Quality Magazine

Color Imaging: Color: It's Not Always What You See

January 10, 2012
Color is a versatile, non-contact method to identify materials and material variations and its wave properties are commonly used for gauging.

Concave grating spectrometer covers wavelengths from 360 to 825 nm. Input via fiber optic cable. Spectra detected by 2048 pixel linear CCD detector and output communicated via USB 2.0 and RS-232. Exposure time from 1 ms to 65 seconds. Source: Ocean Optics


All materials have color.

If you shine light onto a material (solid, liquid or gas), the amount of light that is reflected, transmitted and absorbed depends upon the color (or wavelengths) of the light and of the material. The colors can be in the ultraviolet, visible and infra-red.

The wavelengths of the light reflected, absorbed and transmitted, as well as the light emitted by the light sources, are the direct consequence of processes involving the electrons and molecules.

In emission from an atom, an electron in a higher-quantized energy level drops down to a lower-quantized energy level. The energy lost by the electron can be emitted as a light photon with its energy (and wavelength) equal to the energy difference between the upper-electron energy level and the lower-electron energy level.

Absorption by an atom can take place in the reverse direction: a light photon (or quantum of light) with the appropriate energy is incident on the atom and absorbed by an electron in a low energy state. The electron is then boosted to a higher energy state by an energy difference equal to the energy in the photon.

If the energy of the photon does not match the energy difference between the two energy levels of the electron, then absorption does not occur. Vibration and rotation modes of molecules provide other acceptable lower-energy differences (usually in near infra-red) that provide probes with different energies.

In a solid material (e.g., a semiconductor), the energy levels allowed to electrons are broadened (This occurs since the electrons now feel the electric field of its own atom’s nucleus as well as electric fields from near-by atoms). So, instead of narrow-band emission and narrow-band absorption, the wavelength spreads (or bands) are wider. This, of course, affects both the absorption bands of semiconductors used in detectors, as well as when they are used as light sources. In semiconductors, life is more complicated: for example, controlled impurities used to dope the material can add other optical features.

These properties of light are particular to different materials and are analogous to fingerprints of the materials for identification purposes. The specificity of properties of different materials can be used to characterize different materials for process and product control.

In addition, light has wave properties as well as the particle-like quantum properties discussed previously. The wave properties are extensively used for non-contact gauging.

Emphasis here is on vision and sensing applications. The wavelengths of the light discussed will be from the ultraviolet (180 nm to 400 nm), through the visible (400 nm to 700 nm) and near infrared (700 nm to 2,000 nm).



Point-and-shoot spectrometer. Spectrometer covers wavelengths from 380 to 780 nm. Spectrum from image is detected by pixel linear CCD detector. The output is displayed on the unit and can be communicated via USB 2.0 and RS-232. Exposure time is from 3 ms to 6 seconds. Object size, with auxiliary lens, down to 36 microns. Source: Photo Research

Lighting

As with other aspects of illuminating materials and products:

The best lighting provides the best images.

The best images provide the best signals.

The best signals provide the simplest, fastest and most reliable signal processing.



The stand-alone word “lighting” or “illuminating” sets up the wrong mental image. Lighting is not just shining light, as in lighting a room (even room lighting has different applications, such as room lighting for offices, living rooms, bedrooms).

I prefer “targeted lighting,” which exploits the optical properties of the target (or object of interest). Use of color can also exploit differences between information-containing signal and background light to improve the signal-to-noise ratio.

It can use not only color/wavelength/spectral properties, but also geometric properties (micro and macro dimensions, texture and texture axes) and induced optical properties (timed fluorescent excitation with fluorescent markers and intrinsic fluorescence).

We can see how different colors and purity are produced, selected and detected. This provides a basis for choosing different light sources, color selectors and detectors.

First, examples of practical applications of targeted lighting are discussed.



Examples of Practical Applications

Using broad spectral regions to determine materials and variations

Determine extent/completion of thermal annealing of automobile brake linings by sensing changes in near infra-red reflection.

Determine area of aluminum precipitated on silicon crystal using a different value for each of the absorption edges of the two materials.

Detect presence of inorganic material contamination in raw mixed soup vegetables for major soup manufacturer using general differences in optical properties between plant (organic) and inorganic materials.



Using spectral lines (narrow spectral regions) to discriminate against background

Identify presence of helium on sun, 93 million miles away.

Confirm presence of annealing gas in expensive whisky bottle during heat treatment.

Sense & image the presence and chemical activity of molecules by attaching fluorescent markers to molecules (optogenetics).

Count rapidly moving cool steel tubes in factory with very variable overhead & sun light by using small, low-power laser, narrow band filter, sensor and computer.

Gage diameter of glowing rods leaving annealing oven using narrow laser-line illumination with narrow-band filter for laser-line to eliminate background light.



Using dimension of light wavelengths for gauging (N.B. 500 nm = 0.5 microns; 25 microns = 1 mil = 0.001 inch; yellow is about 600 nm)

Gage diameter of small-diameter wire and fibers from diffraction pattern produced by single wavelengths from parallel laser beam.

Gage particle size and surface roughness from small-particle scattering.

Gage strain in structures by using fiber-optic sensors. The fiber optic cables have diffraction gratings embedded in them, so that small strains change the spacing of the grating apertures; this changes the diffracted wavelengths seen by the wavelength sensitive detection modules and the change is a measure of strain.

High-resolution, multi-wavelength dimensional gauging with interferometers.

Use polarization properties of thin transparent extruded films to detect holes and defects.



Optical Tools

As seen in the examples, color applications vary from simple to sophisticated.

Most applications require a light source and photodetector(s) that use optical properties of the target. The simplest is when the target is opaque, as in sensing when a person walks through an entrance. The most complex depend upon specific features of the target.





Light Sources

The most common light sources are incandescent and fluorescent, with LEDs expanding rapidly in all areas. LEDs are small, inexpensive, rapidly-switched, semiconductor devices that operate for longer times but have wider spectral lines/bands than lasers.

Low-power lasers for sensing provide very parallel beams of light with very narrow spectral lines and can be the size of AA batteries with built-in optics and electronics.

Small semiconductor lasers were used in several of the examples.

Narrow-band spectral lines are available from low-pressure gas light sources at different wavelengths.

Sodium doublet lines (at 589.0 nm and 589.6 nm) provide the illumination from yellow highway light. They have been used as precision wavelength sources for factory and laboratory use.

A handy bright ultraviolet line at 253.7 nm from a low-pressure mercury source (with quartz windows and eye protection) can be used for both sensing and UV curing.





Optical Filters

Optical filters can provide wavelength bands of interest and discriminate against unwanted background light. They are available in band-pass, high-wavelength pass, low-wavelength pass and narrow-band filters.





Broad-Band Optical Filters

A simple set-up for detecting defects and foreign matter may use a white-light (many wavelengths) light source, an optical filter appropriate to the materials and a photodetector or camera sensitive to the light transmitted by the filter.

Since the materials detected are solid, then it would be likely that the bands detected would be broad, so that broad-band (many wavelengths) optical transmitted filters can be used.





Instruments for Selecting Narrower Wavelength Regions of Interest

To obtain narrow spectral lines from broader sources, narrow-band filters, monochromators, spectrometers and interferometers are used. Each has their own characteristics and variations.

Monochromators and spectrometers separate the different colors, from light sources, across large wavelength regions; narrow wavelength bands are produced.

Interferometers are often used to measure the light intensity of different wavelengths (e.g. the structure of spectral lines in very narrow spectral regions)



Order of Battle

For determining which wavelengths to use for materials in products, use a monochromator or spectrometer to provide information on which wavelength region is most useful (i.e., gives the highest signal-to-noise ratio). Then, in practice, use a filter or filters that do the job.

It can be sufficient to simply have a number of filters that cover the spectral region of interest and try them one at a time (i.e. kind of a poor-man’s monochromator). Then, in practice, use an appropriate filter (or filters) to do the job at lower cost and smaller space requirements.





Signal Processing

Different color applications can require not only different spectral resolutions (or how well two colors can be distinguished), but also means for detecting very low color/light signals that are close to, or within, the background noise.

Simple photosensors can be used for higher intensity light, but more sophisticated optics are required for higher spectral discrimination.

For very low signals, it is common to modulate (at a given electrical frequency) the source of the incident light before it illuminates the target of interest and then to detect the modulated signal (of that electrical frequency) that has been modified by the target.



Summary

Color is a versatile, non-contact method to identify materials and material variations. Wave properties are commonly used for gauging. These provide:

fingerprints of the materials for defect detection and identification purposes.

non-contact gauging of features, particularly those with small-dimensions

improvements in the signal-to-noise ratio due to background.



It is used in virtually every industry to determine/confirm quality for high-volume production and to determine/confirm quality for low-volume, high-value products. It is used for product and process control. V&S

Tech Tips

The wave properties of color provide “fingerprints” of the material for defect detection and identification.

Color imaging is an effective non-contact gauging tool, particularly small dimensions.

The use of color can also exploit differences between information-containing signal and background light to improve the signal-to-noise ratio.