illumination



The LED Coax Illuminators are available in 1 watt and 5 watt versions. The 1 watt comes in white, warm white (less blue), and red. The 5 watt comes in the two whites, no red, green, cyan, blue, and royal blue. Available power drivers include a manual, digital / with computer control, and an OEM board for computer control for the 1 watt, and a computer control and OEM board for the 5 watt.



External Coaxial Illumination

Image: External Coaxial Illumination

The goal with coaxial illumination is to get the reflected rays back into the lens system. When using the last lens as a condenser, the downward ray is limited by the diameter of the lens.

If a much larger field coverage is required, some applications utilize an "under the lens" coaxial system, as pictured above. The obvious drawback to this type of system is that, to get a much larger downward ray, the illuminator optics also become large and this reduces the available clearance under the lens.

The Uni-Lite Illuminators below, reduce clearances by 88.9mm (3.5 inches) and 50.8mm (2 inches) respectively, and are used only with the Zoom 6000, 12X, and Precise Eye.

Image: External Coaxial Illumination