The phenomenon where colors appear to be different depending on the surrounding colors or lighting conditions is known as color metamerism. This occurs because the human visual system processes color based on the light spectrum, and different spectra can produce the same perceptual color. Understanding metamerism is important for fields like photography, design, and printing to ensure colors are accurately reproduced across different mediums.
What Is Color Metamerism?
Metamerism refers to the phenomenon where two colors match visually under one light source, but when viewed under a different light source, they no longer match. This happens because the light spectrum affects how we perceive color. Our eyes contain photoreceptors that respond to different wavelengths of light. The combination of signals from these photoreceptors allows us to see a range of colors.
However, it is possible for two different spectral power distributions to produce the same response in our photoreceptors and therefore appear as the same color. These two colors with different spectra are called metamers. While they look identical under one light source, when the light spectrum changes, the distribution of wavelengths also changes. This alters how the photoreceptors respond and reveals the colors are actually different.
How Does Our Visual System Process Color?
To understand metamerism, it is important to understand some basics about how we see color. There are two main components involved:
- Light spectrum – The wavelengths and intensity of light emitted from a source
- Photoreceptors in our eyes – Known as cones, that respond to different wavelengths
We have three types of cones that are sensitive to short (S), medium (M), and long (L) wavelengths of visible light. By comparing the signals from these three cone types, our visual system can differentiate between millions of colors. This is known as trichromatic color vision.
When light enters our eye, the cones are stimulated in proportion to the amount of light at each wavelength. If two light spectra produce the same relative stimulation of the three cone types, they will be perceived as the same color, even if the spectra are different. These stimuli are metamers.
Metamerism and Light Sources
The light source plays a key role in metamerism. Under one light source, two colors may match due to equal cone stimulation. But if the light spectrum changes, the distribution of wavelengths and cone stimulation will also change, revealing the spectral differences between the metamers. For example:
- Daylight has a full spectrum of wavelengths, allowing us to differentiate more colors
- Indoor lighting is often deficient in longer wavelengths, causing color shifts like making whites look yellow or blue
- Fluorescent lights have spiky emissions at certain wavelengths that can cause wide color disparities
As a result, when viewing metameric color samples under fluorescent versus daylight illumination, dramatic color mismatches may be observed. The metamers look identical under one light but very different under another.
Examples and Applications
Metamerism comes into play with many real-world applications. Some key examples include:
- Textile color matching – Dyed fabrics can appear different colors under alternate light sources due to metamerism. This causes issues when trying to match colors across production batches.
- Paint color matching – Paint companies maintain precise illumination conditions when matching paint colors to avoid metamerism effects.
- Color printing/photography – Getting consistent color reproduction across different printers and papers requires managing metamerism.
- Color vision deficiency – Color blindness is caused by issues in cone photoreceptors, leading to increased susceptibility to metamerism.
Any field dealing with color reproduction faces challenges with metameric failure. Two materials engineered to match colors under one light source may appear obviously mismatched under real-world illumination.
Avoiding Failures in Color Matching
To avoid metameric issues, here are some steps that can be taken:
- Use a light source with full spectrum output like natural daylight
- Compare color samples under multiple light sources
- Use measurement devices like spectrophotometers to compare spectral data
- Select colorants that have similar spectral reflectance curves
- Limit the number of colorants used in reproductions
By understanding the causes of metamerism and taking steps to mitigate it, color matching and reproduction can become more accurate and consistent across different media and lighting conditions.
The Science of Metamerism
On a deeper scientific level, metamerism relates to the trichromatic nature of human vision. The phenomenon arises because the absorption spectra of the three cone photoreceptor types overlap significantly. This allows very different combinations of light wavelengths to produce a color match.
Mathematically, metamers can be described as a pair of stimuli with different spectral power distributions (S1 and S2) that have equivalent color matching functions when projected onto the three cone sensitivity curves:
M1(λ)S1(λ) = M2(λ)S2(λ) for λ = 380 to 780 nm
Where M1 and M2 represent the color matching functions for the observer’s cones and λ is wavelength.
This equation indicates that metamers S1 and S2 produce identical triplet values for the three cones despite having different spectral power distributions. The color match only holds for a given observer and light source.
Individual Variation in Metamerism
There are also individual differences in susceptibility to metamerism due to factors like:
- Variation in photopigments altering cone sensitivities
- Lens and macular pigment filtering light reaching the cones
- Deficiencies in color vision affecting cone function
A color match between two metameric samples can look very different to people with even subtle photoreceptor abnormalities. The range of wavelengths perceived can also shift with age as the eye’s optics change.
This means metameric samples appearing as a good color match to most people may look mismatched to certain individuals. This poses challenges for industries aiming to achieve uniform color appearance across their customer base.
Test Methods for Metamerism
To evaluate the metameric properties of color specimens, the following clinical tests are often utilized:
- Visual inspection – Samples viewed alternately under different light sources
- CIE Illuminant Series – CIE standardized white light sources A, C, D50, D55, D65, etc.
- Fluorescent versus daylight – Exaggerates metameric failure
- Instrument measurement – Spectrophotometers quantify spectral data
These tests help quantify metameric properties and identify samples prone to color mismatch. Minimizing metamerism is key for successful color reproduction across different lighting environments.
History of Research on Metamerism
Initial discovery of metamers dates back to the 18th century when chemists noticed color changes in chemical solutions under candlelight versus sunlight. In the 1930s, Wright and Guild carried out extensive studies on color matching and metamerism. They introduced the term metamerism in reference to color matches with different spectral reflectances.
Later work by Stiles, Wyszecki, Vos and others led to advances in the quantifying and modeling of metameric stimuli. Research is still ongoing today to better understand the neurophysiology of color processing in the retina and visual pathways.
Conclusion
In summary, metamerism presents an interesting challenge in color perception and reproduction. Two colors can appear identical under one light source, yet look strikingly different when that light source changes. Careful color management and testing methods are required to minimize unpleasant surprises from metameric failure.
While metamerism can cause headaches in photography, design and other fields, the complexity of color vision ensures this phenomenon will continue to be both useful and problematic. A deeper appreciation of the nuances of human color perception can lead to innovations in fields from digital displays to interior design to vision science.
| Light Source | Metameric Pair Example | Perceived Color |
|---|---|---|
| Daylight | Fabric A vs Fabric B | Color Match |
| Fluorescent | Fabric A vs Fabric B | Color Mismatch |
