Colors are not as simple as they seem. Every human sees and perceives color differently, and how we respond to different colors… well… differs. For example, did you know that green is used for night-vision goggles because the human eye is able to recognize most shades of that color? Or that the food industry utilizes red and yellow to evoke the taste buds and stimulate appetite? If you didn’t immediately think of McDonald's after that example I’d be surprised. Color is important to how we see and interact with the world. Being able to accurately reproduce color on branded items across a variety of packaging types brings to question… how do we measure success?
How We Measure Color
Color measurement is a vast and complex subject that involves multiple scientific & industrial concepts. It encompasses several fields such as imaging, colorimetry, and, photometry. In the early 1900s, The International Commission on Illumination (CIE) was formed to standardize these fields. CIE introduced the idea of Delta-E in 1976 when they highlighted the issue of color difference. It’s a single number that denotes the 'distance' between two colors. It helps us understand the perception of the human eye towards color difference. Studies show that a dE of less than 1.0 is not perceivable by most humans. When applied to print, an acceptable variance in color is set around a dE of 2.0. So, any dE less than 2.0 is acceptable print quality and largely unnoticeable while any dE above 2.0 is unacceptable and visibly lacking in quality.
dE Calculates:
Degree of deviation of print or proof from the original
Drift of a device
Effectiveness of a particular profile for printing or proofing
The below table will help you understand the concept of Delta-E
Delta E Perception
<= 1.0 = Not perceptible by human eyes.
1 - 2 = Perceptible through close observation. (acceptable printed range)
2 - 10 = Perceptible at a glimpse.
11 – 49 = Colors are more alike than opposite (ex: color families, blues, yellows etc.)
100 = Colors are exact opposite (ex: Black and white)
Images sourced from sherwin-williams.com
COLOR MEASUREMENT INSTRUMENTS
Historically, experienced colorists and engineers would perform a color measurement, but more recently, instruments and devices have replaced old methods. The goal is to save time & effort, reduce labor costs, and apply scientific methodology to improve efficiency. Color measurement instruments are available in great variety. These instruments take the reading of the amount of visible light passing through or reflecting from an object. These can be divided into three main types which are explained below.
Densitometers
A color measurement device that is used to measure the optical density (degree of darkness) of a reflecting surface or a semitransparent material. In actuality, it measures density instead of color. Its basic working principle is that a sample is placed between a light source and a photocell. The light source is then targeted on the photocell and it determines the density by the difference in the readings of the two components.
There are three types of densitometers
Transmission densitometers: which measure transparent materials, such as film negatives or glass
Reflection densitometers: which measure the amount of light being reflected from a surface, such as ink on film or paper
Combination densitometers: measure both reflection and transmission densities
Colorimeters
A light-sensitive device that measures the absorption and transmission of light passing through a liquid sample. A light source, a photocell, and a cuvette that contains the sample are the three main components of a colorimeter. Its working is based on Beer-Lambert's law, which states that the medium concentration and absorption of light transmitted through the medium are directly proportional to each other. Other than research and quantitative analysis in various fields, colorimeters are utilized in food ingredients, textile products, building materials, chemical solutions, beverages, and many others.
There are two types of colorimeters
Color densitometers: which are used to measure the density of colors.
Color photometers: which are used to measure the reflection and transmission of colors.
Spectrophotometers
An optical instrument that is used to measure the intensity of light relative to the wavelength. It helps the engineers or scientists to draw a spectrum of white light and quantify the light returned from the sample. Spectrophotometry is considered one of the most valuable approaches of quantitative analysis in many fields such as physics, chemistry, chemical & material engineering, clinical applications, and biochemistry.
CALIBRATION
Modern instruments give efficient performance when first manufactured, but this efficiency is reduced with time, heavy usage, or with variations in the operating environment. Therefore, the performance of an instrument should be monitored, and it should be calibrated regularly. The comparison between a standard measurement and known measurement taken by an instrument is known as calibration. Un-calibrated instruments affect measurement accuracy during sample preparation and presentation which reduces product quality.
As a standard procedure, instruments should be calibrated
After exposure to any environmental hazard (Dust, water, fire, etc.)
Daily after each shift (Under heavy usage)
After any electrical or mechanical shock.
Periodically (annually, quarterly, monthly, etc.)
Recertified by the manufacturer annually or biannually per their recommendations
Following are a few steps that we can take to minimize the calibration errors in instruments
Routine maintenance
Cleaning & Aligning of the optics
Ensure handling by the expert staff
Minimum moving and shifting