07.11.2013
By: David Schreiber, Claudius Piske

Measuring intensity in watts rather than lumens

What are the right wavelengths for the LED lighting on any given machine vision system?

"vicolux SFD30/9" und "vicolux SFD42/12" series dome lights

It is not hard to find a suitable camera system along with the matching optical components for most machine vision applications. In many cases, the correct size and geometry are also known or already specified. However one question often remains: which wavelength is most suitable? This article shows how appropriate catalogue entries can make it simple to use and compare the non-trivial lighting technology units of measure.

Now that LEDs are used almost exclusively as the light source in machine vision applications, determining which wavelength is the most suitable has taken on major importance. LEDs have a narrow-band spectral intensity distribution and how they act on the detector can vary greatly depending on the color. When viewed with the human eye for example, a red LED must produce around three and a half times as much light output to appear to be as bright as a green LED. Brightness perception is highest for green light, as is shown by the sensitivity curve for daylight vision 1 V(λ) in Fig. 1. The maximum of 683 lm/W is reached at a wavelength of 555 nm.

Fig. 1: The differences between the spectral sensitivity of the human eye and a selection of standard image machine vision are shown on the graph.

The machine vision system "eye"

The detector in a machine vision system, particularly the sensor, is comparable to the eye. The CCS and CMOS sensors on these systems have spectrally distributed sensitivity which overlaps with the eye's sensitivity to some extent. The only difference is that the photo effect on the silicon chip has a wider bandwidth than the photo receptors in our retina.

The brightness of LED components and lighting in the visual (visible) range is normally expressed in lumens (lm) which are photometric units. Lumens are used as the unit of measure because the main application of LEDs is "general use for illumination purposes" in which case the light is perceived and used by humans. In machine vision, visible light must be regarded as being only a portion of the electromagnetic spectrum and consequently light intensity should be expressed in radiometric units such as watts. Radiometric and photometric units are both listed in the Vision & Control product catalogue. This is done so that proper consideration can be given to the following parameters which have an effect on the resulting image brightness in the machine vision system: •

  • Lighting efficiency •
  • LED spectral intensity distribution •
  • Reflection/transmission of the object under evaluation •
  • Spectral transmission of the lens •
  • Spectral sensitivity of the detector

It is necessary to consider the spectral and radiometric characteristics of the lighting from the eye safety standpoint as well. An evaluation is done in house by Vision & Control for all vicolux lighting systems based on DIN EN 62471.

Application scenario

In order to achieve high illumination intensity, selection of the "right wavelength" can be crucial, especially in high-speed inspection applications. Sufficient exposure must take place at the image sensor in a very short time. The following machine vision task will be used to illustrate the effects which changing the wavelength has on the system's image sensor.

A colorless fluid is filled into vials made of colorless glass at a pharmaceutical company. A machine vision system checks the fill level and cap tilt. During a system retrofit, the machine vision unit is being optimized so that it can handle twice the cycle rate. The resolution and accuracy of the overall system must remain the same.

Total-system lighting comparison

Fig. 2 show a brightness comparison for the vicolux FDL60x90 transmitted light system at different wavelengths (blue, green, red and infrared) at the Sony ICX204AL image sensor under continuous light. The spectral sensitivity of the standard CCD sensor is very similar to that of the human eye. The lighting is operated at 6W at each wave length. The lens is an entocentric lens with a 25 mm focal length. The caps on the vials are about 50 mm high and about 15 mm in diameter. They are spaced at about 15 mm on a material transport system. An image scale of about 1:3 is needed to fully inspect the object. The vials are filled at a rate of about 600 units per minute, so the conveyor speed is roughly 0.3 m/s. In order not to exceed the allowable motion blur of around 30 µm (about 10 µm in the image), the maximum allowable sensor exposure time for the sensor is 100 µs. The vicolux FDL60x90 lighting system operating in flash mode is a good choice for such short exposure times. Flash and continuous modes are a standard feature of vicolux lighting systems.

Fig. 2 (center) shows the results of comparative brightness testing on lighting systems operating in flash mode using the same sensor and lens (adjusted aperture setting). The LEDs on the infrared lighting system, especially at the peak wavelength of 850 nm, can be made to light up very "bright" because of the short flash times. The intensity of infrared lighting increases even faster compared to red lighting.

Fig. 2:shows a comparison of transmitted light for the vicolux FDL60x90 in four different colors (blue, green, red and infrared) using the Sony ICX204AL image sensor under continuous light (top) and in 100 µs flash mode (center). The images at the bottom were taken using a MT9V034 CMOS sensor, also operating in 100 µs flash mode. (Lab photos with stationary objects)

If the machine vision system has a CCD sensor, red light is advisable to produce the maximum grayscale level in the resulting image. If the system has a CMOS sensor, infrared lighting could be the better choice. Fig. 2 below shows a comparison of four illumination colors taken with the MT9V034 CMOS sensor (also 1/3 inch). In relative terms, the sensitivity of CMOS sensors is considerably higher in the near infrared range (e.g. at IR 850 nm) compared to CCD sensors. (The factors obtained are guideline values and can vary for different LED lots).

Comparison using other LED types

It is important to keep an open mind, however, when looking at the results of the example described above. Substantial efficiency gains, based for example on thin-film technology, have been made in recent years in InGaN chips which are used to fabricate white LEDS, especially power LEDs. InGaAlP chips for red LEDs used to be at the leading edge of technology but more efficient technologies are now available. vicolux®-SFD30/9 and vicolux®-42/12 series dome lights are one example.

Using power LEDs as the basis of comparison, the results are markedly different from those obtained with the lighting discussed in the previous section. At a working distance of 5 mm, the illuminance of red lights in this product series is 58 klx. The figure is 98 klx for the white version. Expressed in the equivalent radiometric units, the numbers are 325 W/qm for red and 319 W/qm for white. This shows the enormous increase in efficiency compared to red lights.

The comparison in Fig. 3 shows that in contrast to the first example, the white light version is brighter when a CCD sensor is used and blue light produces roughly the same results as red light.

Fig. 3: Brightness comparisson for vicolux-SFD30/9 and vicolux-42/12 series lights (power LEDs) in four different colors, measured with different detectors.

When making comparisons between CCD and CMOS sensors, it is important to keep in mind that amplification of the sensor signal in the camera can vary and that the figures presented above were calculated based on standardized quantum efficiency.

Summary

"Don't trust your eyes." The comparison of photometric data and subjective image and brightness perception can sometimes be very different from what the camera in a machine vision system "sees".

It is therefore advisable to estimate brightness on the basis of radiometric units. Vision & Control provides both radiometric and photometric intensity data to make things easier for you. The CCD and CMOS comparison value which is also listed helps you select the most suitable illumination wavelength for standard sensors. For non-standard sensors, the relative sensor effect can also easily be determined with the aid of the illumination spectrum and quantum efficiency of the sensor.

appeared in Markt&Technik 43/2013