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Visual Observing Situation Model

HunterLab instruments measure the color as it relates to the quality of the product. To be able to begin to measure and quantify color we have to know more about why colors appear as they do. As humans we all see and perceive color differently; this is because the human perception of color is a psychophysical response. The visual observing situation model shown above illustrates the three components necessary for the perception of color.

The first component of the visual observing model is a white light source. This light provides the spectral energy required for viewing color, without light we cannot see colors. The next component needed in the visual observing model is an object. This object modifies the spectral energy from the light source. Colorants, such as pigments or dyes, in the object selectively absorb some wavelengths of the light while reflecting or transmitting others. The last component of the observing model is the human observer. The observer is a human eye that receives the light off or through the object and then the brain provides the perception for the vision.

In order to build an instrument that will quantify human color perception each item in the visual observing situation model must be quantified. These three items are all able to be quantified as a table of numbers.

This is a series post. We will go into further detail on quantifying white light sources, objects, and observers in subsequent posts.

FAQ: “You used the acronym CIE.  Could you tell me what that is? If it is a document, could you please let me know where I can access it?”

CIE stands for “Commission internationale de l´éclairage” which is the French version of “International Commission on Illumination”.

The CIE is a standards body that defines and maintains the global rules for light and color measurement. This is like the CIPM (Comité international des poids et measures) who defines and maintains metric dimensions, weights and volumes. It is composed of approximately 38 member committees from countries around the world.

The CIE rules are incorporated into industrial test methods defined by approximately 30 international industrial standards bodies, the most prominent of which are:

• ASTM – American Society of Testing and Materials, West Conshohocken, PA USA
• ISO – International Organization for Standardization, Geneva, Switzerland.
• JIS – Japanese Industrial Standards are available from JSA – Japanese Standards Association, Tokyo, Japan.

More background information is at the CIE web site and at Wikipedia.

HunterLab does not supply glass or plastic filter transmission standards but here are some sources.

HunterLab would always encourage a client to regularly use the diagnostic options that come with our sphere instruments to verify instrument performance over time.

1. Short Term Repeatability on the White Tile to verify the instrument electronics are in good health.
2. Green Tile Test for verification that the instrument has not changed in long term photometric performance.
3. Didymium Filter Test for verification that the instrument has not changed in long term wavelength performance.

Additional transmission filters can provide an extra level of validation particularly related to the measurement of transparent solid samples (go here for transparent liquid standards). Here are three situations where having one or more additional transmission filters could be helpful:

1. When your product is a transparent solid, having a more stable, durable, cleanable and uniform glass filter to serve as a PQ Performance Qualification check filter can validate the measurement process. Each day, after standardization, operators can read the check filter and compare to baseline values measured in the same color scales as the product. If the read values match the baseline values closely, the operator and instrument are performance qualified to proceed to measure production samples.
2. Additional confidence in the measurement process can be gained by measuring the color of a uniform glass filter, similar in color to the product, over time. It does not have to be the same color, just in the same area of color space. Documenting the filter results over time helps verify that the product color you measure today is the same as measured weeks, months or years ago.
3. When you are conducting an inter-instrument agreement study of transmission color measurements taken on a transparent solid product at multiple sites, inclusion of a glass filter can allow outlier sites to be easily spotted. The glass (best) or plastic filters will be more uniform and stable than the product. All sites should agree closely on a uniform glass filter of similar color as the product. If not, the usual problem is that outlier sites do not have their instrument configured correctly for the measurement. If a site is an outlier on the check filter, their product measurements are also suspect. Stopping the test at this point and figuring out what is wrong is the best path forward.

The reference document that defines the visual EP Opalescence scale is:

EP 2.2 Physical and Physico-Chemical Methods for color and opalescence

EP – European Pharmacopoeia, Section 2.2 Physical and Physico-Chemical Methods, Unit European Pharmacopeia, Strasbourg, France (1997: 15-16) www.pheur.org

This method describes the visual evaluation of scattering or opalescence in near clear liquids, typically pharmaceutical, relative to distilled water being a perfect clear.

There are two types of physical liquid standards for visual turbidity or opalescence – Formazin solution (with or without stabilizer) and polymer beads (polystyrene micro spheres). The Formazin solution is the historical liquid scattering standard but the polymer beads is considered more stable and homogenous.

Section 2.2.1 Clarity and Degree of Opalescence of Liquids in the EP 4th edition defines a Formazin Primary Opalescent Liquid Suspension (rated at 4000 NTU per EP 5th edition) as a solution of hydrazine sulphate solution and hesamethylenetetramine solution which is stable for 2 months stored in glass.

The EP 4th edition further defines a Formazin Standard of Opalescence (rated at 60 NTU per EP 5th edition) as a dilution of 15.0-ml of the Formazin Primary Opalescent Liquid Suspension (4000 NTU) to 1000.0–ml of water. This suspension must be freshly prepared and stored for no more than 24 hours.

To make the EP Reference suspensions or OP – Opalescence standards, the Formazin Standard of Opalescence (60 NTU) is mixed with distilled water in the following proportions to define 4 levels of liquid EPOP Opalescence Standards. Distilled water is nominally a fifth EPOP standard defining no opalescence or scattering.

Table 2.2.1-1 EPOP StandardsIIIIIIIV Formazin Standard of Opalescence (60 NTU)0.0 ml5.0 ml10.0 ml30.0 ml50.0 mlDistilled Water (fill to 100.0 ml mark)100.0 ml95.0 ml90.0 ml70.0 ml50.0 mlNTU Rating361830

At my new quality assurance job, one of our product quality checks is to measure the color of our product and to ensure that we are producing our product that is within a preset acceptable color range. We use an instrument, a spectrophotometer, that reports Hunter L, a, b; XYZ, and L*a*b scales. I’ve always thought that color was measured in terms of RGB, like the way computer monitors and TV screens describe color. I’ve even created custom colors for fonts on my computer by manually adjusting the RGB quantities. Can someone explain why we would measure color using Hunter L, a, b; XYZ, and L*a*b instead of RGB?

It depends on what you want. Do you want White, Yellowish White, or Bluish White?

In simple terms, the primary difference between CIE Tristimulus Scales (Hunter L, a, b; XYZ, and L*a*b scales) and RGB is their purpose within the color world. RGB is a device dependent method of producing color and is not exact enough to be used to describe a color for quality control purposes. CIE XYZ color scales represents the true color of an object, while RGB describes a flat solid color representation of the average color of an object is displayed on a screen. One is used to provide color directionality (RGB), the other is used to precisely quantify a color (Tristimulus values). let’s illustrate…

Let’s take a drive to the Lincoln Memorial

Let’s pretend the Lincoln Memorial is not a physical object but rather a specific color, let’s say white, since in fact it is made of a very specific white concrete. To get there, should I use RGB or Tristimulus values? This will depend on how close to the Lincoln Memorial, or its specific color of ‘white’ you want to get. Using RGB to measure the color white and expecting analytical precision would be like trying to get to the Lincoln Memorial without the exact address and a GPS/map to guide you. While you may know that the Lincoln Memorial is located in Washington D.C., getting to the specific address would be a challenge.

RGB is very much like this in that you might know the general area of red, blue, green, or in this case ‘white,’ but getting to a precise color takes more than a general direction. Much like GPS, which uses three-dimensional physical coordinates that can guide you to within three feet of the desired address, CIE Tristimulus scales provide three-dimensional color coordinates to give you the exact address of a specific color with extreme precision. While RGB might drop you off on the Mall without any further direction, tristimulus coordinates will direct you precisely to a specific color with decimal precision, much like GPS will guide you to the Lincoln Memorial within three feet.

Color theory is all around us — in the products we use, the images we see and the nature in our backyards. Understanding color theory is critical to anyone in design, and it plays a crucial role in product development and brand imagery. Even color theory psychology is a growing trend. So what is it exactly?For starters, modern color theory has no shortage of definitions, but generally, it refers to the concepts behind our perceptions of color. So, to understand the basics of color theory, you must know how colors work.

What Is Color?

The way we perceive color makes it seem like the objects around us all have inherent visual properties attached to them. For instance, an apple is red, grass is green and the sky is blue, with little deviation.

What we find out, however, is that color is a matter of perception. It requires three things:

• An object
• A light source
• An observer

The light source can be natural or human-made, and the observer doesn’t necessarily need to be a living creature — it can also include machines like cameras and spectrophotometers.

When light bounces off an object, photons and electrons interact, with electrons absorbing or reflecting the light. In the process of reflection, the electrons release specific wavelengths of energy that correspond to certain colors, which our brains process.

How Do We See Color?

While light reflections release specific wavelengths, our brains interpret them as the colors we know and love. First, the reflected light enters the eye through the cornea. Then, a lens focuses it into the retina, the layers of nerve cells at the back of the eye.

The retina contains cells called photoreceptors, mainly rods and cones, that detect light waves.

• Rods activate in low or dim light and do not process colors.
• Cones activate in bright environments and contain specific pigments that correspond to our perceptions of red, green and blue.

These photoreceptors weave their way to the brain, connecting the communication patterns from the neurons in the retina to the brain.

Color vs. Appearance

In the common vernacular, color is typically a characteristic of appearance. For our purposes, appearance refers to surface features, like gloss and texture.

As light reflects off an object, it can take a few differing paths, based on the smoothness of the surface.

• Specular reflection: Specular reflections occur on surfaces with relatively few imperfections. You might see specular light in a still lake. It would reflect the trees and the clouds above it, so you could make out the image.
• Diffuse reflection: A diffuse reflection is much more common. It happens when the surface is rough or textured. The light reflects in random directions and won’t maintain an image of the source light.
• Combinations: Both specular and diffuse reflections can occur at the same time. Usually, this happens with scattered light distributed in a specific group. Semi-gloss and textured metals are an example of these mixtures. You may be able to see a low-resolution reflection of the image around it.

These surface characteristics can influence how we interpret color. For instance, coarse texture is going to reflect the light in more directions and appear lighter in color than a smooth, high-gloss surface. This effect occurs because less of the light is reaching your eyes when it scatters.

Pink is a calming color associated with love and affection. It also has a relatively recent history as a feminine color associated with womenswear. Read on to learn more about the color pink.

Facts About the Color Pink

Here are a few facts about pink:

• Madame de Pompadour, one of Louis XV’s lovers, liked the hue so much that the Sèvres porcelain company created and named a specific pink shade for her.
• Former first lady Mamie Eisenhower loved the color pink and had pink decor throughout the White House.
• Elvis Presley had a pink 1955 Cadillac.