Equiluminant

Equiluminant (pronounced ee-kwuh-loom-uh-nuhnt)

(1) In optics, the quality of two or more objects or phenomenon being equally luminant.

(2) Figuratively, two or more people being judged equally illustrious, attractive, talented etc.

1860s: The construct was equi- +‎ luminant.  As an adjective, luminant means "that which illuminates; that which is luminous" while as a noun it describes "an illuminating agent".  Luminant was from the Latin verb lūminant, the third-person plural present active indicative of lūminō, the construct being lūmen, from the Proto-Italic louksmən, from the primitive Indo-European léwk-s-mn̥, from the root lewk- (bright) +‎ -ō (appended to form agent nouns).  The accepted synonym is isoluminant and equiluminescent is the alternative form.  When used figuratively, although it would make no sense in science, the comparative is “more equiluminant” and the superlative most “equiluminant”.  Equiluminant & equiluminescent are adjectives and equiluminance is a noun; the noun plural is equiluminances (which some list as non-standard).

The prefixes: equi-, homo-, peri- & iso-

The prefixes “equi-”, “homo-”, “peri-” & “iso-” are all used to in same way suggest a concept of sameness or equality, but by tradition and convention, are used in different contexts to produce different meanings or emphasis:

Equi- is used to indicate equality, evenness, or uniformity and is often seen in mathematical, scientific & technical publications to describe something is equal in measure or evenly distributed such as equilateral (a shape having all sides of equal length, equidistant (being at equal distances from two or more points) & equilibrium (a state of balance where opposing forces or influences are equal).  Homo- is used to imply “same” or “alike” and thus sameness or (sometimes by degree) similarity.  In technical use it is a standard form in biology, chemistry & the social sciences to indicate sameness in kind, structure, or composition and by far the most common modern use is in the now familiar “homosexual” which in many jurisdictions is now a proscribed (or at least discouraged) term because of negative associations (“homo” as a stand-alone word also having evolved as a slur used of, about or against homosexual men).  The uses of the prefix are illustrated by homogeneous (composed of parts or elements that are all of the same kind, homologous (having the same relation, relative position, or structure) & homonym (in linguistics words which sound the same or are spelled the same but have different meanings).  Iso- is used to denote equality, uniformity, or constancy in terms of specific characteristics like size, number, or configuration and is most used in scientific and mathematical publications.  Examples of use include isometric (having equal dimensions or measurements, isothermal (having constant temperature) & isosceles (having two sides of equal length).  Peri- is used to denote “surrounding or enclosing”, or “something near or around a specific area or object”, examples including perimeter (the continuous line forming the boundary of a closed geometric figure), periscope (an optical instrument for viewing objects that are above the level of direct sight, using mirrors or prisms to reflect the view & peripheral (relating to or situated on the edge or periphery of something.

So equi-focuses on equality in measure, distance, or value, homo- focuses on sameness in kind, structure, or composition, iso- focuses on equality or uniformity in specific characteristics or conditions while peri- :focuses on surrounding or enclosing, or being near or around something.  For most purposes equi- & iso- can be used interchangeably and which is used tends to be a function of tradition & convention.

Equiluminant colors

An example the equiluminant in blue & orange.  In color the text appears at the edges to "shimmer" or "vibrate".  When re-rendered in grayscale, because the value of the luminance is so close, the two shades become almost indistinguishable.

In optics, “equiluminant” is a technical term used to colors with the same (or very similar) luminance (brightness) but which differ in hue (color) or saturation (intensity).  The standard test for the quality is to convert a two-color image to grayscale and, if equiluminant, the colors would appear nearly indistinguishable because they share the same level of “lightness”.  It’s of some importance in fields as diverse as military camouflage, interior decorating, fashion, astronomy and cognitive psychology.  In the study of visual Perception, when colors are equiluminant, the human visual system relies primarily on the differences in hue and saturation rather than brightness to distinguish them and can create unique visual effects and challenges in perception, as the brain struggles will in most cases struggle to segregate colors based solely on luminance; essentially, there is a lack of information.

An enigmatic abstraction (2024) by an unknown creator.  This is an example of the use of non-equiluminant shades of orange & blue.

In art and design, the qualities of is used to create visual effects, the perception of some “shimmering” or “vibrating” at the edges where colors meet actually a product of the way the different hues are perceived by the brain to be “less defined” (a process not dissimilar to the “grayscaling”) and thus “dynamic”, lending the impression of movement even in a static image, especially if seen with one’s peripheral vision.  While a handy device for visual artists, that’s something of significance in the world of practicalities because the close conjunction of equiluminant colors can make certain visual tasks more difficult, most obviously reading text or distinguishing shapes and objects.  All that happens is the luminance contrast can means there’s a perception of fuzziness at the edges of shapes which means people can suffer a diminished ability to distinguish fine details and the smaller the object (text, numerals or geometric shape), the more acute the problem.  The phenomenon has been well researched, scientists using the properties in equiluminant colors to study how the brain processes color and the findings have been important in fields like instrumentation the production of warning signs.

Richard Petty's 1974 NASCAR Dodge Charger (left) and 3 ton Super-Duty Jack, produced under licence by the Northern Tool Company (right).  1974 was the last year in which the big-block engines were allowed to run in NASCAR; the big-block era (1962-1973) was NASCAR's golden age. 

Richard Petty (b 1937) used a "reddish orange" to augment his traditional blue when he switched from Plymouth to Dodge as the supplier of his NASCAR stockers in the early 1970s.  His team was actually sponsored by STP rather than Gulf and STP wanted their corporate red to be used but in the end a "reddish orange" compromise was negotiated.  However, when he licenced the Northern Tool Company to sell a "Richard Petty" jack, the shade used appeared to be closer to the Gulf orange.  

At scale, equiluminance doesn’t have to be obvious for it still to have desirable “side effects” and while it’s often noted two specific hues: (1) the blue Llewellyn Rylands pigments 3707 (Zenith Blue, replicated by Dulux as “Powder Blue”) & (2) the orange Rylands pigments 3957 (Tangerine, replicated by Dulux as “Marigold”) are “equiluminant colors”, that their use in combination appears so often on cars, motor-cycles and other stuff with wheels is due less to the claim the shades seem at the edge to “vibrate” that the striking combination appearing on some of the Gulf Oil sponsored Ford GT40s and Porsche 917s during sports car racing’s golden era which ended in the early 1970s.  Given the surface area involved, the effect is probably imperceptible which viewed at close range but the science does suggest that at speed (and these were fast machines), at the typical viewing range found on racetracks, there was what the optical analysts call “visual pop”, something which heightens the brain’s perception of motion,

Ford GT40 chassis# 1075, winner of the 1968 & 1969 Mans 24 hour endurance classic in Gulf livery.

Gulf Oil however didn’t explicitly have the equiluminescent in mind when they choose to adopt the combination.  Originally, the Gulf GT40s were painted in the their corporate colors of dark blue & orange but Gulf was an acquisitive conglomerate and in late 1967 it took over the Wilshire Oil Company of California, the corporate colors of which were powder blue and orange, something which Gulf’s management thought “more exciting” and better suited to a racing car.  The change was made for the 1968 season with the Fords now running as five-litre (302 cubic inch) sports cars, governing body having banned the seven-litre engines the cars previously had used (under a variety of names, motorsport has for decades been governed by some of sport’s dopiest regulatory bodies).  In the Gulf colors, GT40 chassis #1075 won the Le Mans 24 hour endurance classic in 1968 & 1969 (repeating the brace Ford had achieved with the seven-litre (427 cubic inch) machines in 1966-1967), the first time the same car had achieved victory twice.  In 1968, the same car won the BOAC International 500, the Spa 1000-kilometer race, and the Watkins Glen 6-hour endurance race, while in 1969 it also took the Sebring 12-hour race, a remarkable achievement for a race car thought obsolescent.  The livery has since been much replicated.

1971 Porsche 917K in Gulf livery.  The fins were added to improve straight-line stability and were strikingly similiar to those which appeared on some late 1950s US Chryslers although the aerodynamic properties of those were dubious.

Interestingly, the team painting the GT40s were aware of the issue created by equiluminant colors and as a matter of professional pride didn’t want it thought they’d created something with “fuzzy edges” so deliberately they included a dark blue hairline-border around the orange, reducing the optical illusion to ensure everything looked immaculately smart when photographed.  When the Gulf team in 1970 switched to using Porsche 917s for the World Sports Car championship, they adopted the expedient of a black line of definition between the blue & orange so the whole enduring appeal of the combination lies just in the striking contrast and relies not at all on the equiluminance.

Ford GT Heritage Edition First Generation (left) and Second Generation (right). 

Little more than 100 GT40s were built but Ford noted with interest the ongoing buoyancy of the replica market, thousands of the things built in a number of countries.  In the twenty-first century, the company decided to create their own replicas although the new GT (2004-2006) was hardly a clone and although it shared the basic mechanical layout and the shape (although larger) was close, it was a modern machine.  The car wasn’t called GT40 because the rights to the name had ended up with another company and Ford declined to pay the demanded price.  Over 4000 were built and one special run was a tribute to the 1968-1969 cars in Gulf livery, 343 of the “Heritage Editions” produced.  A second generation of GTs was produced between 2016-2022 and was very modern, the demands of the wind-tunnel this time allowed to prevail over paying tribute to the classic lines of the 1960s.  Although the supercharged 5.4 litre V8 didn’t return and the new car used a turbocharged 3.5 litre (214 cubic inch) V6, it outperformed all its predecessors over the last 60-odd years (all the original GT40 chassis built between 1964-1969) including the 7 litre (427 cubic inch) monsters which won at Le Mans in 1966 & 1967 so it took decades, but eventually there really was a "replacement for displacement".  The V6 also was used also in pick-up trucks which doesn't sound encouraging but versions of the small & big block V8s used in the GT40s also saw similar service, the latter even first appearing in the doomed Edsel.  Production of the second generation was limited to 1350 units, 50 of which were “Heritage Editions” in the Gulf colors, one of several “limited editions”.

Lindsay Lohan who when swimming uses both orange and blue, sometimes as solids (left and centre), sometimes in combination (right).