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You have probably experienced it. A color that looked perfect in the production hall suddenly turns out to deviate at the customer's site. Or a textile match that was flawless in the light booth falls apart as soon as the clothing hangs in the store. The cause is almost always the same: the light source.
Light is not neutral. Every light source has its own spectral composition that directly determines which colors you can perceive — and how you perceive them. For everyone who works professionally with color, understanding light source properties is not a theoretical luxury. It is the basis of reliable color assessment.
Light fulfills three functions simultaneously in the perception process, and each of those functions has practical consequences.
Carrier of color information. The spectral composition of the light determines which color information is available at all. If part of the spectrum is missing from the light source, the colors that need that part are simply invisible.
Frame of reference for adaptation. Our visual system continuously adjusts to the prevailing lighting conditions — the mechanism we call color constancy. The light source acts as the calibration point. Switch light sources and the calibration point changes, shifting the entire color perception.
Stimulus for the visual system. Light activates the photoreceptors in the retina. The intensity and spectral distribution determine which receptors are engaged and to what degree. Too little light, and color discrimination decreases. Too much, and glare blurs the image.
Three functions, one conclusion: the light source is not a boundary condition — it is an active part of every color assessment.
Not all light sources are equal. Three measurable properties determine how a light source influences color.
The spectral power distribution describes how much energy a light source emits at each wavelength in the visible spectrum. It is the fingerprint of the light.
There are two main types. Continuous spectra — such as sunlight and incandescent lamps — distribute energy smoothly across the entire spectrum. Discontinuous spectra — such as fluorescent lamps and many LEDs — show peaks at specific wavelengths with valleys in between.
The difference is directly noticeable. A continuous SPD generally gives a more natural color rendering because all wavelengths are represented. A discontinuous spectrum can "miss" certain color nuances because the energy is lacking exactly where the material reflects. Modern LEDs with broad spectra reduce this difference, but the principle remains.
Color temperature — expressed in Kelvin — describes the apparent color of the light itself. The concept is based on the color an ideal black-body radiator would emit at a given temperature.
In practice we work with three zones:
| Zone | Temperature | Application |
|---|---|---|
| Warm white | 2700–3000K | Classic incandescent lamps, mood lighting |
| Neutral white | 3500–4500K | Offices, general work spaces |
| Cool white | 5000–6500K | Daylight, professional assessment lighting |
A common misconception: color temperature says something about the color of the light, but nothing about the quality of the color rendering. A lamp of 6500K can have poor color rendering. Color temperature and color rendering quality are two independent quantities.
The CRI measures how accurately a light source renders colors compared to a reference source — daylight for cool sources, an incandescent lamp for warm sources. The scale runs to 100.
| CRI value | Meaning |
|---|---|
| 100 | Perfect rendering, equal to the reference |
| 90+ | Excellent — minimum for professional color assessment |
| 80–89 | Good for general use, insufficient for critical color work |
| < 80 | Unsuitable for any form of color-dependent assessment |
The practical rule of thumb is clear: if you work in production, quality control or design, CRI 90 is the lower limit. For critical applications — think of color matching in textiles or coatings — you aim for 95 or higher.
To make color assessments comparable, the CIE (Commission Internationale de l'Éclairage) has defined a series of standard illuminants. Three of them dominate practice.
CIE D65 is the main standard: 6500K, an approximation of average northern daylight. D65 is the universal reference for color assessment in virtually all industries — from textiles and coatings to plastics and graphic media.
CIE A simulates incandescent lighting at 2856K. This standard is used to check how colors look under warm artificial lighting — relevant for products used in domestic environments.
CIE F11/TL84 represents typical store lighting: fluorescent, around 4000K. Indispensable for the fashion and retail industry, where the product is ultimately assessed by the consumer under this type of lighting.
Working with multiple standard illuminants is not a superfluous step. It is the only way to counter one of the most persistent problems in color quality: metamerism.
Metamerism is the phenomenon in which two colors appear identical under one light source, but visibly differ under another. It is no rarity — it is one of the most common causes of color complaints in the chain.
The explanation is spectroscopic. Two materials can have different spectral reflectance curves that happen to produce the same color stimulus under a specific light source. Switch the light source — and with it the spectral power distribution — and those curves no longer coincide and the difference becomes visible.
The classic example: clothing that is perfectly matched in the store (under TL84), but suddenly differs clearly at home under an incandescent lamp. The materials were never truly equal — the lighting masked the difference.
The solution is systematic and simple: always assess under multiple relevant light sources. Start with D65 for the primary assessment, then check under the light source that simulates the end-use situation. Document metamerism observations and communicate them clearly to suppliers and customers.
The choice and importance of light sources differs per sector.
In the textile industry, D65 is the standard for matching and approval, supplemented with TL84 to simulate retail conditions. The challenge lies in the fact that different fiber types — cotton, polyester, nylon — inherently have different spectral properties. A metameric match between two fiber types is the rule rather than the exception.
In the coatings industry, D65 is supplemented with illuminant A to evaluate the difference between outdoor and indoor situations. Automotive coatings add the complexity of effect pigments: metallic and pearlescent paints change color with the viewing angle, making multi-angle assessment under multiple light sources necessary.
In the food industry, it is about the difference between production and presentation. A piece of cheese or meat is assessed under D65 in the laboratory, but the consumer sees the product under store lighting. Packaging material adds an extra variable — the color of the product changes visually due to the transmission properties of the film.
The knowledge about light sources translates into a compact number of practical measures:
Respect the warm-up time. Light sources in viewing booths need at least 30 minutes for stable output. Assessing with a cold lamp yields unreliable results.
Plan maintenance. Reflective surfaces in the light booth gradually get dirty. Tubes and LED modules degrade. Annual calibration of color temperature and CRI is not excessive — it is basic maintenance.
Introduce a multi-light-source protocol. Start with D65, check under the relevant end-use lighting, document and communicate. Three steps that do not eliminate metamerism problems, but do make them manageable.
Check the intensity. The standard for color assessment lighting lies between 1000 and 2000 lux on the assessment surface. Too little light reduces color discrimination; too much causes glare and fatigue.
In many organizations, lighting is treated as a facility — something that is simply there. But for color-dependent processes, the light source is a measuring instrument. An instrument that must be calibrated, maintained and used correctly, exactly like a spectrophotometer or a scale.
Investing in the right lighting and accompanying procedures yields measurable results: fewer rejections due to metamerism, faster approval cycles, fewer discussions in the chain and ultimately higher customer satisfaction.
The light source is the first link in reliable color assessment. Anyone who controls that link has the basis in order.
This article is based on insights from "Kleurbeoordelen in de Praktijk" by Mark Kotterink. The full technical elaboration is available at ColorExpertsHub.
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