The Forensic Analysis of Color
Written by Mike Nofi   

An understanding of the science behind the measurement of color is important for forensic analysts. Color comparison is frequently performed when dealing with forensic paint samples—such as from hit-and-run cases or from paint trans-ferred from a vehicle to a crowbar. Forensic color measurement using tools such as microspectrophotometry has been around for many years. However, the relatively recent introduction of advanced, color-shifting pigments has made this topic even more complex and demands a full understanding of how light and viewing angle can affect analyses.

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Color-shifting Microflakes


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Color-shifting microflakes are becoming more popular in the marketplace. Examples range from banknote security applications to decorative pigments—including automotive paints, cosmetics, fabrics, leather, plastics, and floor coverings. The color-shifting properties of these pigments are based on thin-film light interference.

Analysts presented with these new microflakes face a challenge in characterizing the color and appearance of the technology in various applications and product forms.

The unique property of optical thin-film interference microflakes is the ability to change color with the observer’s viewing angle, a quality called gonioapparent color. The observed color is also affected by the viewing environment, including the type of illumination (for example, diffuse illumination vs. direct illumination).

While “real-world” environments can have combinations of various types of lighting, direct lighting is best for viewing microflake effects. This greatly simplifies the analysis of visual color. Direct lighting is used to simulate the color appearance under direct sunlight, or single light-source illumination. This is the type of lighting used by most spectrophotometers, devices that measure the intensity of the part of a ray of light that is reflected.

Spectrophotometers that use integrating spheres provide diffuse illumination and are used to simulate the color appearance of these materials under overcast or foggy skies. The type of measurement geometry used in a spectrophotometer measures a specific color and appearance property of color-shifting microflakes.

Measuring Geometries

Direct 45°/0° geometry—This is the most popular geometry for measuring diffuse colored samples. It correlates well with the way the human observer views color by using an overhead light with the observer viewing the sample at 45°. This geometry does not work on colors with mirrored surfaces and is of limited use for measuring gonioapparent colors.

Gloss angle geometry—With this geometry, the angle of incidence is equal to the angle of view. This is used for mirror surfaces. This geometry works for measuring mirrored-surfaced gonioapparent colors.

Diffuse d/8° geometry—This technique incorporates an integrating sphere to diffusely illuminate the sample. The sample is viewed at a near-normal angle of 8°. This type of geometry is becoming increasingly popular in the color industry. It tends to factor out non-uniformity in the sample, giving a more averaged color measurement. It has the benefit of being somewhat insensitive to unflat samples and samples that exhibit “directionality of surface” (i.e. samples that look different when rotated, such as corduroy fabrics).

Aspecular X-Rite/BYK Geometry—A more recent line of multi-angle instruments is available for measuring metallic pigments. These instruments usually feature five or six measurement angles. The sample is illuminated at 45° and measured at multiple angles away from the specular angle. Interference color-shifting microflakes have an extremely strong specular color component. Therefore, measurement angles more than 30° away from the specular angle do not give useful color-shift information. It does, however, provide useful information about the “flop” or change in lightness as you view the paint further away from the specular gloss angle. Multi-angle instruments are not the best choice for measuring the multi-angle colors of interference microflakes. They are, however, useful in measuring and controlling the orientation of the flakes.

Aspecular 10° geometry—Measuring the dynamic color performance of interference color-shifting microflakes requires a measurement geometry that is currently only available with goniospectrophotometer instruments. The Aspecular 10° geometry includes 11 angles of illumination and view. The geometry is similar to multi-angle gloss geometry, with the exception that for each illuminating angle, the viewing angle is 10° aspecular. This is done to avoid the gloss component associated with clear-coated painted samples. Aspecular 10° geometry is used to measure dynamic color area (DCA), the measure of the gonioapparency of the color or interference color-shifting flakes. DCA metric can be described as a measure of the dynamic “colorfulness” of a paint or pigment.

Color by Diffraction

Recently, a new type of pigment based on interference by diffraction has been introduced. When white light is incident on a grating surface, it is separated into different wavelengths’ components. If angle of incidence is not normal to the surface, zero-order (or specular) reflection is created. The diffraction grating also creates first-order diffracted lights surrounding the zero-order reflection. Similarly, second- and higher-order diffracted lights can be created at higher angles.

Diffractive geometry—For diffractive types of pigments, a new geometry—called “diffractive”—has been created. Currently, only goniospectrophotometers are capable of measuring the full performance of diffractive paints and pigments.

Color-Measurement Equipment

Spectroscopy

Spectroscopy is used to analyze how light interacts with materials. It includes analysis over a broad range of wavelengths including the ultraviolet, visible, near infrared, and infrared.

Spectrophotometer

A spectrophotometer is a device that measures the intensity (as a function of wavelength) of that part of a ray of light that is reflected, absorbed, or transmitted when the ray of light interacts with the material. The material could be a solid, powder, fluid, or gas.

Colorimeters

Colorimeters are used to measure the color of an object and are usually limited to the visible light spectrum. They are extremely accurate within narrow color ranges. With the exception of multi-angle colorimeters, the illumination/ viewing geometry is fixed to ensure uniform conditions for measurements.

Goniospectrophotometers

A goniospectrophotometer is a spectrophotometer where the illumination/viewing geometry is selectable, allowing the user to create multiple combinations of angles of incidence and view.

This system assesses the ways objects distribute light by measuring the amount of light reflected or transmitted as the directions of incidence and view are changed. Goniospectrophotometers are ideal for measurement and analysis of gonioapparent materials such as metallic and pearlescent paints and opaque-interference coated pigments.

The fundamental procedure for evaluating the color of a reflecting or transmitting object is to obtain spectrophotometric data for specified illuminating and viewing conditions. Using this data, absolute color standards and color tolerances can then be applied to control variations in color appearance.

Microscopy

Microscopy is the examination of objects by means of a microscope. Photomicrography is the recording of images, usually magnified more than 40x, produced by a microscope on a digital camera CCD or CMOS detector array. The most familiar form of microscopy in the forensic evaluation of color is a microspectrophotometer—an instrument that combines the capabilities of a spectrophotometer with those of a microscope. This is useful for measuring the color of very small particles.

Visual sample evaluation

The ability to perform a visual observation to verify a color match between sample pairs is needed to validate instrumental color measurements.

To accurately view color, a color-viewing booth is needed to provide standardized viewing conditions to simulate real-world appearance. Traditional light booths provide overhead lighting with the samples viewed inside a box enclosure that is painted a medium neutral grey.

What Does this Mean for Forensics?

Color can be complex and is affected by the type of light that objects are viewed under. Color appearance can depend on: the type of illumination (direct vs. diffuse), angles of illumination, angles of view, background differences and the texture characteristic of specimen.

Today, accurately measuring color in various ways is key in forensic analysis given the advances towards color-shifting pigments. For example, the goal of colorimetry in forensic science is to quantify the color of an object for the purpose of material identification to determine if a match exists between two materials found at two different locations. The intent is to tie these two materials to the same source.

Often in forensic science, very small materials are available as evidence in a crime. In some cases, only single flakes of particles such as cosmetic glitter or paint chips can be found at the crime scene. A microspectrophotometer can obtain a “spectral signature” from the particle that may be matched to the suspect and the victim. This information is used in a court of law. Without measuring the color of these microscopic materials there is no direct way to link the suspect to the victim.

Another important application of the microspectrophotometer is to provide analysis on suspected counterfeit materials. By analyzing individual flakes from the genuine article and comparing these to the suspect article a determination can be made as to authenticity.

For More Information

To learn more about color-shifting pigments and how they work you can visit this site.

This article was adapted from “Metrology Color Lab: It’s All About Measuring Color and Appearance,” a paper from JDSU.

About the Author

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it is the Metrology Lab Manager for the Flex Products Group in the Advanced Optical Technologies business segment of JDSU.

 
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